text:
Meaning of 'kalapas'
What did the Buddha really meant with 'kalapas' (those ultra small particles of existence, that arise and pass away at an incredible speed)?
Is there a correlation between those kalapas and the emptiness of all things (sunnata)?
Can those kalapas be considered as the 'source' for all manifestations?

Wikipedia claims that "the Buddha never directly speaks of kalapas".

What is it that you mean by the emptiness of all things?

All phenomena exhibit selflessness.  However, if one is trying to explain a river to someone who has only an understanding of droplets, it would not be wrong to say that the river is many many many droplets together.  It would not be right, though, either.

Can one experience "kalapas" in the present moment without it being a mind made concept by the one experiencing it?

One source for you to look into is "Knowing and Seeing" by Pa Auk Sayadaw; kalapas are discussed therein. http://www.paaukforestmonastery.org/books/knowing_and_seeing_rev_ed.pdf

kalāpa in the sense you are thinking is used in the commentaries. The word is generally used in the suttas to mean "bundle", as in yavakalāpa - bundle of grass, or dhanukalāpa - sheaf of arrows. I can't pinpoint the first usage of the word to mean "smallest particle  of matter", but the Visuddhimagga uses it in this way:

“But since neither of these is a fact, you should therefore give up conjecturing
  the difference to be in the supporting primary elements. Just as the natures of visible
  objects, etc., are dissimilar from each other though there is no difference in the
  primaries that form a single group, so too are eye-sensitivity, etc., though no other
  cause of their difference exists.”
-- Vism XIV.45 (Nyanamoli, trans)

The Pali here is ekakalāpe - "in a single group". The meaning is that each instance of matter is made up of a single group of characteristics.
The abhidhammattha-sangaha also uses the term:

Ekuppādā ekanirodhā ekanissayā sahavuttino ekavīsati rūpakalāpā
  nāma.
There are twenty-one material groups inasmuch as they arise together, cease together, have a common basis, and occur together.
(Bodhi, trans)

Bhikkhu Bodhi's explanation is:

Material phenomena do not occur  singly,  but  in  combinations or
  groups known as rūpakalāpas, of which twenty-one are enumerated. Just
  as all the cetasikas possess four characteristics (see II, §1), so too do
  the material phenomena  in a group. All  the material phenomena in  a
  group arise together and cease  together.  They  have  a common base, namely, the conascent great essentials, which are the proximate cause
  for the derivative phenomena as well as for each other. And they all occur
  together from their arising to their cessation.
(From A Comprehensive Manual of Abhidhamma, pp. 252-3)

Nyanatiloka gives the following explanation of rūpa-kalāpa in his Manual of Buddhist Terms:

'corporeal group', material unit, designates a combination of several physical phenomena constituting a temporary unity. Thus, for instance, the so-called 'dead matter' forms the most primitive group, consisting only of 8 physical phenomena, called the 'pure eightfold unit' or 'octad' (suddhatthakakalāpa), to wit: the 4 elements (the solid, fluid, heat, motion); colour, smell, taste, nutriment (pathavī, āpo, tejo, vāyo; vanna, gandha, rasa, ojā).
In Vis.M., and elsewhere, it is also called ojatthamaka-kalāpa, 'the octad with nutriment as the 8th factor'.
The simplest form of living matter is the '9-fold vitality unit' or 'life-ennead' (jīvita-navaka-kalāpa), formed by adding 'vitality' to the octad.
Seven decades, or units of ten (dasaka-kalāpa), are formed by adding to the 9-fold unit one of the following corporeal phenomena: heart (physical seat of mind), sex, eye, ear, nose, tongue or body. -
See Vis.M. XVIII, 4; Compendium of Buddhist Philosophy (PTS), p. 164, 250; Atthasālini Tr., II, 413f.

Finally, Mahasi Sayadaw talks about rūpa-kalāpa in the context of rebirth as a human in his discourse on Paticca-Samuppada:

With the arising of rebirth consciousness there occur simultaneously three kammajā-rūpakalāpas or thirty rūpas. These are rūpas that have their origin in kamma, viz., ten kāyarūpas, ten bhava-rūpas and ten vatthu rūpas. The nine rūpas, to wit, the solid, fluid, heat, motion, colour, smell, taste, nutriment and life together with the kāyapasāda (body-essence) rūpa form the ten kāyarūpas; bhava-rūpa and the solid, etc form the group of ten Bhavūpas. Bhavārūpa means two germinal rūpas, one for manhood and the other for womanhood.
...
This embryonic rūpa has the size of a little drop of butter-oil scum on a fine woollen thread. It is so small that it is invisible to the naked eye. It does not exist by itself. We should assume that it arises from the fusion of the semen (sukka) and blood (sanita) of the parents. If we reject this view, it will be hard to explain the child's resemblance to his parents in physical appearance. It is also said in the suttas that the physical body is the product of the four primary elements and the parent's semen. Moreover, the piṭaka specifies three conditions necessary for conception, viz., the parent's intercourse, the menstrual discharge of the mother and the presence of something qualified to become an embryo. Thus it is clear that according to the scriptures, the embryonic kalāla has its origin in the fusion of parent's semen and blood.
...
According to Western biologists, it is the fusion of the mother's ovum and the father's spermatozoa that gradually develops and becomes a child. The original embryo is so small that it cannot be seen with the naked eye. The findings of these scientists fairly agree with what the Buddhist books say about conception. Without the help of microscope or other instruments but purely by means of his intellect the Buddha knew how life begins with three kalāpas or thirty rūpas as kalāpa on the basis of parents' semen and blood. This was the Buddha's teaching 2500 years ago and it was only during the last 300 years that Western scientists discovered the facts about conception after long investigation with microscopes. Their discoveries bear testimony to the Buddha's infinite intelligence. However, they are as yet unable to reveal the genesis of thirty rūpas probably because the extremely subtle kammajarūpas defy microscopic investigation.

UPDATE (to reflect your new questions):
Is there a correlation between those kalapas and the emptiness of all things (sunnata)?
The theory of the kalāpa has no direct connection with the theory of suññatā; it simply describes the composition of individual groups of matter. Indirectly, though, since suññatā is very much connected with anattā (being devoid or "empty" of self), the fact that rūpa is made up of individual states that arise and cease without remainder is indeed an important part of the realization of this truth. E.g.:

Having thus discerned by knowledge of contemplation of reflection that
  “All formations are void” (see S III 167), he again discerns voidness in the double logical relation thus: “This is void of self or of what belongs to self” (M II 263; Paṭis II 36).
When he has thus seen that there is neither a self nor any other [thing or
  being] occupying the position of a self's property, he again discerns voidness in the quadruple logical relation as set forth in this passage: “I am not anywhere anyone’s owning, nor is there anywhere my owning in anyone (nāhaṃ kvacani kassaci kiñcanat’ asmiṃ na ca mama kvacani kismiñci kiñcanat’ atthi)” (M II 263).
-- Vism XXI.53 (Nyanamoli, trans)

Can those kalapas be considered as the 'source' for all manifestations?
kalāpa, or more properly rūpa-kalāpa, are physical, so no they are only a source for certain other phenomena. "source" is a problematic term anyway, since the causal nature of reality is far more complex than simply A => B.

Is there a correlation between those kalapas and the emptiness of all things?

@StoppingEgo, it's empty as in empty of a self and empty of any inherent value.

@StoppingEgo answer updated to reflect your new questions

The meaning of Kalapas in the context of how you form your question (What did the Buddha really mean by Kalapas) cannot be answered. In short, the only correlation between Kalapas and Sunnata (in the form that 'emptiness' is presently understood) is that neither are original teachings of the Buddha.
Kalapas -
Kalalaps are first mentioned in the Abhidhamma, written in the 5th - 11th century. An excellent summary of misrepresentation of Buddhas original teachings by Christopher Titmuss (What the Buddha did not teach):

Sub atomic particle (kalapas). The Buddha did not teach development of
  the power of concentration (samadhi) in meditation in order to
  experience sub-atomic particles. The word ’kalapas’ does not appear
  anywhere in the suttas. The Buddha did not support such a
  materialistic view or non materialistic views.

Sunnata - In the only Sutta where the Buddha discusses Sunnata directly (a translation can be found here Cula-suññata Sutta), he uses it as way to describe seeing existence as it is i.e. empty of personal interpretation or overlay:

Thus he regards it as empty of whatever is not there. Whatever
  remains, he discerns as present: 'There is this.' And so this, his
  entry into emptiness, accords with actuality, is undistorted in
  meaning, & pure.

The meaning of Kalapas

How A Kalapa Is Formed
A group of people association is called a club. In Pali kalapa also means a group. As said before rupa (matter) cannot exist in isolation. Appropriate units of matter combine to exist as a group or an aggregate. Such a group with common properties, existing together, disappearing together, is termed 'one kalapa'.
Matter In Combination
The four fundamental elements - pathavi, tejo and apo - together with vanna (appearance), ghanda (scent), rasa (taste), oja (essence of nutrition) are the eight classes of matter found always in coexistence. A lump of soil is an aggregate of eight classes of matter too. It has a certain appearance and a specific smell and taste; it can be touched and felt. The same is true for water, wind, fire, heat, light etc. They are all aggregates of the eight classes of matter.
The Size Of A Kalapa
Each kalapa is so minute that you cannot see it with your naked eye. Even the finest dust particle is an aggregate of a large number of kalapas. A bacterium, which can only be seen with the most powerful microscope is composed of countless kalapas formed by kamma, citta, utu and chara (*ahara). Therefore the minuteness of a kalapa is beyond description. (Reference may be made to text on Abhidhamma for classes of kalapas and their names).

Before we can get to the understanding of Kalapas, we need to understand some of the characteristics of Rupas and their causes. The "much easier and readable definition of Rupas" happens to be in the introduction to this Pathana.
Rupas and their characteristic

.. there are in total 28 paramattha rupa which are all realities and can be sensed by one of our 6 senses.
Rupa or vanna is sensed by eye only,
      sadda is sensed by ear only,
      gandha is sensed by nose only,
      rasa is sensed by tongue only and
      pathavi, tejo, vayo three of mahabhuta rupa are sensed by body only.
Vanna, gandha, rasa, oja or ahara, pathavi, tejo, vayo and apo always co-exist and they are called avinibbhoga rupas and other 20 rupas are called vinibbhoga rupas.
All other rupas that is 5 pasada rupas and 16 sukhuma rupa or subtle materials are all sensed by mind only; that is 5 pasada rupas and 16 sukhuma rupa can never be sensed by any of eye, ear, nose, tongue, and body.

Rupas and their relationship with causes

There are kamma that can cause kammaja rupas.
These kamma are:

12 akusala (* unskilful )kamma,
8 kamavacara kusala (* skilful human) kamma, and
5 rupavacara kusala (* skilful Brahma) kamma.
Even though there are 28 paramattha rupas, they do not arise in isolation. At least some arise together. And they vanish together. They have to arise on mahabhuta rupas and they co exist in a form called rupa kalapa. There are 21 rupa kalapas. 9 kalapas are kammaja rupa kalapas, 6 are cittaja, 4 are utuja and 2 are aharaja rupa kalapas.

Now we can classify kalapas according to their causes

Kammaja rupa kalapas are:

cakkhudassaka,
sotadassaka,
ghanadassaka,
jivhadassaka,
kayadassaka,
ithibhavadassaka,
punbhavadassaka,
vatthudassaka, and
jivitanava kalapa.

In cakkhudassaka kalapa, there are 8 avinibbhoga rupas, jivita rupa and cakkhu pasada rupa altogether 10 rupas arise together. Other kammaja kalapas are the same and just to replace cakkhu with their correspondent rupa. Jivitanava kalapa is a combination of 8 avinibbhoga rupas and jivita rupa itself.
6 cittaja rupa kalapas are:

suddhattha kalapa which comprises 8 avinibbhoga rupas only,
kayavinattinava kalapa which comprises 8 rupas and kayavinatti rupa,
vacivinatti saddadassaka kalapa which is compsed of 8 rupas, vacivinatti and sadda,
lahutadi ekadassaka kalapa which is combination of 8 rupas and 3 lahutadi rupas,
kayavinatti lahutadi dvadassa kalapa which comprises 8 rupas, 3 lahutadi and kayavinatti rupa, and
vacivinatti sadda lahutadi terasaka kalapa which includes 8 rupas, 3 lahutadi, vacivinatti and sadda rupa.

4 utuja kalapas are:

suddhattha kalapa,
saddanava kalapa,
lahutadiekadassaka kalapa which comprises 8 rupas and 3 lahutadi rupas, and
sadda lahutadi dvadassaka kalapa which comprises 8 rupas, 3 lahutadi rupa, and sadda rupa.

2 aharaja rupa kalapas are:

suddhattha rupa lakapa and
lahutadiekadassaka rupa kalapa which comprises 8 inseparable rupas and 3 lahutadi rupas.

From the 4 utuja rupa kalapas, suddhattha rupa kalapa and saddanava rupa kalapa can arise outside of beings or sattas. All other rupa kalapas only arise as ajjattika.

Well in can only get easier when you have read it for about 10 times remembering the pali names that go along with it.
As Kalapas arise and pass away it does not posses and inherent existence nor an existing self so in that way it is empty.
All material manifestations produced by the 4 causes of kamma, citta, utu, ahara are in the form of kalapas. So yes! As there are no material manifestation that are not produced by these 4 causes (with ref. to the Abhidhammma, of course).
(* my additions/comments/terminology)

As Buddah said the smallest unit of the universe is a "Suddashtaka" it contains following 8

Hardness
Air
Liquid
Heat (Hot/Coolness)
Colour
Smell
Taste
Energy ( Ojah)

And each any every Kalapa are consists with 1 or more of this Suddashtaka.
Each and every thing in this universe consists of these things.

If I search for "Suddashtaka" I only find this previous answer of yours. If you can, please add a reference, for example to a sutta in which the Buddha said this, or to a book in which an author talks about it, or to a dictionary entry.

Yeah I have been studying for a few years and have never come across such a unit nor amalgamation of properties into a single unit.

suddhattha kalapa which comprises 8 avinibbhoga rupas only - Patthana Dhamma by Htoo Naing - Chapter 3. Suddhattha - Pali, Suddashtaka - Sanskrit

Kalapas are sub atomic particles. Gautama the Buddha when sitting at the age of 35 under a bodi tree, sat in the ultimate adithaana meditation. He sharpened his awareness to the extent that he tore apart and experienced an atom and its particles. What he also experienced is the impermanence of these particles as in they were arriving and passing away. He also experienced these as vibrations or wavelets. This revealed the impermanence of matter or the universal law of the atomic universe.
These can be reconciled with science as the disappearance of particles during a phenomenon called tunnelling where a particle disappears and appears somewhere else.
96% of the universe is non atomic. Also called dark energy or dark matter. 99.99% of the remaining 4% of the atomic universe contains interparticle space. The space between say elections and nucleus. And space between proton and neutrons as they move around. Thus the rest is particles and sub particles or quarks. These are also impermanent. They disappear when not observed or not interacting with other particles.
THUS THE UNIVERSE IS MADE UP OF...,,NOTHING.
The last bit is being considered as scientific fact apparently.
Watch here https://youtu.be/WMhJgdpj1d0

"He sharpened his awareness to the extent that he tore apart and experienced an atom and its particles." Where is this written in the Buddhist scriptures? Thanks


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Any advice for beginners about Vipassana traditions?
There are many Theravada Vipassana traditions according to this wikipedia page and more according to this answer.
My questions are:

Are any, or all, of these traditions suited to beginners?
And why?
Are any of them more especially suited for beginners?
If you recommend a tradition for beginner, how can one begin it: books; weekly lessons with a teacher; ten-day retreats; Youtube; ... ?
Please reference (i.e. quote, cite, or link to) advice, from any of them, that's especially intended for beginners.

Some of the traditions include:

Ledi Lineage (Ledi Sayadaw, U Ba Khin, S.N.Goenka, Anagarika Munindra, Dipa Ma)
Mahasi Lineage (U Narada, Mahasi Sayadaw, Nyanaponika Thera, Bhikkhu Bodhi, Goldstein, Salzberg)
Thai Forest Tradition (Ajahn Chah, Ajahn Sumedho, Thanissaro Bhikkhu, Kornfield)
Pa Auk Sayadaw Lineage (based on Visuddhimagga)
Mogok Sayadaw
Anagarika Munindra (combined Goenka and Mahasi methods)
and more

I'm pretty sure each of the traditions mentioned will claim they are for beginners... I can't imagine you getting an objective answer to this question.

I'm going to make this a wiki and ask contributors to post information helpful for beginners in each tradition - please only talk from experience/authority/citable sources, and please don't brag about your tradition! Objective info only please.

I'm personally very interested in this. In my tradition - Triratna - we say that insight isn't suitable for beginners at all. I'm aware this is controversial. So I'm interested how different traditions approach this

I agree with yuttadhammo. I'd also add that far more important than nit picking over the vipassana tradition, would be picking one and actually committing to it and being consistent with your practice. Don't get so caught up researching and learning that you forget to practice.

I'm happy that I didn't know Vipassana wasn't for beginners when I was a beginner. But I am still a beginner.

@yuttadhammo I edited the question to try to match your suggestion.

Beginners would benefit most from having an attitude of being a beginner. According to the Zen tradition of the Soto branch, the object of practice is not to achieve great knowledge, powers or concentration but to discover and weed out things that block the experience of what Shunryu Suzuki calls beginners mind. He is not talking to Soto Zen students. He is talking to Buddhists everywhere in all traditions. Every Buddhist tradition can lead to that fresh perception of reality that awakening reveals. If a tradition only provides you with a set of beliefs and does not point the way to awakening then keep looking. Everyone has the right to choose their own path, and if it serves its purpose or does not help you, move on.
www.arvindguptatoys.com/arvindgupta/zenmind.pdf
I ask myself daily, am I ready to be a beginner. There are no advanced students in Buddhism. We are all beginners. Some beginners have walked the path longer and are aware of the inner mysteries to a greater degree. The Buddha never said I am enlightened and you are just mere mortals. He recognized the Buddha consciousness in all.
When I came to Buddhism I attended meditations in as many traditions as possible. Sometimes I clicked when I read a book but did not click at a local meditation. It does not matter what path you choose as long as it is one that is in harmony with your inner self and one that you practice every day in your meditation and in your living. It is a path of yielding and melting away any sense of separate self so we do not need to attain or accomplish great things
"just flow like the river
going to the sea,
flow around every bend and turn
Until all that's left is eternity."

Especially if you are a beginner it is extremely important that you get the basics right. Vipassana is the insights that you make in the Dhamma Path, and for that you have to have a very good grasp of the True Dhamma. We should not think that we have the ability to do it our way. There are places where the Buddha has stated directly: “Do it this way and the result will be this.”  In this present era we are at a disadvantage when it comes to the practice of Dhamma. The wisdom to see this ‘otherness ’started deteriorating from the time of the Buddha.
Therefore we must think of approaching the Dhamma from a convenient place. The best method of approaching Dhamma is to use a system indicated by the Buddha. There are places where he has directly indicated, “If done this way, these will be the results”. We should act thus.
The Supreme Buddha said that there are five factors which if practiced extensively will result in being unemotional (with the world at large), to ridding oneself of egoism, to perfect wisdom and to Nibbana. What is it that prevents us from coming to this?  We have still not distanced ourselves from the ways of the world. We have been adopting a methodology which does not give rise to dispassionateness or profound disapproval.
Many people claim to be meditating. They claim to be studying the Dhamma. They are carrying out the four activities of listening to Dhamma, discussing Dhamma, Samatha and Vipassana.
In spite of performing those four activities pride, arrogance, thinking too much of themselves, looking down on others surface in them. That means although they claim to be performing the aforementioned four activities they have been doing something else. Something appearing as those four.
What happens if they had correctly done the four activities of listening to Dhamma, discussing Dhamma, Samatha and Vipassana? it is a upward path to progress up to Arahantship for the individual who practices these four activities. To proceed up to Arahantship one must establish in himself with understanding the quality of dispassionateness. This quality has to get built up.
If one practised listening to Dhamma, Dhamma discussions, Samatha and Vipassana properly, dispassionateness with understanding should take place in him. That should happen. But in case of some people the dispassionateness with understanding does not take place while vanity and pride surface. Then what has gone wrong? Doing something which appears as correct thinking or correct thing. That is where the mistake arose.
It is due to not accepting what is given in the preaching. Or not carrying out what the Buddha has preached. Something which appears like that is carried out. If what the Buddha preached had been done, the results preached by him should materialize. Why doesn’t the anticipated result materialize? What has been done is not the relevant thing. Something else appearing like that has been done. That is why the mind turns towards aspects like vanity and pride.
If vanity, pride placing oneself above others, looking down on others arise in a person developing concentration after meditation it is due to his not doing the correct thing. That would not have happened if he had done the correct thing. If one practices the five factors indicated in this preaching he will invariably develop dispassionateness with understanding. The Buddha has stated ‘definitely’. It happens invariably.
That is what we should get done. If we practise the five Dhamma factors dispassionateness will definitely arise with understanding. If dispassionateness arises in one’s mind Vi-raga arises. Raga starts to fade away. Desire fades away. Bonds that tie you down starts to unravel. His mind starts to calm down. That is referred to as Upasamaya.
If the mind someday becomes calm, it is possible to develop special wisdom in such a mind. Developing special wisdom in that manner is referred to as Abhinna. Understanding the four Noble Truths completely with such a calmed mind is called Sambodhaya. One who understands the Four Noble Truths in its entirety attains Nibbana. That fortune dawns on the person who trusts this Dhamma with confidence.

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What is zen sickness?
I was reading that the Zen monk Hakuin suffered a debilitating condition called zen sickness before his enlightenment. This seem to be a direct result of his practice.
Does anyone know what this was? Has there been a retrospective medical diagnosis of this? Are there equivalent stages of the path in other traditions? Should I be worried?
Thanks

Could it be connected to this: https://www.theatlantic.com/health/archive/2014/06/the-dark-knight-of-the-souls/372766/ ?

Also in his own words: https://buddhismnow.com/2015/09/12/zen-sickness-by-zen-master-hakuin/

there's another word for something similar to zen sickness, but i forget what ?

I have done some research online and read articles about zen sickness.
There are a couple of problems when doing research, online, on an ailment which, can be "bearing" on enlightenment, because there are lots of  "me too is having this ailment", in other words "I too, am close to being enlightened"; And suddenly, zen sickness symptoms multiplied: psychological depersonalisation, etc.. are linked to zen sickness and the definition of zen sickness becomes vague and fuzzy.
Even when I confine my readings to only very few recorded accomplished zen/chan masters, who actually do intensive meditations, their descriptions of zen sickness differ, and that is perhaps due to incorrect attribution ( the "me too" phenomena) of an experience to zen sickness. (I decided, eventually, not to provide links because it would create more confusion/debate on it, so don't take my word on it).
The zen sickness of Master Han Shan, IMHO, seems to be a genuine description, but I don't agree to some of the discussion, especially the last but one paragraph which begins with:

The situation of Zen sickness is not something you should desire, and certainly not something you want to force into occurring. Since it is a trouble or affliction, you want to avoid it if you can, but the important thing is that there is a remedy if it occurs. Think of it in terms of ....

It is not something you can force or get into easily unless you want to reprogram your own mind, but that's my view.
Theravada Tradition
From my reading and analysis it would correspond to an Anagami in the Theravada tradition working towards Arahatship, where at that stage the most prominent problem is restlessness.
IMHO, at the path stage of an Anagami it has seen emptiness clearly and enjoys the bliss of holding onto emptiness. To get to the fruit of Anagami it has to let go of that hold on emptiness. Once it has decided to let go of emptiness the bliss, vanished. The bliss makes the mind concentrated, I quote "the purpose of bliss is concentration"(.. in some sutta).
With the letting go of emptiness & bliss the mind will just grab at any thought,tune,occupations to concentrate the mind, because if it didn't, the restlessness of the mind is unbearable( we are here talking about a very sensitive fine mind; restlessness is one of the last five fetters ).
Why then can not an Anagami phala(phala sounds better than fruit, which reminds me of fruitcake) go back to holding onto emptiness again and get the bliss back? The answer is that it can't, but I won't go into that.
In Theravada we would call it restlessness rather than a (zen) sickness. In the articles on zen sickness I've read, the recommendation is lots of sleep, or get drowsy with alcohol, if permitted, and then get to sleep. But if restlessness is, like I say, unbearable sleep is not an option.
The answer is learning how to abide in emptiness after letting go of emptiness. How to do that is another topic.
The deep state of samadhi in which Master Han Shan was in for 5 days is, in the Theravada tradition equivalent to sanna-vedayita nirodha ( ceassation of perception and feeling) in which when one awakes, enlightenment is gain (but there are controversies, some say back to Anagami phala if it wasn't attained), I haven't a clue!
So it's a bit of way to go before you encounter it and when you encounter it just learn to abide in emptiness, then the restlessness will not go away but lessen. But if you practice jhannas the mind will lead you into sanna-vedayita nirodha for 7 days, like Master Han Shan. The you can tell us about it!

The article you referenced seems to be a description of a completely different sickness and different prescription (cure) to the one described (for example) here. Why do you say one reference (i.e. the one in your answer) is more credible than another (e.g. the one in this comment)?

This is the right answer but a good further reading on this phenomenon and how it relates to the Path go to page 244 of "How to Measure and Deepen Your Spiritual Realization." It really is just a phenomenon of uncontrollable psychological creativity that if sought after can be maddening but if passed through would mean you have reached a higher stage of overall transformation. In the book, it is described how some masters would get their students drunk to just get them sleeping through the stage otherwise the hyperactivity can cause psychological problems.

@ChrisW - In your link Hakuyu, who cured him, said it was a psychosomatic problem. In wiki of Hakuin  it says: "what in modern terms would be a nervous breakdown". So it's an error of attribution, unless we take "nervous breakdown" as another description of zen sickness, which I ruled out.

Robert Persig once said, "Suddenly I realised that the person who had come this far was about to expire. I was terrified, and curious as to what was coming. I felt so sorry for this guy I was leaving behind. It was a separation. This is described in the psychiatric canon as catatonic schizophrenia. It is cited in the Zen Buddhist canon as hard enlightenment." I wondered whether so-called 'hard enlightenment' is a/another form of zen sickness: but this one would be characterized as catatonia not as restlessness, and depersonalization, etc.

@ChrisW - IMHO one can define zen sickness as one specific type that is encountered on the way towards enlightenment or any form of mental imbalances that happened when practicing zen, related or unrelated to enlightenment. I opted for the first definition

@ChrisW If we take the definition that  zen sickness occurs near full enlightenment then using the Theravada 4 model stages, personalities and ideas of self have  already been abandoned at the first stage, then any mental delusions regarding self, I would rule out.

I would be worried. Happened to me and many other intensive meditators. "Meditation sickness" occurs to those meditators embedded in an ideology or worldview that values silence and stilling thought as superior or as more valuable than thinking or acting. This devalues thought, thinking, and acting in the world. And, these value/devalues are what is associated with "meditation sickness".
Celebrated Zen master Guiding Zongmi, warned that excessive focus on meditation and achieving "inner stillness, and non-critical/non-analytical study of the scriptures led to meditation sickness.
Robert H. Sharf, Professor of Buddhist Studies in the Department of East Asian Languages and Cultures, as well as Chair of the Center for Buddhist Studies, at the University of California, Berkeley, wrote of meditation sickness:

"Meditation sickness" excessive focus in meditation on achieving
  “inner stillness” (ningji), especially when unbalanced by an
  engagement with the scriptures, leads to a state described as “falling
  into emptiness” (duokong), which is, in turn, associated with
  “meditation sickness” (changing). The term meditation sickness was
  used by various Buddhist masters as a critique of practices they
  deemed detrimental to the path, notably techniques that emphasized
  inner stillness—they seem to have been targeting practices that
  cultivated a sort of non-critical or non-analytical presentness." --
  from p476 Is mindfulness Buddhist? (and why it matters), Robert H.
  Sharf, University of California, Berkeley, Transcultural Psychiatry 2015, Vol. 52(4) 470–484

Zen Sickness, by Zen Master Hakuin mentions one cause of it was access striving. In Buddhism you should pratice in a balanced manner.

Does anyone know what this was? Has there been a retrospective medical diagnosis of this?

From what I have gathered from Zen Sickness this is fatigue due to excessive striving and beyond what the body can bear.
Also possible that this is linked to insight knowledges.

Are there equivalent stages of the path in other traditions?

Another way these symptoms can have arisen is higher insights: The Progress of Insight - Awareness of Fearfulness to Knowledge of Re-observation
Also some mild forms of restlessness can be due to low level defilements and excessive thinking and pondering about the Dhamma which is mentioned in (Yuga,naddha) Paṭipadā Sutta.

Should I be worried?

Yes if it is excessive striving. No if it insight knowledges.

The are few borderlines between mystical (enlightenment) and psychotic experiences.
Read 'Psychotic and Mystical States of Being: Connections and Distinctions', the scholarly paper by Caroline Brett, PhD, in the journal of Philosophy, Psychiatry, & Psychology.
The paper is available free on this website http://www.yogapsychology.org/art_psychoticandmystical.html. Or, if you have access to journals through your library you can download a PDF of Brett's article.)
Brett argues and provides significant evidence that psychotic and mystical experiences (enlightenment states of mind) are fundamentally indistinguishable. She gives some critical distinctions:
"...the psychotic invests the conceptualization of the experience with great emotional significance, because it serves the purpose of contextualizing and reifying the sense of identity, which is under threat."
Before enlightenment: chop wood, carry water; after enlightenment: chop wood, carry water. (Zen proverb)

this was a good read, but some of the statements seem overly precocious, such as "attention is turned inward, away from involvement in practical or worldly matters, so that the process of perception itself is the focus of attention"...

Hakuin describes it himself in 'Yasenkanna' - the problem seems to have been that his focus was too cerebral, and the remedy given by the sennin was to direct his energy to his lower body.
https://terebess.hu/zen/Yasenkanna2.pdf

I think it's basically impossible to know. According to this page, Hakuin, described it as such:

my heart fire began to rise upward against the natural course, parching my lungs of their essential fluids. My feet and legs were always ice-cold: they felt as though they were immersed in tubs of snow. There was a constant buzzing in my ears, as if I were walking beside a raging mountain torrent. I became abnormally weak and timid, shrinking and fearful in whatever I did. I felt totally drained, physically and mentally exhausted. Strange visions appeared to me during waking and sleeping hours alike. My armpits were always wet with perspiration. My eyes watered constantly.

The article includes a footnote explaining some of the context of such claims about "heart fire":

This was a basic notion in Chinese medical lore. Cf. the statement in the encyclopaedic compilation Wu tsa tsu (Five Assorted Offerings, the section on “Man”), by the Ming scholar Hsieh Chao-che: “When a person is engaged in too much intellection, the heart fire burns excessively and mounts upward.”
Torei’s Biography (1710, Age 25) lists twelve morbid symptoms that appeared: firelike burning in the head; loins and legs ice-cold; eyes constantly watering; ringing in the ears; instinctive shrinking from sunlight; irrepressible sadness in darkness or shade; thinking an intolerable burden; recurrent bad dreams sapping his strength; emission of semen during sleep; restlessness and nervousness during waking hours; difficulty digesting food; cold chills unrelieved by heavy clothing.

He lists very specific ailments that are difficult to tie to particular problems connected with meditation practice, and without further accounts from others claiming to have the same thing, how can we know?
Willoughby Britton at Brown University is investigating problems that are arising in people following Mindfulness at the moment, and it's possible that Hakuin's overzealous nature led him to experience what he did (as her research seems to indicate a connection with overly intense practice).
You can read about it here: The Dark Night of the Soul

This is always how I understood it.  I think most people underestimate how stressful periods of intense practice can be.  In addition to all the psychological shit that can arise, there are the visceral, bodily experiences of stress that come along with it.  Most of that comes about from not tempering concentration with relaxation.  To use some TCM terminology, it's akin to too much fire and not enough water.  Very good answer!

@xxxx Thanks. Interestingly I suffered a GREAT deal, much in the same way that people in the Atlantic article I linked to. It took me three years to feel as though my personality had fully rebuilt itself after a depersonalization. I'd still love to know what I did wrong and how I can avoid it happening again in the future (if possible). Let me ask you: How do you temper concentration with relaxation? By just being? Ie. Letting your mind wander? Thanks.

Considering the Vipassana emphasis on this site, I know I'm going to get a lot of flack for this, but you most often see this kind of reaction in mindfulness-based traditions. Really, all this is is insight gone haywire.  The Buddha taught a coupling of samatha (or calming) meditation and insight. The two are inseparable. A good way to think about this is that where Vipassana is the sail and the rudder that steer you through the ocean of delusion, samatha is the keel that adds stability to your mind and allows you to safely explore greater depths.  So my advice - do some samatha practice!

I'm grateful for this thread. There are many great resources in the links.
The Dark Night of the soul article is particularly resonant. When I teach students meditation, I explain that these practices should come with a warning label, much like on every pack of cigarettes. There are real dangers in entering into this path of serious practice.
I found solace and many helpful tips on not destroying your life after awakening experiences in Daniel Ingram's book: "Mastering the Core Teachings of the Buddha" which he says he wrote so that others don't mess up their lives after these experiences as he did. https://www.mctb.org/
Kenshos happen. And like Hakuin's first one, they can distract from further progress and lead to narcissistic inflation of self-accomplishment and spiritual materialism.
Find a good teacher you can stay in contact with during the post-retreat integration and subsequent return to the rest of your life.
I still get occurrences of "Zen Sickness" after waves of intense wakeful experiences. No big deal. Just keep bringing it all back to chopping wood and carrying water. IMHO, Dilgo Khyentse's instructions on the "Every Day Practice" are perhaps the most cogent, compact pedagogy for the whole path. See this article on my website: The Everyday Practice by Dilgo Khyentse Rinpoche.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Meditating with an overwhelming emotion
It gets really hard for me to meditate when there is something really important happening in my life, something that triggers overwhelming emotions such as fear, anger or total despair. Avoiding them is not a solution, they don't simply go away.
I try the usual method to deal with them (seeing for what they are and repeting "fear...fear...fear..."), but it seems those big emotions and events can block or even stop my meditation. I'm 100% sure I'm not the first one to face this "challenge", so any advises on how to deal with it would be great.

I personally would recommend insight meditation, learn the 3 universal characteristics- impermanace, suffering and non-self. Hopefully once you realise such insight, you might question yourself, and ask: In a 100 years times, when i am dead, will the things that are causing me stress today, will they mean anything?
One day,very soon, we will all be dead, and the stress, struggles and fighting through this life/existence will end. The reason why i'm saying this is because it will mean nothing!! Live your life, let go of things you can't change and let go of the things you can change... let it all go... just live and be happy, because one day, very soon, you will be dead and the only difference will be on your death bed, when you ask yourself... was i a happy person or an unhappy person? The choice is yours my friend!
Metta

You may try anapanasati meditation to try and help bring about more calm of mind. Here is one reference for this : https://www.youtube.com/watch?v=YKFl4E3YhlI
But these fears and feelings and such will resurface through this calm once you begin again to practice vipassana. So while useful, anapanasati may only be a crutch, albeit a potentially useful one. This is no different than being stopped by pain while sitting and having your leg fall asleep and give rise to pain. No matter how intense the pain gets, your meditation will not bear good fruits unless you see reality through in any form it manifests itself.  Avoiding things is rarely the solution. An unkempt wound festers. And because you want them to, these "blockages" you speak of wont go away, . The only way to make peace with them and part ways is to look at them and let them be as they are, however they are.
Here is more food for thought: https://www.youtube.com/watch?v=UFh7aeq7QaY

I feel like I should add to this by saying, be kind to yourself when facing these obstacles. It is alright if you can't sit with them in their entirety right away. Just don't let that discourage you. Try to endure longer each time, within your own capacity. If you persevere like this you will become mindful.

I found the advice given on www.vipassanadhura.com (If strong emotions arise) very helpful. Specifically related to you question:

Press the fist against the center of the chest (around the heart area) and repeat "knowing, knowing, knowing," being aware of the
  sensation of pressure from your hand. Keep this up until the emotion
  subsides.
If you're sitting, get up and practice walking meditation.
Temporarily switch to a concentration technique until you are able to resume vipassana practice. For example, you might repeat the word
  "Buddho," or another mantra (a mantra is a special word or phrase
  repeated aloud and focused on in concentration meditation).

Summary of other advice given:

The emotions are not 'you'.
Insight meditation neither suppresses or indulges emotions. We acknowledge them by labeling them (fear, anxiety, depression etc.) and then return to the object of meditation.
Emotions, like all phenomena, are transient and will pass.
Emotions usually occur through memory of past events and abiding in the present moment, through an object of meditation, is the ultimate antidote.


text:
Is the attainment of natural wisdom exclusive to Buddhism?
Regardless of the medium, or the path, do you know of any other traditions, or religions, or ways to attain wisdom? Wisdom as it's known in buddhist tradition: Seeing things as they are.

Panna, wisdom, in buddhism, is insight into reality: impermanence, suffering, and non-self. So I believe we can answer this question. Yes, there are other religions or traditions which may teach true wisdom. But they will all, regardless of their name, teach the same thing as Buddhism. Namely, the nature of reality as suffering, the cause of suffering, and the path leading to the cessation of suffering.

By asking about "other traditions or religions" wouldn't this be off topic for Buddhism.SE?

@Wanderer. When asking about other "traditions" are you then talking about traditions/schools within Buddhism?

If we equate "true  wisdom" with nibbana, I think parinibbana sutta has an interesting answer: "In whatsoever Dhamma and Discipline, Subhadda, there is not found the Noble Eightfold Path, neither is there found a true ascetic of the first, second, third, or fourth degree of saintliness. But in whatsoever Dhamma and Discipline there is found the Noble Eightfold Path, there is found a true ascetic of the first, second, third, and fourth degrees of saintliness."

@Robin111 actually this question addresses Buddhisn directly.

Dhamma isn't exclusive to Buddhism. As the story goes, wherever there are apple trees, there are apples; but one can't expect to find apples where there aren't apple trees. Likewise, can't expect to find wisdom teaching where wisdom is not being properly taught.

While sitting on one side the wanderer Subhadda said this to the Gracious One: “Those ascetics and brahmins, dear Gotama, who have a community, a group, who teach a group, well-known, famous, religious founders, agreed upon as good for the people, such as: Pūraṇa Kassapa, Makkhali Gosāla, Ajita Kesakambala, Pakudha Kaccāyana, Sañjaya Belaṭṭhaputta, Nigaṇṭha Nāṭaputta, have they all understood, according to their own avowal? Have they all not understood? Or, have some understood? Or, have some not understood?”
“Enough, Subhadda, stop this: ‘Have they all understood, according to their own avowal? Have they all not understood? Or, have some understood? Or, have some not understood?’ I will teach the Teaching to you, Subhadda, listen to it, apply your mind well, and I will speak.”
“Very well, reverend Sir,” the wanderer Subhadda replied to the Gracious One, and the Gracious One said this: “Wherever, Subhadda, the Noble Eightfold Path is not found in a Teaching and Discipline there a true ascetic is not found, there a second true ascetic is not found, there a third true ascetic is not found, there a fourth true ascetic is not found.
But wherever, Subhadda, the Noble Eightfold Path is found in a Teaching and Discipline there a true ascetic is found, there a second true ascetic is found, there a third true ascetic is found, there a fourth true ascetic is found.
-- Mahāparinibbāna sutta, DN 16

Of course not!
In buddhism there are beings called paccekabuddhas, which are beings who attain enlightenment in a period where the dharma is not available in this world. Since this is possible, it's also possible that a practitioner of another tradition attains enlightenment, or at least some of the stages of enlightenment.
But when the Buddha said, in the Satipatthana Sutta that "This is the direct path", also translated as "This is the only path", he meant that any person who wants to attain enlightenment has to have direct knowledge of what he was teaching in that sutta. In short, that the body, sensations, mind states and mental objects are impermanent, unsatisfactory and are not-self.
What this means is that truth is truth, it's not buddhist truth. Whoever knows truth, he is enlightened. But he has to see the truth anyway, because there is no other way to liberation aside from directly knowing the truth.

"You shall know the truth, and the truth shall set you free." John 8:32

Indeed. Perhaps we shouldn't be too naive and think all religions are the same. But any person who is sincere, resolute and disciplined in search of the truth will be traveling mostly in the right direction. Different religions, or even different sects of buddhism may send us in somewhat different directions when we begin, but the commited search for truth is the core, imo. And these commited seekers may attain stages of enlightenment irrespective of their religion.

Nonduality is part of all religions and cultures, and exists back through time, as best I can tell, so it is "naturally" part of the human experience. It does not depend on a deity or scriptures or dogma, or any other artifact or particular: it depends only on increasing self-development and self-awareness.
That there are as many descriptions of it as describers, and differing opinions on how to achieve it, should not deter anyone. "You are unique, just like everyone else." Self Inquiry is the path to Nonduality (along with anything else that you personally find helpful), and is increasingly well-known and practiced by peer-led groups around the world. I co-lead one where I live.
Gone, Gone, Gone Beyond, Gone Completely Beyond, Wisdom, So Be It! (The Mantra of Prajnaparamita - Supreme Wisdom - from The Heart Sutra - which explicitly disavows and overturns all of the basic tenets of Buddhism.)

Is the attainment of natural wisdom exclusive to Buddhism?
No.
Do you know other traditions, or religions or ways to attain wisdom?
Yes.
Buddhist wisdom is not an enterprise owned by any cultural stream.
All spiritual attainments are beyond name and form.
Watching your mind (which includes the Four Foundations of Mindfulness) is recommended in almost every religious school and any self-developmental seminar worth a penny.
I could go into a long tirade as to why spiritual self-development is similar across all religions but there are a limitless explanation on the phenomena of shared experiences (kundalini, chi, visions, emptiness, heat, sweating, etc.).
I cannot do this topic justice though because there is a huge burden of proof that is already there.
So where do I recommend you look to get a detailed answer to both of your questions?

Read Spiritual Paths and Their Meditation Techniques ($)
Read How to Measure and Deepen Your Spiritual Realization - A Short Multi-disciplinary Course on Evaluating and Elevating Your Meditation Progress and Spiritual Experiences ($$$)
Read all the articles here. There are many guidances of other non-Buddhist techniques you may apply to grow spiritually. (free)

--
Now, I am assuming that you did look into explanations there above. Now are full of understanding of the 5 Stages of the Spiritual Path that are common to all cultivation schools. Now you know for sure that the spiritual path is something beyond any prayer to any deity... it is developing Pure Presence and cultivating other features of Being-As-Is.
Now, I would like to tell you that despite the fact that various cultural streams have many of the same features as Buddhism... and even similar or even equivalent practices... (Early) Buddhism is still better.
It is the most organized, systematic method available.
Buddhism, especially Theravadin Buddhism, is the "only" direct path. It is what the Buddha first spoke after all...! It contains within it the bare essentials to practice -- the "extras" are supportive practices like to improve your morality practice Brahmavihara.
Although it may seem that some sects in some religions are exclusive, all the various methods within Buddhism and in all religions--fall under the 8/9 samadhi/dhyana/jhana that our Lord Buddha experimented with and shared.
"The Surangama Sutra" or the more adaptable "Twenty-Five Doors to Meditation - A Handbook to Entering Samadhi" both expand and explain this in a practical way.

Sorry to be off-topic: I noticed you removed the [tag:nonduality] tag which @nocomprende added to the question. Though I don't know what "nonduality" is exactly, nor "natural wisdom", if you have time would you mind posting on this meta-topic to explain why you removed that tag, ideally clarify or come to consensus with nocomprende about when and where that tag should or shouldn't be used?

A well known Vipassana meditation teacher, Mr. S.N. Goenka always refers to the Dharma (the truth or seeing things as they are) in a non denominational way.

Dharma is not Hindu nor Buddhist,
  not Sikh, Muslim, nor Jain;
  Dharma is purity of heart,
  peace, happiness, serenity.
  —Hindi doha of S. N. Goenka
Two thousand years ago in India, there were two distinct traditions. One tradition gave importance to the purity of Dharma. The other gave importance to sectarian rites, rituals, religious ceremonies, external appearances, and so on. In those days the tradition of pure Dharma was quite strong, but slowly it became weaker and weaker, and eventually vanished from India. What was left had no trace of pure Dharma. It is very unfortunate that we have lost Dharma. When one speaks of Dharma in today’s India, the question that arises in the audience’s mind is: "Which Dharma? Hindu-dharma, Buddhist-dharma, Jain-dharma, Christian-dharma, Muslim-dharma, Sikh-dharma, Parsi-dharma, or Jewish-dharma? Which Dharma?"
It is a great pity that we have totally forgotten pure Dharma. How can Dharma be Hindu, Muslim, Christian, Jain, Parsi, or Sikh? This is impossible. If Dharma is pure Dharma, it is universal. It cannot be sectarian. Sectarian rites and rituals differ from one sect to another. The so-called "Hindu-dharma" has its own rites, rituals and religious ceromonies; its own beliefs, dogmas, and philosophies; and its own external appearances, and disciplines, such as fasting. It is the same with the Muslim-dharma, Christian-dharma, Sikh-dharma, and so on. But Dharma has nothing to do with all these. Sectarianism is divisive. Dharma is universal: it unites. Source: The Gracious Flow of Dharma - by S.N.Goenka

Mr. Goenka's meditation centers around the world teach insight meditation in a completely non religious setting. Rules include not being able to bring religious items or continue with spiritual practices while you are there.

During the course it is absolutely essential that all forms of prayer, worship, or religious ceremony — fasting, burning incense, counting beads, reciting mantras, singing and dancing, etc. — be discontinued. All other meditation techniques and healing or spiritual practices should also be suspended. This is not to condemn any other technique or practice, but to give a fair trial to the technique of Vipassana in its purity.

I believe this to be an example of a practice of learning to attain wisdom, as it's known in the Buddhist tradition, outside of Buddhism.


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Finding The Right Buddhist "Vehicle" / Sect For An Individual
I know there are three vehicles in Buddhism: Therevada, Mahayana, and Vajrayana. It seems there are so many sects of buddhism. Each of these three vehicles has sub-sects, (Zen, Pureland). Do the majority of Buddhist study each in depth in order to choose which fits them best in order to study the sutras accordingly? I find comfort in reading small pieces of sutras from each sect, though I've always loved Vajrayana and Zen. Is there a large difference?
On a side note: is the eightfold path is acknowledged by all three sects?

The introduction to Buddhism by country -- Eastern Buddhism (Mahayana), Southern Buddhism (Theravada), Northern Buddhism (Vajrayana) -- suggests it's often a matter of which country you are in ... and, I would expect, which language you understand. I suppose that until recently (e.g. 100 years ago) it would have been difficult for most people to study from more than their own (local) sect.

Side note: the answer to What teachings do all schools of Buddhism share? says "yes"; a few other answers e.g. here (and maybe elsewhere on this site) suggest that path is not as central in later sects. The eightfold path was explained in the first turning of the wheel of dharma.

From biographies I've read on Wikipedia I think that usually at least the founder of each sect has studied in depth from the materials and/or other teachers available.

Absolute Zen is considered the direct path. Most people shy away from it. "Too difficult" they say, when it was actually 'too simple'. Other Buddhist schools complicate the matter of Buddhism to 'entice' the neurotic mind because that is what a conditioned mind responds best to: mental objects, ideas and various theories.

@NeuroMax What is Absolute Zen? Thanks

@user663837 - Ah, well, you got me there! The term 'absolute' bastardises the very essence of Zen. It's a poor term I devised to point to the meaning of Zen. The paradox is that Zen cannot be caught with words; it cannot be held down by language, concepts or theory. For that reason, people see it as difficult. That's where one should attend.

Demographically the majority of Buddhists today (nearly 99%) are born in Asia into Buddhists families and would likely follow their family or local tradition.
But assuming your question is regarding those of us who have converted to Buddhism in primarily non Buddhist countries, there may still be no consensus on what the majority of these Buddhist converts do to select a tradition or sect.
One important consideration is whether one wishes to practice with a local group. If so, it might be more practical to gather information on local Buddhist centers/monasteries/temples and visit each to see where one might comfortably fit in and have an opportunity to study and practice along with other like minded local people rather than spending time learning in depth about traditions which may not be represented locally.
If one is more interested in studying and practicing alone or in online communities, the possibilities are far less limited and studying at least the basics of each of 3 major branches and the major sub sects within the major branches could give one enough information to see which branch really peaked ones interest to begin a more in depth study into a particular branch. This answer links information on the various canons used in different traditions.
Finding a teacher whom one really feels they can learn from, whether a present day teacher teaching locally, online, or through books/videos or a historical teacher teaching through their writings, can certainly influence an individual's choice of sect too. When teachings resonate, the energy to learn more grows.
Your question is interesting to me because it's the same thing I wondered when I first began learning about Buddhism. The information on the various schools seemed overwhelming. I started with a local group to make things easier for myself. Then spotted some interesting online resources which made me think beyond the local group. Then found the energy to begin to study the different branches in a basic way and eventually found the tradition that really resonates for me. But I think if I had just sat down and tried to read up on everything all at once, it wouldn't have made much sense. :)

I first was interested in Buddhism at the age of 15, but actually started meditating that year with Hindu yogis from India.
For some time, I practiced in a Japanese school of Buddhism but after seven years  but I realized I had joined a cult. That was very disappointing. Not only did they never meditate, but their practice consisted solely of  chanting one mantra. When I asked about emptiness, for example, they would just tell me that this was not important.
Later I found myself attending the Zen Center in my community, and then I would periodically attend the insight meditation Center in my city. The one day I met a Tibetan teacher and I realized that this was my path.
There is a somewhat mystical or maybe even silly belief in the Dharma community that one finds oneself exactly where one should be.
I have found that many Catholics and Jews seem to enjoy the Tibetan practices, probably because of the ritualistic aspects. Who knows. It is not necessary to know everything about each school, for example, because the basics of Buddhism are 1, to be compassionate and loving towards all sentint and beings, and 2, a belief in awakening: that is to say that we can recognize our own true nature, the awakened state, or enlightenment.
Whichever path you choose, all you have to do is walk that path and practice. Some individuals are more cerebral, and enjoy reading, deliberating philosophy, and other "academic" endeavors, and they may find themselves enrolled in a Tibetan University for 12 years, in order  for them to obtain the equivalent to a doctoral degree, called a GESHE.
As such,you may meet a Tibetan teacher who will be referred to as "Geshe-la," an honorific title which denotes that he has received a PhD in Tibetan studies. most monks and nuns in University, at least in the Tibetan system, will study for many years.
I wish you the best in your search for awakening.

Each of the sects alive today originated to suit their time and place - they were a result of the personalities of the teachers, and the receptiveness of the population. There were other sects that have since perished and faded  {source}, but enlightened a great many when they were popular.
The truth of nirvana can be attained through a great many ways, and the Buddha himself never offered a single formula for all to follow.
Some find samadhi or  meditation more appealing, some find sila or ethical behavior more appealing, and some best respond to philosophical inquiry or prajna. It depends on our respective past lives, past merits and available opportunities.
For some, especially those on the path of wisdom or prajna, examining several schools maybe necessary. They are cautious by nature, and are fond of extensive verification. Their quest of inquiry may take them to several teachers, and even several religions. As long as their intent is to realize the truth, there is nothing wrong with this. In fact, the Buddha too was like this, and over his several lives, studied with a great many teachers.
It works best if we find a teacher or school that resonates, that changes us deeply. We use what works, discard what doesn't.
Some people have bad allergies from good medicine, it is not the fault of the medicine or the individual.
We must ultimately be very practical about our choices. If we are not too hungry, we can wait until we find a restaurant we like, but if we are really hungry, we eat wherever we get food.

Theravada is the most orthodox and conservative tradition, as it tries to stick as closely as possible to the original teachings of the historical person of Gautama Buddha. Also the Theravada monastic order tries to maintain the original monastic rules from the time of the historical Buddha, although the Buddha allowed his followers to change the minor rules. It doesn't officially recognize any other "vehicle" apart from itself. There was only one turning of the Dharma wheel, according to Theravada.
The Theravada monastic order has done a good job maintaining the Pali Canon and its commentaries for 2500 years. It's true that later Theravada writings introduced exaggerations, but in general, most of the body of writings including texts and commentaries are kept close to the original.
The teachings of the Pali Canon (especially the Sutta Pitaka and the Vinaya Pitaka), the primary text of the Theravada tradition and from Theravada's perspective, the original teachings of the historical Buddha, focuses on the four noble truths, the noble eightfold path, the three marks of existence and dependent origination. Its focus is empirical and soteriological, not ontological or metaphysical.
In terms of culture and practice, Theravada temples introduced new innovations in terms of rituals, idols, chants and practices possibly to cater to the lay masses in Theravada countries who are not knowledgeable of the teachings. These are entirely optional of course.
Mahayana includes Vajrayana. The Mahayana tradition positions itself as the "Greater Vehicle" that teaches the path to Buddhahood, instead of just Arahantship, and presents itself as the compassionate path calling for the liberation of all sentient beings, as opposed to the "Lesser Vehicle" which is only concerned with one's own liberation.
The Mahayana tradition also introduced the second and third turning of the Dharma wheel, introducing teachings that it presents as being more advanced to the teachings of the "Lesser Vehicle" (which is limited to the first turning of the Dharma wheel).
The second turning of the Dharma wheel introduces the Bodhisattva vow, bodhicitta and Madhyamika emptiness.  The third turning of the Dharma wheel introduces Tathāgatagarbha (also known as Buddha nature), the basis-consciousness (ālaya-vijñāna), the doctrine of cognition-only (vijñapti-mātra) and the "three natures" (trisvabhāva). Metaphysics was introduced into Buddhism through these, that is not in Theravada (which is strictly soteriological and empirical).
There are many more new introductions by Mahayana like the concept of non-abiding Nirvana (continued worldly existence after death, after full enlightenment), new Bodhisattvas with supernatural abilities, like Avalokiteshvara (compared to Theravada's recognition of only Maitreya) and the trikāya doctrine which says that Buddhas have three bodies, the Dharmakāya (ultimate reality), the Saṃbhogakāya (divine incarnation of Buddha), and the Nirmāṇakāya (physical incarnation of Buddha).
On top of this, Vajrayana (including Tibetan Buddhism) added tantric practices. Tibetan Buddhism also introduced the concept of tulkus (reincarnations of past teachers who are emanations of Bodhisattvas), dharma protectors, trances and mediums etc. Culturally, Tibetan Buddhism added mandalas, torma, singing bowls etc.
Some Mahayana and Vajrayana schools, like Pure Land and Nichiren, also introduced practices like mantra chanting for being reborn into Buddha realms after death or to become enlightened and so on. On the other hand, some other schools like Zen and Ch'an are focused on meditation.
Of course, the Theravada tradition does not recognize these newer teachings and practices, and sees them as departing from the original teachings of the historical Buddha. On the other side, Mahayana (including Vajrayana) sees its teachings as being more advanced, but still grounded on Theravada's teachings of the first turning of the Dharma wheel.
Which vehicle is suited to you? I suppose you can study them all and it's up to you to choose how many times you want to turn the Dharma wheel in your practice.
To understand how the Buddhist sects evolved, please read the book "Sects & Sectarianism" by Ven. Bhikkhu Sujato.

I'm going to make the natural analogy here, and talk about actual vehicles:

Theravada is like mounting a horse: progress is slow, steady, personal, and organic
Mahayana is like running a railway: You don't want the train to leave until everyone gets onboard
Vajrayana is like buying a sports-car or a jet: it's meant to be a fast-track process, to get one from here to there as quickly and directly as possible.

Each has its strengths and weaknesses; each has its insights and peculiarities. Which of them one chooses — assuming one isn't born into one or the other or the other — becomes a matter of focus, intention, and taste.
Sects are the evolution of cults: they are the teachings of some specific individual within one of these major tropes that have been codified over time into a system. If you are looking at a particular sect, you should know what the originator teaches, and you should use discretion: do you feel aligned with that teaching? Do the sects followers seem to follow it, or are they deluding themselves? Usually a sect arises because a particular teacher has a particular insight into how the dharma can best be taught or understood by a particular type of person. You should ask yourself whether you are that type of person, or whether this particular teaching might lead you (personally) astray. It's a difficult, self-reflective assessment, but difficult self-reflection is in many ways the nature of Buddhist practice, so even looking at and rejecting a particular sect might be an important step on your path. Just always bring it back to basics: does this teaching draw me (personally) into or away from cravings and discontentment? I'm not going to tell you you can't go wrong, because you can. But there's nothing wrong with 'going wrong' if it teaches you how to 'go right'.

Dharma provides the answer at the right time.
Advice may come from many of the well educated and the well wishers; however your path is your own and when the inspirational spark of enlightenment strikes like a bell you will know your decision - as is the nature of Dharma.
It could be symbolic or it could be something more mundane, however you will know when another answer to your question is unsuitable.
Cultivate in harmony brother


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Overwhelmed feeling when visiting Temple
Is there a reason why I get this overwhelmed feeling and want to cry when I visit the Temple?

Is it a happy feeling or a sad feeling?

Hi Alice and welcome to Buddhism.SE. We've put together some information to help you get started here.

Do you like the Temple? Is it beautiful in your eyes? Judging by your response, it sounds as if it is very meaningful to you. I know that when I visit my local Zen center, entering the meditation hall causes such a reaction (along with a deep feeling of peace).
Crying is a natural human response to many situations and stimuli. This article offers a socio-biological reason for the crying response.
Interestingly, at least according to Wikipedia, crying in Buddhism is considered unskillful, as it indicates clinging/attachment.


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Buddhist practice and physical exercises
What is the right mindset we should have towards physical exercises according to buddhist practice ? e.g. Weight training , cardio , running etc.
In my case I'm spending most of my day sitting at desk , so having some exercise is necessary to be healthy . But would this consider as clinging to body? I can understand having supplements such as protein shakes could be considered as clinging.

A similar question is Physical Exercise as a Monk?

On YouTube, Ask A Monk: Physical Exercise

This question is a super repeat.
Also here is my answer.

I don't see this question as a repeat. The question of whether exercise is appropriate for a monk is different than the question of how a Buddhist lay follower should approach exercise with the right mindset.

It is ok(ethically) to do physical exercises even for the purpose of being attractive. Lay people are not expected to follow the rules of monks. Such conflicts occur mostly when you try to live a monk's life as a lay person. Is it clinging/attachment that motivates you for physical exercises? Absolutely! But it's similar to asking if it is ok to have love affairs or get married.  They all involve unwholesome mind states. But you are mostly expected to keep to the 5 precepts as a Buddhist layman. So you should focus more on the 5 precepts as they can be easily broken if you are not mindful. If you want to live a more noble life, you can get ordained. Things become conflicting and confusing when you try to walk on the fence, not picking either lifestyle.
In fact, it is clinging that makes you do the job you do. It is clinging that keeps you in lay life. If you don't have clinging, you can't live as a householder. You have to go get ordained within 7 days or die(parinibbana).

What did you mean by this sentence: "You have to go get ordained within 7 days or die(parinibbana)"?

An Arahant cannot stay in lay life more than a week.

Where can i read about this?

Not sure where exactly it's mentioned. Probably in the commentaries. But it's the common understanding.

Health is the greatest gift, contentment is the greatest wealth, a trusted friend is the best relative, Nibbana is the greatest bliss. -
  Dhammapada 204

For a lay person, moderate exercise, moderate diet and good hygiene can be considered as part of the middle way that the Buddha taught in the Dhammacakkappavattana Sutta, as it contributes towards the maintenance of one's health, which is beneficial to practice. You also cannot practise Right Livelihood without good health. The purpose of Buddhism is to escape suffering. And with poor health, you would only be causing suffering for yourself.

"There are these two extremes that are not to be indulged in by one
  who has gone forth. Which two? That which is devoted to sensual
  pleasure with reference to sensual objects: base, vulgar, common,
  ignoble, unprofitable; and that which is devoted to self-affliction:
  painful, ignoble, unprofitable. Avoiding both of these extremes, the
  middle way realized by the Tathagata — producing vision, producing
  knowledge — leads to calm, to direct knowledge, to self-awakening, to
  Unbinding.

On the other hand, excessive exercise either for vanity or as an addiction (see here and here for the case of running addiction), are not considered skillful. In case you are afflicted with exercise addiction, the Buddha has a remedy in the Sukhamala Sutta:

"Even though I was endowed with such fortune, such total refinement,
  the thought occurred to me: 'When an untaught, run-of-the-mill person,
  himself subject to illness, not beyond illness, sees another who is
  ill, he is horrified, humiliated, & disgusted, oblivious to himself
  that he too is subject to illness, not beyond illness. And if I — who
  am subject to illness, not beyond illness — were to be horrified,
  humiliated, & disgusted on seeing another person who is ill, that
  would not be fitting for me.' As I noticed this, the healthy person's
  intoxication with health entirely dropped away.

In the case of monks, the Buddha explains in the Sukhamala Sutta:

"Drunk with the intoxication of youth, a monk leaves the training and
  returns to the lower life. Drunk with the intoxication of health, a
  monk leaves the training and returns to the lower life. Drunk with the
  intoxication of life, a monk leaves the training and returns to the
  lower life."

Despite this, I can still find some example of a monk trying to get light exercise, in the Meghiya Sutta (below). This leads me to think that light exercise without "intoxication of health" might still be considered acceptable to monks.

Then in the early morning, Ven. Meghiya adjusted his under robe and —
  carrying his bowl & robes — went into Jantu Village for alms. Having
  gone for alms in Jantu Village, after the meal, returning from his
  alms round, he went to the bank of the Kimikālā River. As he was
  walking up & down along the bank of the river to exercise his legs,
  he saw a pleasing, charming mango grove. ...

Without knowing for certain, i'd be willing to go out on a limb and claim he was doing walking meditation. One of the reasons for doing walking meditation is to keep the body in good order and aid in digestion. Which is why he would be walking after having had his morning meal. I don't see this as an admission for monks strictly being able to exercise.

Walking meditation is useful for many reasons. it strengthens ones concentration and mindfulness before doing sitting meditation, and the added bonus of it promoting health and aiding digestion makes it all the more useful for ones practice. This monk wasn't walking up and down the bank of a river trying to develop his calf muscles.

The mind is no different than the body.  The state of one has a direct impact on the state of the other.  If you are out of shape and inflexible, you simply won't be very well prepared for sustained mediation sessions.  You are either going to fall asleep on the cushion (i.e. sloth and torpor) or your back, your legs, and/or your hips are going to be in agony.  You certainly won't be able to maintain the upright and invigorated positions necessary for deeper mediation.
Don't get me wrong, some discomfort is inevitable.  In fact, a little discomfort is even encouraged.  But why compromise your sits when you can do something to combat the grosser disturbances that an out of shape body could cause?  Obviously, don't go overboard with it.  You also shouldn't select exercises that could cause you difficulties on the cushion (heavy back squats are just one example).  You aren't clinging to the body by exercising.  You are maximizing the benefit of the time you spend on the cushion!
And let's not forget - some taoist yoga, Vedic yoga, and Shaolin kung fu were all [to one extent or another] designed to bolster meditation practice!

Exercise can be healthy or delusional according to your intentions. If you believe that bodies can be beautiful and having a beautiful body (or a body that is somehow different than it is) will bring you true happiness; than that doesn't really accord with true happiness in Buddhism.
If you're trying to stay healthy to have the energy and vigor to study and practice with ardor, what could be wrong with that? Unless you go overboard and lose your wholesome intention of course.
One thing about sitting at work; see if you can possibly switch up your work station to stand. I did this 10 months ago and it feels very energizing. (I'd rather spend my sitting time on a mat at home!) If your employer is open to the idea, some creative re-arranging of your work station might be easier than you think. Best wishes.


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What is/was the role of agamas in Chinese and Japanese history?
Recently I read/heard someone saying that the agamas are/were dismissed and downplayed in Chinese and Japanese Buddhism throughout history.
I understand that Mahayana texts received much attention in these regions, but I wonder if the (presumably) lack of importance given to the agamas are simply due to them being neglected among the vast literature, or if they were explicitly disliked (for a specific reason?) or considered inferior, or obsolete.
EDIT: Trying to be clearer: I'm looking for historical evidence for either chinese/japanese appreciation of the agamas, or evidence of reasons for chinese/japanese to neglect these books across the centuries.

They are the practice of the sravakas (voice hearers, desciple), hinayana (though not Theravada) and not the practice of the Bodhisattva, so it is ignored, as not worthy of practice.
Mahayana views in Sravaka link

In the 4th century abhidharma work Abhidharmasamuccaya, Asaṅga describes those who follow the Śrāvaka Vehicle (Skt. śrāvakayanika). These people are described as having weak faculties, following the Śrāvaka Dharma, utilizing the Śrāvaka Piṭaka, being set on their own liberation, and cultivating detachment in order to attain liberation.[29] While those in the Pratyekabuddha Vehicle (Skt. pratyekabuddhayānika) are portrayed as also utilizing the Śrāvaka Piṭaka, they are said to have medium faculties, to follow the Pratyekabuddha Dharma, and to be set on their own personal enlightenment.[30] Finally, those in the Mahāyāna (Skt. mahāyānika) are portrayed as utilizing the Bodhisattva Piṭaka, as having sharp faculties, following the Bodhisattva Dharma, and set on the perfection and liberation of all beings, and the attainment of complete enlightenment.[31]
According to Vasubandhu's Vijnanavada teachings, there are four types of Shravakas:[32]

The fixed
The arrogant
The transformed
The converted (to "Bodhi" or Buddhism)

The transformed and the converted (Buddhist) are assured in the Lotus Sutra of eventual Nirvana.

SO SAYS MAHAYANIST....hmmmmm???

The author of your quote is Asaṅga who was the founder of Yogācāra school in India. In the Hinayana article I read about Hinayana and Mahayana both being practiced at the same monastery (as reported by Chinese pilgrims to India).

@ChrisW Yes, that is true. That was sometime ago and now dhamma has evolved..

Yes. But what I wonder is (a) whether such books containing critical remarks were part of the chinese canon; (b) if the target of criticism (e.g Śrāvaka Piṭaka) was identified as the agamas; and (c) the chinese took those criticisms to heart. The fact that texts existed, including that of Asaṅga, making general remarks (e.g. about some Srãvaka Pitaka "what books might those be?" a chinese ask) still doesn't quite explain to me if and why the agamas were neglected.

The path that is shown by the agamas (roughly the pali canon) lead one ONLY to arahatship and in Mahayana's reckoning, not fully enlightened, Whereas the path of the Bodhisattva leads to full enlightenment. That's why the council of arahants meant not so much to the Mahayanist. It is about how enlightened one wants to be, arahant, bodhisattva, buddha?

@Samadhi These remarks do not explain chinese attitude towards the agamas, nor my points above. From the standpoint of someone not familiar with this period (post-nikaya, 7th century on) and place of the buddhism history, it's possible that the agamas were in fact studied by the chinese at certain times. It's also possible that the agamas were neglected, but only because specific suttas became more popular, among the vast number of books. Thus, the criticism delineated in texts (like Asaṅga's) could have played no role in the trends that developed in china even if available in chinese earlier.

If the Lotus sutra is disparaging about the arahants and the practices of the arahants, don't you think that would have bearing on the adherents of the traditions and the popularisation of the Lotus sutra?

First chapter on how the sravaka disciples have yet to gains enlightenment. summary


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Are we taught to try to understand our fears?
On page 13 of Good Questions, Good Answers it says,

The Buddha taught us to try to understand our fears, to lessen our desires and to calmly and courageously accept the things we cannot change.

Is it true that we're taught to try to understand our fears?
Assuming it's true, do you suppose it means nothing more or less than the first noble truth: i.e. that dukkha is fear and vice versa?
Or does it mean something else?
Are there techniques to "understand our fear"?

I'm not reading the book, but i'd imagine it means to see it as everything else is; impermanent, suffering, and non-self. To see your fears as they arise, persist, and cease.

Yes, we are taught to understand our fear and see where it comes from.
It comes from the projections of our mind.

As fear is based on something that we think may happen in the future, it is clearly a mental process which tries to predict the future - in that sense, the reason of fear is a projection of our mind.
  We can be afraid to fall, but once we are falling, we are afraid to hit the ground, once we hit the ground, we may fear we have a bad injury, once we know we have a bad injury, we may fear the pain and the consequences of not being able to work for some time or become disabled etc. So one could say that fear is always based on something that has not happened yet, and is therefore a fantasy of our mind rather than fact.

There are some fears which are wholesome:
Hiri and Otappa which are fear of shame and fear of unwholesomeness are called "Guardians of the world":

"Two lucid things, o monks, protect the world: moral shame and moral dread. If these two things were not to protect the world, then one would respect neither one's mother, nor one's mother's sister, nor one's brother's wife, nor one's teacher's wife ...." (A. II, 7).

There is another fear which results from the practice of vipassana as pointed out by eudoxus:
Knowledge of fear - bhaya-nana

This gives way to the knowledge of fear, (bhaya-nana). In the disappearance of everything examined, the mind at some level begins to realise: there is nothing beneath this parade of changes. There is no foundation. At a fundamental level, there is nothing at all. The result is existential anxiety. In its strong form this can manifest as panic. In its weak form, it can be merely a sense of existential unease, a sense of nothing going right, a sense of helplessness, a sense of loss of control. At this stage of the practice, the meditator’s insight into anatta, not self, usually takes the form of a sense of loss of control. The realisation that "I am not in control of ‘my’ life".

This is not entirely accurate. There are fears which are just produced by the mind without any projections. This is seen clearly with people with childhood trauma, who are anxious throughout their live just becase the fear circuitry in the brain is always activated to smaller or greater degree. In Vipassana, fears may arise (and pronouncedly so in the Bhaya-ñana stage as described in the Vissudhimagga) with no obvious reason; the way to know them is staying present with the fear and acknowledging it, not by avoidance by thinking about causes.(Dealing with fear may be tradition-dependent).

@eudoxos - Thanks for the comment I have added the fears that are wholesome. IMHO in the case of childhood trauma the fearful projections created had been stored in sanna (memory, stored perception) and associated to situations or objects. These objects and situations trigger those projections, sometimes the child is conscious of them and be able to verbalise them, but sometimes just peripheral. :)

Experience of fear/anxiety can be a sign of good progress in vipassana (one experiences what the map says), but I would not call it wholesome just because of that ;) what is wholesome is that the mind trains to be mindful of it, rather than being carried away by it. Both experiences of object-bound fear or unspecific anxiety are not exclusive to Vipassana-practitioners; they just seem to come on average more often at point of the practice.

When training mindfulness (Vipassana), there are 4 foundations of mindfulness: body, feelings, mind, mental objects. "Mental objects" (also "mind objects", or "mind qualities") are usually sub-divided into 5 hindrances and 7 factors of enlightenment. Fear (also under the name of anxiety, worry, depending on translation) is one of the 5 hindrances.
When practicing Vipassana based on 4 foundations of mindfulness (such as in the Mahasi tradition), fear is definitely something to be contemplated (acknowledged), acquainted with, mindful of. By acknowledging it every time it arises, it loses (some of) its magic power it has over an untrained mind, and is seen progressively more and more as a "mental object", something which passes through the mind; thus is the mind purified of fear.
If you want the canonical text, here you go:
From Maha-satipatthana Sutta:

(D. Mental Qualities)
"And how does a monk remain focused on mental qualities in & of
  themselves?
(1) "There is the case where a monk remains focused on mental
  qualities in & of themselves with reference to the five hindrances.
  And how does a monk remain focused on mental qualities in & of
  themselves with reference to the five hindrances? There is the case
  where, there being sensual desire present within, a monk discerns that
  'There is sensual desire present within me.' Or, there being no
  sensual desire present within, he discerns that 'There is no sensual
  desire present within me.' He discerns how there is the arising of
  unarisen sensual desire. And he discerns how there is the abandoning
  of sensual desire once it has arisen. And he discerns how there is no
  future arising of sensual desire that has been abandoned. (The same
  formula is repeated for the remaining hindrances: ill will, sloth &
  drowsiness, restlessness & anxiety, and uncertainty.)


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Is anxiety as hell-bound as cruelty?
On page 24 of Good Questions, Good Answers it says,

The gentle, loving type of person tends to be reborn in a heaven realm or as a human being who has a predominance of pleasant experiences.
The anxious, worried, or extremely cruel person tends to be reborn in a hell realm or as a human being who has a predominance of unpleasant experiences.
The person who develops obsessive craving, fierce longings, and burning ambitions that can never be satisfied tends to be reborn as a hungry spirit or as a human being frustrated by longing and wanting.

I have three questions:

If a person is all three (i.e. gentle, loving, anxious, and longing) then what determines their future rebirth?
Why are 'anxious' and 'cruel' both reborn in hell?

Isn't someone who is cruel more harmful than someone who is anxious, and therefore more 'deserving' of hell?
Is an anxious person reborn in hell because anxiety a hellish (unpleasant) experience, i.e. if you choose to live a life of anxiety you're therefore inclined to choose to be reborn in hell?

Do cruelty and/or anxiety correspond to kilesas? The introduction to the Wikipedia article says,

Kleshas include states of mind such as anxiety, fear, anger, jealousy, desire, depression, etc.

... but I don't see anxiety (nor cruelty) listed later in the article, e.g. in the list of 10 defilements.

If a person is all three (i.e. gentle, loving, anxious, and longing)
  then what determines their future rebirth?

There's classification of Karma, according to the priority of effect

WEIGHTY (GARUKA) KARMA.
PROXIMATE (ASANNA) KARMA OR DEATH-PROXIMATE KARMA
HABITUAL (ACCINA) KARMA
RESERVE OR CUMULATIVE (KATATTA) KARMA

So it depends on what comes forward at the time of your death to give your next birth.

Why are 'anxious' and 'cruel' both reborn in hell?

Because they are both unwholesome mind states.

Isn't someone who is cruel more harmful than someone who is anxious,
  and therefore more 'deserving' of hell?

That again depends on the strength of it. Killing a mosquito is cruel. Embarrassing someone to have fun at some point in your life is also cruel. But a lifetime of anxiety can potentially be much more harmful to the person. There was a monk who kept all the pātimokkha rules to the letter. Once when he was going on a wooden raft, he raised his hand to touch a leaf of a branch that was leaning towards him. Since the raft was still moving while he was holding the leaf, the leaf was broken. Then the thought came to his mind that he has broken 1 of the Vinaya rules. He kept worrying about this for the rest of his life. Because of that, after he died, he was born as a cobra at the river bank.

Is an anxious person reborn in hell because anxiety a hellish
  (unpleasant) experience, i.e. if you choose to live a life of anxiety
  you're therefore inclined to choose to be reborn in hell?

You don't get to choose where you will be reborn. It depends on the Karma that comes forward to give the next life. If a Karma caused by anxiety comes into fruition at the time of death, it will lead you to a bad destination since it is unwholesome.

Do cruelty and/or anxiety correspond to kilesas? But I don't see
  anxiety (nor cruelty) listed later in the article

Anxiety, fear, anger, jealousy, cruelty and depression are instances of Dosa. Which means aversion in general. Wikipedia only gives a specific instance.
Desire is lobha. Greed, craving, lust are instances of it.

(N.B. This is from a Theravada perspective)

If a person is all three (i.e. gentle, loving, anxious, and longing) then what determines their future rebirth?

Karma is a complex system; there are three sets that you have to understand, one mentioned by Sankha:
By order in which they operate:

weighty karma
karma performed at death
habitual karma
other karma

By their function:

productive of new results
supportive of other results
destructive of other results
detractive from other results

By the time it takes them to bring results

in this life
in the next life
in some future life
already finished

So, to say that behaviour X leads to hell is just a simplification - it should be taken as a guide, not a dogma. The Buddha himself said:

“Therein, Ānanda, when a recluse or brahmin says: ‘Indeed, there are evil actions, there is result of misconduct,’ I grant him this. When he says: ‘I saw a person here who killed living beings…and held wrong view, and I see that on the dissolution of the body, after death, he has reappeared in a state of deprivation…even in hell,’ I also grant him this. But when he says: ‘On the dissolution of the body, after death, everyone who kills living beings…and holds wrong view reappears in a state of deprivation…even in hell,’ I do not grant him this. And when he says: ‘Those who know thus know rightly; those who think otherwise are mistaken,’ I also do not grant him this. And when he obstinately adheres to what he himself has known, seen, and discovered, insisting: ‘Only this is true, anything else is wrong,’ I also do not grant him this. Why is that? Because, Ānanda, the Tathāgata’s knowledge of the great exposition of action is otherwise.
--MN 130 (Bodhi, trans)
Why are 'anxious' and 'cruel' both reborn in hell?

Orthodox Theravada theory says that, in general, anger leads to the hell realm, greed leads to the ghost realm, and delusion leads to the animal realm
Anxiety could refer either to worry or fear. If it refers to worry, it is a delusion-based state and will incline one towards the animal realm. If it refers to fear, it is an anger-based state and will incline one towards teh hell realm. Note that simple worry and fear shouldn't lead to either realm, unless it is serious enough to lead one to break precepts - harming others or oneself out of fear or worry. Taking strong anti-anxiety medication might arguably fit the bill (leading to animal realm); killing or stealing could lead one to hell, and could ostensibly be performed out of fear-based anxiety.
Cruelty is anger-based, so yeah, always inclining one towards hell.

Isn't someone who is cruel more harmful than someone who is anxious, and therefore more 'deserving' of hell?

One would think so, probably because we use the word anxiety to refer to a mild form of anger (fear) or delusion (worry). These states can, however, become intense and destructive to the point of suicide, drug addiction, etc., which can in turn easily be more destructive than mild forms of cruelty, like insensitivity to the feelings of others, etc.

Is an anxious person reborn in hell because anxiety a hellish (unpleasant) experience, i.e. if you choose to live a life of anxiety you're therefore inclined to choose to be reborn in hell?

Unwholesome-inclined individuals don't really get the chance to "choose" where they are born. Somewhere, the Buddha is said to have taught that it takes morality for one to have a stable enough mind to actually choose where one is born, IIRC (can't remember the source off-hand). But yes, the general idea is that one's prevalent state of mind inclines one towards hell; it doesn't have anything directly to do with the judgement of others.

Do cruelty and/or anxiety correspond to kilesas?

Answered above.


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What is the proper mindfulness anchor for a stream-winner?
If a practitioner of mindfulness has experienced cessation/fruition should they use the memory of that as their anchor or should they continue to use the perception of the arising and passing away of kalapas? Or does focusing on either one of these cause the practitioner to develop different skills?

I was under the impression that there was no memory or recollection of the experience of nibbana?

Nibbana is free from the five aggregates, therefore it is free from perception and cannot be "remembered", as far as I understand.

Recollection of the memory is not attending to the present but attending to sanna (past perception that has been stored). The best the recollection of the memory can do is inform one of the path that one has travelled and probably one can help other people along the same path.
The continued perception of arising and passing away of dhammas will deepen the insight into the three characteristics. Each stage of the breakthrough occurs because one has clearer and clearer insights of the 3 characteristics and able to let go of further attachments until nothing sticks, and everything slips like "a drop of water on a lotus leaf".
Even at the stage of an Anagami, Khemaka still watch further the arising and passing away of dhammas.