text:
Is the Karma theory an essential component of Buddha's teaching?
Stephen Batchelor claims in several speeches and books that

Buddha never said the Karma and rebirth theory was true,
it is not important to Buddha's teachings, and
when he was a Gelug monk and asked for support for the Karma and rebirth theory, he didn't get any sufficient answers (despite the fact that Gelug Buddhists tend to focus on scientific methods of acquiring knowledge).

According to Batchelor, the Karma theory was a common worldview in India at the time. Siddhartha Gautama's contemporaries believed in Karma like we believe in the Big Bang theory and evolution. Neither of these theories can be proved or refuted with an affordable effort (in order to prove evolution for yourself, you need substantial knowledge of biology plus systemic thinking skills -- both of which are hard to acquire).
Also, neither of these theories affects the main purpose of Buddhist practice -- to reduce suffering. If tomorrow someone proved the Big Bang theory to be definitely true (or definitely wrong), it wouldn't make most of the people happier/unhappier than before.
However, the question about Karma theory affects the Buddhist practice.
If someone believes in rebirth, then they would spend most of their time meditating and preparing for the next lives. This is not just theory -- in some Buddhist communities advanced members spend almost all their evenings in the Buddhist center and almost every vacation on some retreat. It looks like they don't have their own life apart from this practice.
If someone does not believe in the Karma theory, they will focus on making the best of this life (which may be the only one we get). Buddhist practice is a means to live the one life one has as effectively as possible. Basically, you meditate in the evening in order to better achieve your personal goals during the day.
Is the Karma theory an essential component of Buddha's teaching or not?

This link may help: http://www.abuddhistlibrary.com/Buddhism/B%20-%20Theravada/Teachers/Buddhadasa/Kamma/Kamma%20in%20Buddhism.htm

Maybe see also Is rebirth a delusional belief?

"Buddha never said the Karma and rebirth theory was true"
I don't know if he claimed it to be true or not, but it seems like he spoke about karma and rebirth as being true:

"With his mind thus concentrated, purified, and bright, unblemished, free from defects, pliant, malleable, steady, and attained to imperturbability, he directs and inclines it to knowledge of the recollection of past lives (lit: previous homes). He recollects his manifold past lives, i.e., one birth, two births, three births, four, five, ten, twenty, thirty, forty, fifty, one hundred, one thousand, one hundred thousand, many aeons of cosmic contraction, many aeons of cosmic expansion, many aeons of cosmic contraction and expansion, [recollecting], 'There I had such a name, belonged to such a clan, had such an appearance. Such was my food, such my experience of pleasure and pain, such the end of my life. Passing away from that state, I re-arose there. There too I had such a name, belonged to such a clan, had such an appearance. Such was my food, such my experience of pleasure and pain, such the end of my life. Passing away from that state, I re-arose here.' Thus he recollects his manifold past lives in their modes and details. Just as if a man were to go from his home village to another village, and then from that village to yet another village, and then from that village back to his home village. The thought would occur to him, 'I went from my home village to that village over there. There I stood in such a way, sat in such a way, talked in such a way, and remained silent in such a way. From that village I went to that village over there, and there I stood in such a way, sat in such a way, talked in such a way, and remained silent in such a way. From that village I came back home.' In the same way — with his mind thus concentrated, purified, and bright, unblemished, free from defects, pliant, malleable, steady, and attained to imperturbability — the monk directs and inclines it to knowledge of the recollection of past lives. He recollects his manifold past lives... in their modes and details."*
"With his mind thus concentrated, purified, and bright, unblemished, free from defects, pliant, malleable, steady, and attained to imperturbability, he directs and inclines it to knowledge of the passing away and re-appearance of beings. He sees — by means of the divine eye, purified and surpassing the human — beings passing away and re-appearing, and he discerns how they are inferior and superior, beautiful and ugly, fortunate and unfortunate in accordance with their kamma: 'These beings — who were endowed with bad conduct of body, speech, and mind, who reviled the noble ones, held wrong views and undertook actions under the influence of wrong views — with the break-up of the body, after death, have re-appeared in the plane of deprivation, the bad destination, the lower realms, in hell. But these beings — who were endowed with good conduct of body, speech, and mind, who did not revile the noble ones, who held right views and undertook actions under the influence of right views — with the break-up of the body, after death, have re-appeared in the good destinations, in the heavenly world.' Thus — by means of the divine eye, purified and surpassing the human — he sees beings passing away and re-appearing, and he discerns how they are inferior and superior, beautiful and ugly, fortunate and unfortunate in accordance with their kamma. Just as if there were a tall building in the central square [of a town], and a man with good eyesight standing on top of it were to see people entering a house, leaving it, walking along the street, and sitting in the central square. The thought would occur to him, 'These people are entering a house, leaving it, walking along the streets, and sitting in the central square.' In the same way — with his mind thus concentrated, purified, and bright, unblemished, free from defects, pliant, malleable, steady, and attained to imperturbability — the monk directs and inclines it to knowledge of the passing away and re-appearance of beings. He sees — by means of the divine eye, purified and surpassing the human — beings passing away and re-appearing, and he discerns how they are inferior and superior, beautiful and ugly, fortunate and unfortunate in accordance with their kamma..."*

source: http://www.accesstoinsight.org/tipitaka/dn/dn.02.0.than.html
You can certainly find more suttas that will direct you in the same direction of thinking.
"it is not important to Buddha's teachings"
Correct. You don't have to believe in karma and rebirth to achieve complete liberation from suffering.
The main teaching of the Buddha is to do wholesome actions and meditate to find out the cessation to suffering.
"Siddhartha Gautama's contemporaries believed in Karma like we believe in the Big Bang theory and evolution. Neither of these theories can be proved or refuted with an affordable effort (in order to prove evolution for yourself, you need substantial knowledge of biology plus systemic thinking skills -- both of which are hard to acquire)."
Karma exists. Rebirth exists.
Karma does not exist. Rebirth does not exist.
Karma neither exists nor not exists. Rebirth neither exists nor not exists.
All three statements above are true. All of this can be known and seen.
None of the above statements will ever be possible to be 100 % proven with evidence, only with experience, which, unfortunately, is not easy to pass to others.
"Also, neither of these theories affects the main purpose of Buddhist practice -- to reduce suffering. If tomorrow someone proved the Big Bang theory to be definitely true (or definitely wrong), it wouldn't make most of the people happier/unhappier than before."
Your statement is correct. Belief in kamma and rebirth doesn't ALWAYS reduce suffering. Example is an individual who believes in kamma and rebirth very much, but still does such unwholesome actions to end up in a place of deprivation.
Your statement is not correct. Belief in kamma and rebirth reduce suffering. Example is an individual who believes in kamma and rebirth very much, thus he abstains from doing unwholesome actions and ends up in a place of happiness.
Both statements above are true. All of this can be known and seen.
"If someone does not believe in the Karma theory, they will focus on making the best of this life (which may be the only one we get)."
Again your statement is correct, but it can be incorrect for another.
All of this can be known and seen.

You are looking at a wall that has no visible colors, but you are seeing colors on the wall.
You are searching for answers to questions where all answers are true, untrue, true and untrue, neither true nor untrue.
Either one of these views: true, untrue, true and untrue, neither true nor untrue are conditioned by ignorance.
Knowing that all views are true, untrue, true and untrue, neither true nor untrue is cessation of ignorance. Is cessation of clinging and craving. Is complete cessation of suffering.
"Is the Karma theory an essential component of Buddha's teaching or not?"
Buddhist practice is a means to end suffering.
Karma theory is not needed to achieve liberation from suffering.
Don't bother yourself with questions about karma and/or rebirth. You don't need these answers to achieve the end of suffering.
To achieve the end of suffering you need to meditate and look closely at reality as it really is.
Once you will see reality as it really is in its entirety, you will know the answers to all questions.
EDIT:
Think again before marking my answer as the correct answer:
If you know that reality is that there is no kamma nor rebirth, you are thinking wrongly. If that is the case, you don't know the cause of kamma and rebirth, thus you're subjecting yourself to future suffering.
If you know that reality is that there is kamma and rebirth, you are again thinking wrongly. If that is the case, you don't know the path to the cessation of kamma and rebirth, thus you're subjecting yourself to future suffering.
If you know that reality is that there is neither kamma nor rebirth, and that when there is kamma and rebirth they always arise with a cause, you are thinking correctly. If that is the case, you maybe know the cause of kamma and rebirth and the path leading to their cessation, thus there is a chance you're completely liberated from suffering.
If it is not the last, I again say to you: To achieve the end of suffering you need to meditate and look closely at reality as it really is.

recollection of past "lives" (lit: previous homes) is a mistranslation. the meaning of this teachings is here: https://suttacentral.net/en/sn22.79 . Please study it carefully.

Thank you, I've read it. When you translate, you have to put the meaning in the right context based on the entire work, not just on the literal translations of some words, i.e. you have to ask yourself what the writer really meant with his words. Please see this. DN 2: "These beings — who were endowed with bad conduct of body, speech, and mind, who reviled the noble ones, held wrong views and undertook actions under the influence of wrong views — with the break-up of the body, after death, have re-appeared in the plane of deprivation, the bad destination, the lower realms ..."

And this. MN 117: "And what is wrong view? 'There is nothing given, nothing offered, nothing sacrificed. There is no fruit or result of good or bad actions. There is no this world, no next world, no mother, no father, no spontaneously reborn beings; no contemplatives or brahmans who, faring rightly & practicing rightly, proclaim this world & the next after having directly known & realized it for themselves.' This is wrong view." There are many more similar writings in the suttas.

Thank you for taking the time to clarify things. I interpret the suttas from their translations. If the translations are misinterpreted, it would be nice to have them translated correctly to avoid such misinterpretations as I'm doing. You seem to know a lot. I kindly ask you to help me understand what you've found out with your studies. If I understand you correctly, after death there will be complete cessation of suffering irregardless of the views that I hold or unwholesome actions that I've done during my life? Just a yes or no would be enough to help me see in the right direction.

Dhammadhatu, you have this belief: "After death there is the end of experience". Maybe I'll post this as a question.

MN 38 teaches "death" ceases in the living mind that has right wisdom. Please read the last paragraphs of MN 38. Please also read the last paragraphs of MN 140, which state there is no "death" for the arahant free from conceiving "self". Your materialistic physical belief that "death" (marana) refers to the ending of life is not what the suttas teach.

Also, please do not slander me. I do not believe "death" (marana) is the end of experience. Instead, death is a cause of suffering. The Dhamma teaches living arahants do not experience "death" ("marana"). Death is a self-view, The mind free from self-view experiences the Deathless.

I understand. I'm not talking about the "death" (marana) that you're talking. I'm talking about the materialistic physical, the ending of life, death that we see happening every day on television, i.e. death that happens when somebody is killed by pistol, cancer, car accident, etc.

The proper translation is "other world" and "spontaneously arisen" beings. "Beings" is a mental state. Refer to SN 23.2 and SN 5.10. "Death" (marana) means death of self-identity or a "being" rather than death of the physical body. Regards

Thank you. I asked the question here https://buddhism.stackexchange.com/questions/22973/does-self-end-after-death . Please answer there to help me understand better so I can ask for further clarifications, as this comments section is related to a different subject.

There are two things here that I think are worth to have a clear separation: one is what is recorded of the teachings (and how they are understood), and the other is their significance to the path.
Also, notice I'm mainly writing from the point of view of the Pali Canon texts and from a more traditional reading of it.
Karma and Rebirth and the Buddha
There are many interpretations of what was the Buddha's position on these subjects. Among the ones in that vein are:

that karma and rebirth were simply beliefs of his time that the Buddha also had.

I think one problem with that interpretation is that it's diametrically opposed to the kind of mind and attitude the Buddha displays in the sutras. Take the Cūlahatthipadopama sutta for example. In it, the Buddha points the dangers of making claims that could, in reality, be otherwise. His own claims throughout the many sutras seem to be, virtually at all times, very carefully put.
It's hard to conciliate a teacher who teaches his listeners not to make fragile claims (claims that might be wrong) while doing that himself -- that is, it's hard if we take that teacher as a good one.
Another interpretation goes like:

Karma and rebirth were beliefs of his time, and the Buddha repeated those just as means to point his listeners to a deeper truth, but he himself did not agree with those beliefs.

Proponents of this argument usually say these are examples of the Buddha's "skill in means" or that he refrained from opposing his listeners, since they believe those things anyway.
Others would say that another way of presenting this very same argument is that the Buddha lied -- something that he said is not possible for an arahant to do.
It also raises other kinds of questions. For example, it portrays him hesitating about teaching something that, one would think, is "too shocking" to his listeners. And yet, his most shocking teaching is likely anatta. And in the suttas he never appears to hesitate for a second to "shock" his audience with such declarations.
Additionally, it's not that clear that his audience was so overwhelmingly attached to prior beliefs of karma and rebirth as this argument above seem to imply. What history suggests is that, in his time, different beliefs were very popular. Among them, that there are no good or bad deeds ("no karma" doctrine), and that death is "really, the end". That being the case, these ideas likely weren't that shocking.
It's also important to point out that the Buddha's own teaching of karma and rebirth do not appear to be repetitions of prior beliefs, but seem quite original in their particularities -- (to the point the similarities are just on the name, e.g., "karma").
Karma and Rebirth and the Buddha Dharma
"Also, neither of these theories affects the main purpose of Buddhist practice -- to reduce suffering."
Perhaps they do.
For example, in the Buddha's time, there were proponents of "no karma" doctrine. It seems these people believed that there were no intrinsic consequences to their actions, so they could do whatever they want (bad things included) without concern. That is, indeed, the culmination of the full acceptance and belief of "no life after death" view: that morality is artificial (a social construct, an illusion). The ultimate conclusion, therefore, is that it would be foolish to not take illicit advantages when opportunity presents itself and there's no risk of being caught. And that it's foolish to refrain from getting what one wants, regardless if through immoral means.
With that in mind, take the Buddha's teaching to the Kālāmas, where he used a conservative reasoning to make a point for virtuous behavior. In summary, he found the Kālāmas agreeing that good deeds lead to welfare and that bad deeds lead to harm. Then, he proceed to make his case about a person who does good deeds:

“This noble disciple, Kālāmas, whose mind is in this way without enmity, without ill will, undefiled, and pure, has won four assurances in this very life.
“The first assurance he has won is this: ‘If there is another world, and if there is the fruit and result of good and bad deeds, it is possible that with the breakup of the body, after death, I will be reborn in a good destination, in a heavenly world.’
“The second assurance he has won is this: ‘If there is no other world, and there is no fruit and result of good and bad deeds, still right here, in this very life, I maintain myself in happiness, without enmity and ill will, free of trouble.
“The third assurance he has won is this: ‘Suppose evil comes to one who does evil. Then, when I have no evil intentions toward anyone, how can suffering afflict me, since I do no evil deed?’
“The fourth assurance he has won is this: ‘Suppose evil does not come to one who does evil. Then right here I see myself purified in both respects.’
-- AN 3.65

Basically, he is saying that doing good has, at least, benefits here in this very life. And if turns out there are intrinsic harmful effects of evil actions, one is safe from them, while one is also at peace if there isn't. And if there is another life after death, the effects of bad actions won't drag one to a suffering realm. So, in essence, being virtuous is a win-win situation.
Notice the Buddha is not asking the Kālāmas to believe rebirth at face value. He is just teaching them to be intelligent with their choices in a situation where they don't have the knowledge to be certain about things they are uncertain.
Another aspect of the belief (or lack) in life after death is concerned with the commitment to the path: to attain Nirvana. A person who believes in the traditional view of samsara (life, death, another life, ... and on and on)  may become lax in the practice, as you pointed out. To tackle this problem, many times in the sutras the Buddha is seen instigating his disciples to train with urgency.
And that urgency may be developed from the very sight of the possibility that he or she may be reborn again: by understanding how feeble and unsatisfactory our lives are, and the previous ones and the next ones, always subject to suffering, never finally released from it. One of the most inspiring sutras on this matter is the Assu Sutta:

“Long have you (repeatedly) experienced the death of a mother. The tears you have shed over the death of a mother while transmigrating & wandering this long, long time—crying & weeping from being joined with what is displeasing, being separated from what is pleasing—are greater than the water in the four great oceans.
“Long have you (repeatedly) experienced the death of a father… the death of a brother… the death of a sister… the death of a son… the death of a daughter… loss with regard to relatives… loss with regard to wealth… loss with regard to disease. The tears you have shed over loss with regard to disease while transmigrating & wandering this long, long time—crying & weeping from being joined with what is displeasing, being separated from what is pleasing—are greater than the water in the four great oceans.
“Why is that? From an inconstruable beginning comes transmigration. A beginning point is not evident, though beings hindered by ignorance and fettered by craving are transmigrating & wandering on. Long have you thus experienced stress, experienced pain, experienced loss, swelling the cemeteries—enough to become disenchanted with all fabricated things, enough to become dispassionate, enough to be released.”
-- SN 15.3

Finally, to the Buddha (as far as we are talking about what has been recorded about his teachings), fully dedicating the current life to the practice is precisely the best of his recommendation. While he did taught lay people and helped them to maintain their practice (people who were still dedicated to the ordinary life affairs), it is the full commitment that the Buddha praised more -- the abandonment of ordinary affairs of life and full dedication to reach nirvana in this very life.
So, as far as we are talking about what is recorded of what the Buddha said, I don't think they agree with the idea that "Buddhist practice is a means to live the one life one has as effectively as possible. Basically, you meditate in the evening in order to better achieve your personal goals during the day".
It is definitely in agreement that the practice, for whatever extent exercised, have fruits and these are pointed to the "welfare" side of things. But the highest alignment with what the Buddha considered to be "noble" are those who have abandoned all their personal goals, leaving only one remaining: to end suffering.
For more on this theme, see also:

Is rebirth a delusional belief?
Is rebirth/reincarnation central to Buddhism?
Discernment or blind belief?

There is no Buddhism without Karma. Paticcasamuppada (the Law of Dependent Origination) is all about Karma and it's consequences.
Akiriya-ditthi is the false view or opinion of the inefficacy of action: That neither moral good nor moral evil action have any delayed consequences for anyone.
This wrong view was taught by Pūrana-Kassapa during the time of the Buddha.
Today also there are people like Pūrana Kassapa who spread such views. What is more dangerous is that instead of presenting such views as their own beliefs, they try to present them as the teachings of the Buddha and mislead many who are new to Buddhism.

Comments are not for extended discussion; this conversation has been moved to chat.

Let's keep them separate - karma separate from rebirth.
In my understanding, karma is about subtle latent effects of our actions, that create conditions that influence our future experience. In other words, karma is not magic, it's not a moral retribution law, it's just the normal cause-and-effect - albeit the not-so-obvious part of it.
IMO, the teaching of skillful action that lead to good results and unskillful action that lead to bad results is a critical key component of Buddha-Dharma, at all levels from the most mundane to the most esoteric. This point cannot be emphasized enough.
As for rebirth, it is my opinion, based on my study of sutras and practice of insight meditation, that it is a metaphor for the continuity of information-causation, used in its primitive form as a skillful mean to create a sense of accountability, and hope, in the beginner student's mind.
Whether it's justifiable to keep using these archaic constructs in our digital age is a question that will probably outlive both of us, so I will not answer it here. My personal choice so far has been to reveal the meanings behind the metaphors in the hope that it will reduce confusion on the student's part. In my experience, authentic Dharma is valuable by itself, without the fairy dust.

From "Kamma: A Study Guide" by Ven. Thanissaro you can find the summary below. Please read the full study guide for the justification.

The doctrine of kamma (karma — action) is one of the Buddha's central
  teachings. There is a modern myth that he simply picked up his ideas
  on kamma from the worldview prevalent in his day, and that they aren't
  really integral to his message. Nothing could be further from the
  truth. Early Buddhists often cited the Buddha's teaching on kamma as
  one of the prime teachings that set him apart from his contemporaries,
  and a study of his teachings on kamma will show that they underlie
  everything else he taught.

From the essay "A Remedy for Despair" by Ven. Bodhi:

As we grope about for a handle to prevent ourselves from plummeting
  down into the pits of despondency, we may find the support we need in
  a theme taught for frequent recollection by the Buddha: "Beings are
  the owners of their kamma, the heirs of their kamma; they are molded,
  formed and upheld by their kamma, and they inherit the results of
  their own good and bad deeds." Often enough this reflection has been
  proposed as a means to help us adjust to the vicissitudes in our
  personal fortunes: to accept gain and loss, success and failure,
  pleasure and pain, with a mind that remains unperturbed. This same
  theme, however, can also serve a wider purpose, offering us succor
  when we contemplate the immeasurably greater suffering in which the
  multitudes of our fellow beings are embroiled.
Confronted with a world that is ridden with conflict, violence,
  exploitation and destruction, we feel compelled to find some way to
  make sense out of their evil consequences, to be able to see in
  calamity and devastation something more than regrettable but senseless
  quirks of fate. The Buddha's teaching on kamma and its fruit gives us
  the key to decipher the otherwise unintelligible stream of events. It
  instructs us to recognize in the diverse fortunes of living beings,
  not caprice or accident, but the operation of a principle of moral
  equilibrium which ensures that ultimately a perfect balance obtains
  between the happiness and suffering beings undergo and the ethical
  quality of their intentional actions.

The theoretical concepts of karma and rebirth are not helpful for the beginner in his or her practice of mindfulness meditation. Early progress is made independently of one's abstract beliefs. The concept of rebirth explains almost nothing about one's personal experience. From a modern point of view, the theory of karma is a theory of learning. The process of learning is very complex and mostly unconscious, making it a lifetime study. Understanding how mindfulness meditation actually works is very difficult. Buddhist literature does not explain karma or rebirth, while taking it for granted. The actual psychological processes of karma and rebirth are very difficult to understand and require very special states of meditation to investigate. Trying to understand the two concepts in superficial terms is a waste of time and a source of unnecessary concern.

The Buddha taught his special teaching is for the destruction of kamma, as follows:

just this noble eightfold path — right view, right resolve, right speech, right action, right livelihood, right effort, right
  mindfulness, right concentration — is the path of practice leading to
  the cessation of kamma. AN 6.63

The teaching of kamma is for puthujanna (common worldlings) that are unable to give up self-view or 'acquisition'. MN 117 states kamma is a defiled (impure) mundane or worldly teaching for unenlightened common people to make merit (be moral ) & avoid self-harm, as follows:

And what is the right view with effluents, siding with merit, resulting in acquisitions?...There are fruits & results of good & bad actions. MN 117

In other words, 'kamma' is not a teaching that is unique & special to the Buddha. Therefore, kamma is not an essential teaching, as shown in the first two sermons of the Buddha, where the method to liberation was explained as ending craving. When a person lives without craving, they automatically do not engage in any harmful behaviours & are also free from suffering.
The Buddha often explained what he taught & never included 'kamma' as his special & unique teaching, as follows:

Bhikkhus, both formerly and now what I teach is suffering and the cessation of suffering. MN 22

What I have revealed is: 'This is Suffering, this is the Arising of Suffering, this is the Cessation of Suffering, and this is the Path
  that leads to the Cessation of Suffering.' And why, monks, have I
  revealed it? Because this is related to the goal, fundamental to the
  holy life, conduces to disenchantment, dispassion, cessation,
  tranquillity, higher knowledge, enlightenment and Nibbaana, therefore
  I have revealed it. SN 56.31

When the Blessed One perceived that the mind of Upāli, the householder, was prepared, pliant, free from obstacles, elevated and
  lucid, then he revealed to him that exalted doctrine of the Buddhas,
  viz. Suffering, its Cause, its Ceasing and the Path. MN 56

These are the six elements’: this, bhikkhus, is the Dhamma taught by me that is unrefuted … uncensured by wise ascetics and brahmins.
  ‘These are the six bases for contact’ … ‘These are the eighteen mental
  examinations’ … ‘These are the four noble truths’: this, bhikkhus, is
  the Dhamma taught by me that is unrefuted, undefiled, irreproachable,
  and uncensured by wise ascetics and brahmins. AN 3.61

the discourses that are words of the Tathagata — deep, deep in their meaning, transcendent, connected with emptiness (sunnata) ... SN 20.7

Without a theory of the efficacy of action, how does one justify the necessity of right intention, speech and action? Kamma seems like little more than Newton's third law - it doesn't define what 'right action' or 'wrong action' is, rather it states their results find a balance, which is the only reasonable proposition given sunnata (imo).

I didn't downvote your answer. In MN 117 the Buddha is saying that belief in kamma is right view, and the rejecting of kamma a wrong view. It's the same as saying existentialism in most cases creates more wholesome fruits than nihilism. Nonetheless, complete liberation does not lie neither in existentialism nor in nihilism. I think that's the main message of the Buddha.


text:
Monks who are not worthy and worthy
I have seen certain suttas where the Buddha explains about the qualities of monks who are and who are not worthy of gifts, salutation and etc.
Where can I find the Suttas? References are appreciated.

From the Ajañña Sutta (AN 8.13):

"In the same way, a monk endowed with eight qualities is worthy of
  gifts, worthy of hospitality, worthy of offerings, worthy of respect,
  an incomparable field of merit for the world. Which eight?
(1) "There is the case where a monk is virtuous. He dwells restrained
  in accordance with the Patimokkha, consummate in his behavior & sphere
  of activity. He trains himself, having undertaken the training rules,
  seeing danger in the slightest faults.
(2) "When given food, whether coarse or refined, he eats it carefully,
  without complaining.
(3) "He feels disgust at bodily misconduct, verbal misconduct, mental
  misconduct, at the development of evil, unskillful [mental] qualities.
(4) "He is composed & easy to live with, and doesn't harass the other
  monks.
(5) "Whatever tricks or deceits or wiles or subterfuges he has, he
  shows them as they actually are to the Teacher or to his knowledgeable
  companions in the holy life, so that the Teacher or his knowledgeable
  companions in the holy life can try to straighten them out.
(6) "When in training he gives rise to the thought, 'Whether the other
  monks want to train or not, I'll train here.'
(7) "When going, he goes the straight path; here the straight path is
  this: right view, right resolve, right speech, right action, right
  livelihood, right effort, right mindfulness, right concentration.
(8) "He dwells with his persistence aroused, [thinking,] 'Gladly would
  I let the flesh & blood in my body dry up, leaving just the skin,
  tendons, & bones, but if I have not attained what can be reached
  through human steadfastness, human persistence, human striving, there
  will be no relaxing my persistence.'"

From the Ajaniya Sutta (AN 3.94) - please read sutta for elaboration:

"In the same way, a monk endowed with these three qualities is worthy
  of gifts, worthy of hospitality, worthy of offerings, worthy of
  respect, an unexcelled field of merit for the world. Which three?
  There is the case where a monk is consummate in beauty, consummate in
  strength, and consummate in speed.

From the Akkhama Sutta (AN 5.139) - please read sutta for elaboration:

"In the same way, a monk endowed with five qualities is not deserving
  of gifts, deserving of hospitality, deserving of offerings, deserving
  of respect, nor is he an unexcelled field of merit for the world.
  Which five? There is the case where a monk is not resilient to sights,
  not resilient to sounds, not resilient to aromas, not resilient to
  flavors, not resilient to tactile sensations.
"In the same way, a monk endowed with five qualities is deserving of
  gifts, deserving of hospitality, deserving of offerings, deserving of
  respect, an unexcelled field of merit for the world. Which five? There
  is the case where a monk is resilient to sights, resilient to sounds,
  resilient to aromas, resilient to flavors, resilient to tactile
  sensations.

From the Sotar Sutta (AN 5.140) - please read sutta for elaboration:

"In the same way, a monk endowed with five qualities is deserving of
  gifts, deserving of hospitality, deserving of offerings, deserving of
  respect, an unexcelled field of merit for the world. Which five? There
  is the case where a monk is a listener, a destroyer, a protector, an
  endurer, and a goer.

From the Pathama Atthapuggala Sutta (AN 8.59):

Monks, there are these eight individuals who are worthy of gifts,
  worthy of hospitality, worthy of offerings, worthy of reverential
  salutation, the unsurpassed field of merit for the world. Which eight?
The one who has entered the stream, the one who has entered upon the
  course for the realization of the fruit of stream-entry, the
  once-returner, the one who has entered upon the course for the
  realization of the fruit of once-returning, the non-returner, the one
  who has entered upon the course for the realization of the fruit of
  non-returning, the arahant, the one who has entered upon the course
  for arahantship.

Also, Ratana sutta called Sangha Ratana (sangha is worthy like a gem). See sangha unseat I katha in path of purification for more detail.

Akila Hettiarachchi, and those interested,
It's general dangerous to approach topics of who "deserves my goodness" and questions are always better put in a way "how am I worthy of goodness I receive", "how do I best give back the goodness received", and in this manner the most teachings on who is worthy of special gifts are put by the Buddha to his disciples: to urge them to practice well.
For a householder it is difficult, most impossible to know who is a person count as the unexceled field of merits, which is maybe best shown in the Paṭisalla Sutta
One may find some useful hints in regard of giving alms, which counts for reverence similar under the answers to [Q&A] Details on what may be given as alms (Tips for alms-giving)

A warning: Be careful of that what ruins a fool who having gained knowledge! He/she might think in ways like: "The monks must... His duty is... so and so he should receive..." Having gained much knowledge about possible duties of others, not really know practice and as it really is, and forgetting on focussing on wn mindstate, which is the A&O of all here, one makes nothing then demerits, distroys his goodness, with every act one believes to make merits at least. Generosity that is path-supportive requires not only wisdom but also right view. A normal person or even a fool is not able to go or came as even far as this.

So it's general good to be more positive "selfish" and think about ones own duties and good states of mind. The kamma of the receiver is at least his/her and you, especially as householder, are not obligated to take on the rule of police or judge, yet it is at the same moment good and importand to judge of who is a worthy receiver of special gifts and reverence, and that is maybe an importand point, since every being is worthy of the share and giving of the four requisites (food, shelter, cloth, medicine) in need an there are proper times for such.
On the matter of the importand judgement one may find an extended discussion after some basic and importand teachings on giving here: Dana (Generosity, Charity) "Abhidhamma in daily life "

A very good and importand question, Ta Ron raised, since it is broadly not so good understood and especially within the group of people who already have faith in the benefit of Dana an see even reason why such is good made, it is always a topic, Ta Ron, and my person explains this again and again...

To often one with dicerment can see that especially literary learned people, but without having it really practiced to understand, if they are fools, simply destroy all there goodness (past merits), when ever they learn more, since they use everything to give their defilements (here stinginess) excuses and ground to stay so.
Of two people, one good literary informed, but lacking on genuine goodness, and another person, even lacking totally on literary knowledge, but with genuine goodness, the secound can not only be expected doing further undertakings for accumulation of merits, but also that he/she successes in it, while it is not sure for the first to even start to try.
In regard of reverence and respect, one may find tips and reverences here: Respect and Veneration
Given that:

VERSE 108: In this world, one may make sacrificial offerings, great and small, all the year round [to ordinary people], in order to gain merit; all these offerings are not worth a quarter of the merit gained by worshipping the Noble Ones (Ariyas) who walk the right path.

It's good to practice in areas where such can be susbected, and its most importand to do such on places where people have joy in giving and sharing merits, rather to see good deeds and joy with it as a tabu and childish. Something that is of course not easy not find in modern sociaties.
It's all, incl. the skill of giving, a matter of right view, and associating inwardly and outwardly with it.
A matter that is how ever not spoken of, is also how monks, in this case, are paying gratitude to given special food and it should be a warn sign, if people approach with "all is anicca"

I was talking recently to someone who had given a gift to a lay-run meditation center last year. He came back this year to find that it disappeared. When he asked the people at the center about this they said: "Well, that's impermanence."... Sensitivity through generosity

And the best way to learn things is: Better to Give than to Consume. So if one gives time and effort and goes through all given links and reverences, with respect and gratitude, one can espect to gain really good understanding of the matter of judging of what is worthy and will find out that there is no way than to eager strive by means of giving to become worthy of gifts by oneself and represent one self in the way one has received the goodness of others.

From collection of reverences on Dana:
This was said by the Blessed One, said by the Arahant, so I have heard: "I am a brahman, responsive to requests, open-handed, bearing my last body, an unsurpassed doctor & surgeon. You are my children, my sons, born from my mouth, born of the Dhamma, created by the Dhamma, heirs to the Dhamma, not heirs in material things.
"There are these two kinds of gifts: a gift of material things & a gift of the Dhamma. Of the two, this is supreme: a gift of the Dhamma.
"There are these two kinds of sharing: sharing of material things & sharing of the Dhamma. Of the two, this is supreme: sharing of the Dhamma.
"There are these two kinds of assistance: assistance with material things & assistance with the Dhamma. Of the two, this is supreme: help with the Dhamma.
"There are these two kinds of mass-donations: a mass-donation of material things & a mass-donation of the Dhamma. Of the two, this is supreme: a mass-donation of the Dhamma."
He who, unstinting,
made the mass-donation of Dhamma,
the Tathagata,
sympathetic to all beings:
to one of that sort
— the best of beings, human & divine —
living beings pay homage —
to one gone
to the beyond
of becoming.

Genuine spiritual teachings
May all who are able to rejoice with the merits done here, access highest fruits, and may the Devas inform those who did not heard or seen it yet.
Anumodana!
[Note: This is a gift of Dhamma, not meant for commercial purpose or other wordily gains.]

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text:
Problems with Inactivity
My question is basically as to what constitutes skillful and unskillful activity.  I noticed I return to unskillful craving-related activities when I am idle. However, I cannot ascertain what is skillful in terms of activities. I would think the application of effort is important in terms of an activity, because I am often contemplating/analyzing ideas, but in a slightly lazy way.
Concretely, is idleness bad? For example, I often take public transport, wherein I contemplate certain subjects. But, I often tell myself I should read in public transport instead.
I think to me, some contemplative activity occurs when idle, but it is not focused, nor effortful. Can mere contemplation without much effort or focus be called meditation? Or is it simply wasting time?
Thank you.
EDIT: Also, I would think activity and industriousness generates energy in some sense. What states accompany idleness? (e.g. lethargy, fragmentation, etc.)

Seek activities that awake enthuziasm in you. I keep hearing this these days so I am just forwarding it. I too am of the lazy idle type:) also dont be hard on yourself about this, you are human. Humans are very diverse and all humans belong where they are, how they are.

In Vitakkasanthana Sutta, the Buddha advised:

Just as a skilled carpenter or his apprentice would use a small peg to
  knock out, drive out, and pull out a large one; in the same way, if
  evil, unskillful thoughts — imbued with
  desire, aversion, or delusion — arise in a monk while he is referring to and attending to a particular theme, he should attend to
  another theme, apart from that one, connected with what is skillful.
  When he is attending to this other theme, apart from that one,
  connected with what is skillful, then those evil, unskillful thoughts
  — imbued with desire, aversion, or delusion — are abandoned and
  subside. With their abandoning, he steadies his mind right within,
  settles it, unifies it, and concentrates it.

So, if you're afflicted with thoughts imbued with the three poisons (desire/ greed/ attachment, aversion/ hatred and delusion), then this is the sign of unskillful thoughts. The Buddha advises above to switch to other thoughts which are skillful (which are the opposite of the three poisons), then advises to concentrate the mind (in the context of meditation).
Of course, outside of the context of meditation, one should recognize unskillful thoughts imbued with the three poisons, then switch to other themes.
Concerning idleness or laziness, the Sigalovada Sutta states:

"And what six ways of squandering wealth are to be avoided? Young man,
  heedlessness caused by intoxication, roaming the streets at
  inappropriate times, habitual partying, compulsive gambling, bad
  companionship, and laziness are the six ways of squandering wealth.
"These are the six dangers inherent in laziness: saying, 'It's too
  cold,' one does not work; saying, 'It's too hot,' one does not work;
  saying, 'It's too late,' one does not work; saying, 'It's too early,'
  one does not work; saying, 'I'm too hungry,' one does not work;
  saying, 'I'm too full,' one does not work. With an abundance of
  excuses for not working, new wealth does not accrue and existing
  wealth goes to waste."

Although I couldn't find the exact sutta reference, this essay by Lily de Silva states:

In one sutta (A. III, 293) the Buddha explains how to prepare for a
  peaceful death. One has to organize one's life and cultivate an
  appropriate attitude for this purpose. The instructions given there
  are as follows:
(1) One should not be fond of a busy life involved in various activities.
  (2) One should not be fond of being talkative.
  (3) One should not be fond of sleeping.
  (4) One should not be fond of having too many companions.
  (5) One should not be fond of too much social intercourse.
  (6) One should not be fond of daydreaming.

There is a kind of a middle way advised above, where one should neither be too busy in too many activities, nor daydreaming or oversleeping or over-socializing.
If you have idle time on your hands, you can use it for skillful activities like reading good material (like this book), or by cultivating mindfulness.
In his booklet "How To Meditate", Ven. Yuttadhammo advises in "Chapter Six: Daily Life":

Once one has put aside activities that interfere with clarity of mind,
  one can begin to incorporate meditative awareness into ordinary life.
  There are two ways in which one can meditate on ordinary experience,
  and they should be practiced together, as follows.
The first method is to focus one’s attention on the body, since it is
  the most clearly evident aspect of experience. As in formal
  meditation, the body is always available for observation, and thus
  serves as a convenient means of creating clear awareness of reality in
  daily life. Since the body is generally in one of four postures –
  walking, standing, sitting, or lying down, one can simply become aware
  of one’s posture as a meditation object to bring about clarity of
  mind. .......
When cooking, cleaning, exercising, showering, changing clothes, even
  on the toilet, one can be mindful of the movements of the body
  involved, creating clear awareness of reality at all times. This is
  the first method by which one can and should incorporate the
  meditation practice directly into ordinary life.
The second method is the acknowledgement of the senses – seeing,
  hearing, smelling, tasting and feeling. Ordinary sensory experience
  tends to give rise to either liking or disliking; it therefore becomes
  a cause for addiction or aversion and ultimately suffering when it is
  not in line with one’s partialities. In order to keep the mind clear
  and impartial, one should always try to create clear awareness at the
  moment of sensory experience, rather than allowing the mind to judge
  the experience according to its habitual tendencies. When seeing,
  therefore, one should know it simply as seeing, reminding oneself
  “seeing, seeing”.

From the comments:

OP: Would it be possible to be mindful of discursive thoughts? The bodily and sensory mindfulnesses mentionned don't involve thoughts. Is
  mindulness only perceptual?

This question is answered in Ven. Yuttadhammo's booklet "How To Meditate" in "Chapter One: What Is Meditation" referenced below.
The additional foundations of mindfulness after body, are feeling, thoughts and dhammas (mental objects).
Concerning feelings, if you feel happy or calm, acknowledge it as "happy, happy" or "calm, calm".
Concerning thoughts, when your mind drifts away, usually pondering about the past or future, as soon as you recognize it, acknowledge it as "thinking, thinking". The idea here is to detach from them and not cause you to react to them. For e.g. if you remember some past unpleasant event, it will make you feel unhappy. But through mindfulness, you just acknowledge and let it go. As soon as you realize that you are thinking and acknowledge it, the thinking might stop.
Concerning mental objects, the example given in the excerpt is only the five hindrances. But there are also others. For more information on the Foundations of Mindfulness, please read this essay entitled "The Way of Mindfulness" by Ven. Soma Thera.

When we feel happy, we acknowledge it in the same way, reminding
  ourselves of the true nature of the experience, as “happy, happy,
  happy”. It is not that we are trying to push away the pleasurable
  sensation. We are simply insuring that we do not attach to it either,
  and therefore do not create states of addiction, attachment, or
  craving for the sensation.....
Likewise, when we feel calm, we say to ourselves, “calm, calm, calm”,
  .....
The third foundation is our thoughts. When we remember events in the
  past, whether they bring pleasure or suffering, we say to ourselves,
  “thinking, thinking”. Instead of giving rise to attachment or
  aversion, we simply know them for what they are – thoughts. When we
  plan or speculate about the future, we likewise simply come to be
  aware of the fact that we are thinking, instead of liking or disliking
  the content of the thoughts, and thus avoid the fear, worry, or stress
  that they might bring.
The fourth foundation, the “dhammas”, contains many groupings of
  mental and physical phenomena. Some of them could be included in the
  first three foundations, but they are better discussed in their
  respective groups for ease of acknowledgement. The first group of
  dhammas is the five hindrances to mental clarity. These are the states
  that obstruct one’s practise: desire, aversion, laziness, distraction,
  and doubt. They are not only hindrances to attaining clarity of mind,
  they are also the cause for all suffering and stress in our lives. It
  is in our best interests to work intently to understand and discard
  them from our minds, as this is, after all, the true purpose of
  meditation.
So when we feel desire, when we want something we don’t have, or are
  attached to something we do, we simply acknowledge the wanting or the
  liking for what it is, rather than erroneously translating desire into
  need. We remind ourselves of the emotion for what it is, thus:
  “wanting, wanting”, “liking, liking”. We come to see that desire and
  attachment are stressful and causes for future disappointment when we
  cannot obtain the things we want or lose the things we like.
When we feel angry, upset by mental or physical experiences that have
  arisen, or disappointed by those that have not, we recognize this as
  “angry, angry” or “disliking, disliking”. When we are sad, frustrated,
  bored, scared, depressed, etc., we likewise recognize each emotion for
  what it is, “sad, sad”, “frustrated, frustrated”, etc., and see
  clearly how we are causing suffering and stress for ourselves by
  encouraging these negative emotional states. Once we see the negative
  results of anger, we will naturally incline away from it in the
  future.
When we feel lazy, we say to ourselves, “lazy, lazy” or “tired,
  tired”, and we will find that we are able to regain our natural energy
  in this way. When we are distracted, worried or stressed, we can say,
  “distracted, distracted”, “worried, worried”, or “stressed, stressed”
  and we will find that we are more focused. When we feel doubt or are
  confused about what to do, we can say to ourselves “doubting,
  doubting” or “confused, confused”, and likewise we will find that we
  are more sure of ourselves as a result.
The clear awareness of these four foundations constitutes the basic
  technique of meditation practise as explained in the following
  chapters. It is therefore important to understand this theoretical
  framework before beginning to undertake the practise of meditation.
  Understanding and appreciating the importance of creating a clear
  awareness about the objects of our experience as a replacement to our
  judgemental thoughts is the first step in learning how to meditate.

Would it be possible to be mindful of discursive thoughts? The bodily and sensory mindfulnesses mentionned don't involve thoughts. Is mindulness only perceptual?

Replied in the answer.

Skillful activity is getting your head and your heart to respect each other, and in being mindful as you go about in life. Then your head and your heart can take each other far when you can remain mindful, even amidst the hustle and bustle of everyday life. Your heart needs the help of your head to generate and act on more skillful emotions. Your head needs your heart to remind you that what's really important in life is putting an end to suffering. They need each other.  When you come to this realism, you will dwell in “kaya passaddi”.
To explain the above, I must give you another meaning to words like “Kaya”. Such words come under Adhi-vacana – meaning - the same word has different meanings depending on the sutta, and how it gets used. Here “kaya” refers to action; action of the mind. So, “passaddi” refers to serene, tranquil nature of action. Actions of mind become tranquil only when the mind is devoid of “heat”, resulting from the existence of defilements such as lobha (greed) and dosa (aversion).
There is a tendency of mind to grasp objects. The action of mind (kaya) is always to grasp something. When one’s mind reaches the stage of passaddi (quietude, serenity), the action of grasping takes place in a tranquil manner.  A person who has developed this quietude of mind discerns greed or aversion exactly at the time that these qualities appear in his mind. This discernment which synchronizes with the arising of greed or aversion in mind is the correct way of “anupassana”. Anupassana means “discard according to the principles learned” (“anu” means according to and “passana” means to get rid of).
Let’s take an example where a youth coming face to face with a pretty girl. The sight of a pretty girl is a sensuous sense object for a young guy. If this guy is one who cultivates good or worthwhile qualities in the mind, then, he sense the arising of greed at the sight of the pretty girl. Therefore, he applies “anupassana” in respect of the sense object. Likewise, if this girl walks past the boy completely ignoring him, that could cause anger in him. If the boy could note the anger in his mind, then, he applies “anupassana”. As his sole objective was to maintain a wholesome state of mind, devoid of greed and aversion, he was successful in doing so at the sight of a pretty girl. This is meditation, if you take it in its truest sense of meaning.
When you conduct yourself in this way, in whatever you do, or wherever that you go, then you experience that more often than not, your body calms down and your mind is completely relaxed. Your brain's thinking is slower, your mind is clear and you may even feel slightly drowsy or idle. But worry not, as this is “being intelligent”. Being intelligent signifies being able to exist with a happy, blissful mind. Happiness is the tranquil nature of mind. Happiness arises in mind. It manifests through bodily and verbal actions.  Such a person will go about things in a happy, blissful way.
However, such a mental state is shrouded with our own suffering if we cannot get our head and heart to respect each other, and in being mindful. Once this is done and once the suffering is eliminated, dormant happiness begins to surface in a mind. Then anything that you do constitutes skillful activity.

I think my teacher put this best - "half of Zen practice these days is teaching people how to be a loser!"  In the West (and Japan), at least, we are addicted to work and productivity.  The most "virtuous" among us are those who forgo sleep, work an asinine amount of hours, and generally occupy their time with all manner of industry.  It's at once a delusion and an addiction.  These social norms impel us to monetize every moment.  The also make us wholly negligent of the calm required for effective internal contemplation.
When chopping wood, chop wood.  When carrying water, carry water.  When you're on the bus, just sit on the bus!  Let loose.  Relax.  Let the samatha arise.  Sit back and enjoy the ride.

I find it delightful to read your answers! Very helpful!

Glad I could help!

Would it be possible to be mindful of discursive thoughts? The bodily and sensory mindfulnesses mentionned don't involve thoughts. Is mindulness only perceptual?
"Inactive" needn't mean "mindless".
I gather that "mindful" is supposed to mean "remembering" or "keeping in mind".
I suppose that what you remember, bear in mind, keep in mind, is called "the object of meditation".
Sometimes people use a visual object of meditation: a kasina, maybe a candle, or a mandala, or even a landscape, or  wall.
Or the object of a meditation might be metta bhavana (the development of metta).
I notice that "object" (at least in English) can have two meanings: it's the immediate perceptible thing which you concentrate on and pay attention it; and "the object of something" can mean your purpose or goal, your reason for doing something. In metta bhavana I suppose it's both: you focus on metta in order to bring it into being.
Anther type of meditation or mindfulness in Buddhism is "mindfulness with/when breathing" (anapanasati). I think that has two components, i.e. breathing and being mindful. To the extent that "mindful" means "remembering", though, I don't think that means "remembering to breath" -- I think it means more like, "remembering the object of meditation while being aware of breathing".
Anapanasati is famous: people teach it. Perhaps it's taught in too much detail, e.g. in this answer I tried to answer a question just about the mechanics of "breathing long" (selecting quotes from one 500-page book about anapanasati in general). Forms of meditation are also described in the suttas (and elsewhere) and contemporary teachers (meditation centre, monasteries) teach in person.
Another object of meditation, some else to take as the "object" of meditation, is "letting go" or "non-attachment" (see for example this answer, or the same texts in a bit more detail here).

You're asking what you can do when inactive. I guess in general you can study, do homework ... maybe socialize!
But "inactivity" might be an opportunity to practice the meditation portion of the noble eightfold path.
That (subject) leads to more that's been written on the subject: e.g. Right Mindfulness.

I think to me, some contemplative activity occurs when idle, but it is not focused, nor effortful. Can mere contemplation without much effort or focus be called meditation?
I don't know. And maybe it depends on the school of Buddhism, and the translation (not just between languages but between people.)
For example, "No Attachment to Dust" ends with,

Live with cause and leave results to the great law of the universe. Pass each day in peaceful contemplation.

... which implies that contemplation (or as you put it, "contemplative activity" or "mere contemplation") is recommended.
I guess the flip side though is that "contemplation" might be a cover for thirst and attachment -- if by "contemplation" you meant some "obsessing about gaining something", for example.
Apparently the body needs to eat but shouldn't be encouraged to eat obsessively.
And the eye is used to see, but, they say, sometimes people try to endless fill their eye with sights.
Similarly it's possible for the mind to develop some craving for objects of the mind (ideas and emotions maybe, news and views) ... maybe that would be a "wrong" kind of contemplation ... not ariya, or, not "peaceful".

I would think activity and industriousness generates energy in some sense.
In some sense, yes.
But I'd also try to recommend detachment as liberating.

My question is basically as to what constitutes skillful and unskillful activity.

Skilful is without greed, hatred & delusion:

And what is the wholesome? Abstention from killing living beings is wholesome; abstention from taking what is not given is wholesome;
  abstention from misconduct in sensual pleasures is wholesome;
  abstention from false speech is wholesome; abstention from malicious
  speech is wholesome; abstention from harsh speech is wholesome;
  abstention from gossip is wholesome; non-covetousness is wholesome;
  non-ill will is wholesome; right view is wholesome. This is called the
  wholesome.
7. "And what is the root of the wholesome? Non-greed is a root of the wholesome; non-hate is a root of the wholesome; non-delusion is a root
  of the wholesome. This is called the root of the wholesome.
MN 9

I noticed I return to unskillful craving-related activities when I
  am idle.

This is normal for ordinary people.

However, I cannot ascertain what is skillful in terms of activities.

For the lay person, unskilful is serious bad kamma, such as killing, stealing, sexual misconduct, lying, intoxicants, gambling, porn, etc.
But mild sensual pleasures, such as sports, music, arts, committed sex, etc, is not unwholesome.

I would think the application of effort is important in terms of an activity, because I am often contemplating/analyzing ideas, but in a
  slightly lazy way.

For serious meditators, effort is important. But for the average layperson, effort is important for important duties, such as work. When not working, having a rest or idle time is important. The work of meditation is different to worldly work. Because meditation work creates peace & energy, rest is not required, unlike with worldly work.

Concretely, is idleness bad?

If you need to rest, no, idleness is not bad. For example, I like to have a nap in the middle of the day on weekends.

For example, I often take public transport, wherein I contemplate certain subjects. But, I often tell myself I should read in public
  transport instead.

Contemplation is OK.

I think to me, some contemplative activity occurs when idle, but it is not focused, nor effortful. Can mere contemplation without much effort or focus be called meditation? Or is it simply wasting time?

In Buddhism, we can contemplate on subjects of Dhamma. On public transport, one can sit in formal meditation or samadhi, not thinking about anything. Or, otherwise, one can contemplate on the sense objects seen in a dhammic way.

Practical, Eggman, Nyom Volker, who ever interested:
The Buddha gave his monks the duty:

to either dwell in silence,
3rd of Jhana,
or to talk on Dhamma.

So if not jhanaing,
listen, think, read, reflect, talk...
on Dhamma is quite fine.
Don't 'relaxe' on unskillful causes. When ever possible, instead of letting attention get off, take the hand of an admirable, friend, ni-mitta,
Whether by mind, speech*,
Or the body of breath.
Appamada!
[*]
Food is used to become free of desire after food, so train your hunger all the time and don't let you become a Sea Squirt
[Note: This is a gift of Dhamma, not meant for commercial purposes or other wordily trade]

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text:
How to have a relationship
Every intimate relationship I have ever had has ended in a lot of pain and grief. The last one lasted for 17 years but eventually broke down. That was a year ago and the pain is still very raw at times.
My question is this. If attachment causes suffering then I was obviously very attached but how do you have an intimate relationship without becoming attached? It seems just like the normal human thing to do. You like someone, you fall in love and it hurts when they're gone.
You kind of don't even realise it's happening until it ends and then you feel like you want to die. I'm not sure if it's possible to have a close relationship on the Buddhist path anymore but I also don't want to live life alone. I know Buddha left his family but in these modern times there seem to be plenty of teachers who have partners/spouses.

There may be some relevant answers here: "Do buddhists fall in love?" and "Is it possible to have a romantic relationship without attachment?" .

Also: Any authentic sutta from any tradition that gives guidance on what kind of partner to choose?

Impossible. When there's relationship, there's bonding. When there's bonding, there's pain when it ends.
The only thing you can do to minimize pain is to stay as independent as possible, and be your own source of joy and acceptance. Which means playing an active role in your life, instead of relying on your partner for initiative and values.

Buddhist guidance about how to have a relationships is provided at the following links.
THE PARTNER
(A good spouse)
Samajivina Sutta: Living in Tune
Living Together (1)
Sigalovada Sutta: The Discourse to Sigala --
The Layperson's Code of Discipline
This guidance is not only used for any prospective relationship but also for reflection about where past relationships did not work out.
When painful separation occurs, instead of trying to practise non-attachment, it is wiser to investigate the causes & conditions that resulted in a relationship inevitably not working out.
Contrary to the contemporary method of getting the sex out of the way and then getting to know eachother, the Buddhist way is getting to know eachother's values & future aspirations before engaging in sex.
Generally, the sexual part is what leads to the attachment, striving to maintain the unsuitable relationship & pain. So in Buddhism, the mutual & suitable personal qualities are given priority.

Can you add anything about death?

Why? Thanks..........

About losing a partner to death, @Dhammadhatu.

I think it is not relevant because the question is not asking about this & because successful relationships lead to meaningful lasting love that is often (but not always) impervious to death. Most widows I know only have a temporary period grief because the good thoughts of their past friendship (love) sustains them. I live in a small community with many widows & all I know are doing well.

Because death too might seem to be a problem in life; part of the first noble truth, perhaps; the OP wrote, "You like someone, you fall in love and it hurts when they're gone". Your paragraph which started, "Instead of trying to practise non-attachment..." might have suggested you were aiming for a lasting relationship? AN 4.45 which you referenced talks about how "they will see one another not only in the present life but also in the life to come": given what you've said previously about rebirth, I asked! Your latest comment ("impervious to death") might belong in an answer.

But you don't have to add anything. It was just a question: can you add?

"In the life to come" is a mistranslation of "now & in the future". However, if it was true, why would death cause suffering if there was the faith both partners would be united in future lives? You are engaged in two errors: (i) mistranslation; (ii) misconstruing the mistranslation. I try to teach you about the two types of right view (MN 117) but you get them mixed up. If rebirth was really believed, there would not be concern about death because good karma would lead to a good rebirth. It is best to choose one type of right view rather than mixed them up. Thanks

So it means "now and in the future": no problem. I was asking if you could add anything about death (e.g. what you wrote about widows); but you don't have to.

I have nothing to add because the Buddhist path is based on virtue. Unsuccessful relationships are generally the result of insufficient virtue. Therefore, it is good to develop the relationship virtue and then concern oneself with the wisdom required for separation via death later. This said, most widows I know have children, which makes their recollection of their life meaningful. What I wrote about "impervious" love may not apply to childless relationships. Regards

Your wanting to have a partner is causing you suffering. Meditate on that. There's no romantic relationship without attachment. Attachment leads to suffering.
However, having a relationship is not discouraged for lay people.
Here are some criteria to find a partner who will cause you less suffering.

Is her faith similar to yours?
Does she have similar moral standards?
Is she generous as you?
Does she have similar wisdom?

After getting married you can make the relationship smoother by treating the wife in following ways:

by being courteous to her
by not despising her,
by being faithful to her,
by handing over authority of the house to her,
by providing her with adornments.

Arturia,
If wishing to stay together, meet again and again, this Sutta might help: Living in Tune.
How ever, it can be of course, but not need to make it to ones burden, be caused by previous, and present, actions, that longer lasting relations will not come to be, and it's maybe wiser to conform the 1. Noble Truth, that all related phenomenas are subject of breaking apart, do not last, are subject of suffering and not really worthy to seek for it, hold on it, making it yours.
Then investigate it's cause, of such a phenomena (2. Noble Truth). Seek for it's solution, in the 3rd Noble truth, and having won certain faith, following the 4. Noble truth, in going into relation with the Tree Sublime Juwels, beyound all relations and dwelling possible in relation with the Dhamma for the rest of this live comfortable and in ease.
Support and advices might be found also here: Into the Stream.
There is a lot of freedom in not having any relation, yet of course certain relation has to be seeked for to gain that.
To answer the question: "How to have a relationship" in short: to give food/nurishment (upadana) into it, so a constant required sacrify of what one hold, regards, took as self.
[Note: This is a gift of Dhamma, not meant for commercial purpose or other wordily gains and nurishing relations with it, but for possible benefical relation for liberation]

Difficult relationships are certainly the greatest and most common source of suffering today. Fortunately, over the last century or so, a lot of insight has been acquired about the causes of such suffering. Our problems with relationships are generally related to having grown up in an abusive or dysfunctional family. If your parents were deeply loving people (being valuable friends and deeply sensitive to the needs of others), then chances are you will be able to seek a loving partner instead of a toxic partner simply because the difference between the two will be obvious to you. If your parents were unloving and emotionally abusive, then you may have very low self-esteem and/or be a very angry person. A stable loving relationship requires a great deal of interpersonal awareness, goodwill, insight, and genuine concern for the well-being of one another. Modern couple therapy can be a sophisticated approach to unraveling the complexities of a difficult relationship. I suggest that the concept of "attachment" in the context of this deeper and more scientific understanding of human relationships is a concept of little value when it comes to understanding the psychology of loving relationships between parent and child, between brothers and sisters, between friends, between lovers, and between husband and wife. It provides no insight and no remedy for the difficult times in a loving relationship of any kind. If you are in an emotionally painful relationship, you are better off finding a good psychotherapist or couple therapist rather than trying to apply the concept of attachment to a form of suffering that the Buddha never had to deal with because his students were all monks and nuns!!

The key to emotional relationship mastery is to have extremely high standards and have a practice partner.
I am currently in an intimate relationship with a girl and she is below those standards in terms of appearance, intelligence, and spirituality so I have no attachment and cannot form attachment but purpose the relationship to help each other out in life and also be loving companions.
I have explained to her that this is a temporary situation and I will be extremely happy if she ends up finding someone before we have to break up.
I believe it is good practice to be in this kind of less attached relationship because when you will find someone you really like--it will be harder to not attach. She is also good practice for me in terms of holding to my other tenets in spirituality and life.
The meditation parallel to this situation is doing sitting meditation until you are good at it and then working on doing active meditation when you have mastered sitting to some degree.
Coming back to the subject of relationships, when I find the "ideal partner": Buddhist, intelligent, fair, good career, healthy, etc.
I will be better practiced at managing the dynamics in the relationship (there is management involved in everything whether you like it or not) and also within myself. I would not easily give away as much of my power and purpose as one normally would when committed to their ideal person.
So I know it sounds trashy but I would suggest finding a compliant and good person as close to your ideal person and to have a as a practice partner and to keep improving yourself (and helping them to improve themselves). At the same time you are searching for your ideal partner so you can have a mature form of COMMITMENT not attachment. Search for someone that is worth it to commit to--not attach to.
I suggest reading some dating manuals which will add tremendous value to your social and dating understandings and dispel certain illusions: The Tao of Dating, Sperm Wars, Sex at Dawn and other non-fiction by PHDs.
Sexuality is coupled with many of life's laws which are present in spiritual practice--I won't go into it too much here and need to write a book one day because it is something that can very easily be taken the wrong way and throw apart one's spiritual practice.

Please feel free to correct me if I'm mistaken but that sounds very judgemental and egotistical what you said about this girl being below your standard. Why would you be with someone you felt that way about?  I don't believe that there is any perfect or ideal person. I believe that whoever comes across our path is the ideal person. You might think that she is below your standards spiritually and intelligence wise etc but maybe you're mistaken. Maybe your below hers. Maybe she is wise beyond your comprehension.

This is such horrendously awful advise. It's not trashy, it's just completely shallow and self absorbed. So you just go with someone and use them until you find this so called mythical ideal person who is more at your superior level then you just dump the other one. Yeah great advise!! Not.

This is the cutesy and romantic thinking that causes wives to stay with abusive husbands. It is a conditioned reality. See clearly.

It's hard to square this answer with Samajivina Sutta: Living in Tune ("Lord, ever since Nakula's mother as a young girl was brought to me [to be my wife] when I was just a young boy, I am not conscious of being unfaithful to her even in mind, much less in body..") It sounds more like negging OSLT ("You're inferior, below my standards, we'll have to break up when I find Ms. Right but I'll date you for now."). Maybe that's not what you're saying but it's how I read it,

You read it right and I expressed the same intent to her. She is expressedly happy and thankful enough (and many other women in their twenties who  have been alone for too long) to have a partner for a couple years or so who is interested in helping them become a better woman. And I a better man. It is better than 100 other scenarios including  FWB, being alone and pining, etc.


text:
Is fifth precept included in the Noble Eightfold Path?
In the following video this monk said that consuming alcohol is a latter addition to the fifth precept and hence not part of the Noble Eightfold Path. I wish to know your thoughts on this.
https://youtu.be/VI3dT4aXkfM?t=3163

Does the monk really say that it is "hence not part of the Noble Eightfold Path"?

yes. Why don't you listen to it? The video starts at this particular point.

I only listened from time 52:43 through 53:34 (talking about precepts), and stopped there when his next topic was going to be about the Vinaya.

It is in  counter 52.59. he said liqueur not alcohol.

He says, "So only 7 precepts. Is the liquor precept there? It's not there. So liquor precept was put in later. ... ... Three body precepts and 4 verbal precepts." No mention there of the eightfold path.

Precepts introduce latter does not mean it is not part of the Noble Eightfold Path.

Anyway, in the bit you quoted he doesn't say it's not part of the path. He says it's not one of the "arya precepts", and that breaking it only hurts you, not other people.

Noble Eightfold Path has two aspects of worldly and unworldly. Not only Ariya follows the NEP.

Are you saying that "the worldly [noble] eightfold path" is a synonym for the "the eight precepts"?

Without watching the video, I think it is a late addition.
It was noticeable when writing this answer that several of those core suttas (MN 9; DN 31; SN 55.7) identify four precepts.
It apparently wasn't among the first of the Vinaya rules either (see this origin story).
I do think alcohol is problematic though, in my experience: better to avoid it.

In regard to Vinaya, Buddha is the one who imposed this rule not to consume alcohol.

Yes but he imposed it on bhikkhus: which I guess means that the early/earliest Sangha existed even before he imposed this rule.

I agree. Buddha imposed rules as he goes. It does not mean late additions are less relevant to earlier ones.

Upasaka Sarath,
The custom of asking for precepts, and most explanations about them, and how to train, including all the factor things, are later developments: to sort out something easier to catch by many, even not having found the prerequisite of right resolve.
Instances where the Buddha counted all five are few. Mostly he would speak "only" about the first four, since those are the "real" outwardly actions. Yet he did also often point out the drawbacks of intoxication: it is intoxication which is actually the obstacle, the antidote of right resolve, and it has many layers. He therefore was in all ways eager to encourage on the subject of intoxication: and since (with intoxication) it is not possible to gain right resolve or to have right resolve, it is clear that intoxication is an obstacle for the eightfold path, the most gross: by taking physical food to maintain such.
As for the Silas, conduct that at least provides possible best relations and security in a community, they are not only given for gaining the highest goal, since the Buddha knew that very few beings would strive for it.
If seeking this virtue within the four precepts, one actually finds it in the 3rd: "Kamesu", later "Abrahmacariya".
One also does not find it directly in the main precepts of monks, which also display "only" the four. But again, intoxication, gross or subtle, is always the root of transgressing the basic four.
Look here, who much breaking of the precepts is done but intoxicating with becoming and being on BSe. The more people are addicted to it, use such a substance for relaxation, the more they tend to real worse thing.
At least intoxication makes crazy, and is the cause of an existence of being a fool and/or have crave mental lacks.
So for a virtuous lay-person, following the Buddha, wishing to attain the path within, not only at the end of ones live, having a secure dwelling and most secure future destination, indoxication, that causes heedlessness, lack of conscience, has to be avoided. It's part of the duties for lay people, is sad in the Dhammika ("Buddhist") - Sutta, and of course, a person having entered the stream whould not deliberatly indoxicate himself, in a way that it becomes root for breaking the precepts, would not give deliberatly signs of approve indoxications as well.
The linked Sutta should possible cut away the believe that the Buddha approved indoxitation for what ever aspiration within his Sasana, good wandering on, avoiding downfall or liberation, all paths in this world, following him.
And, being indoxicated, it is not possible to gain path or fruit, therefore, avoiding the indoxication with means which cause heedlessness, is part of the Noble Eightfold Path for having delivering effects.
Appamāda !
(Possible later edition on the answer, as well as space for further questions and discussion, is given consciously here.)
[Note: this is a gift of Dhamma and not meant for commercial use or other wordily gains, also not for getting indoxicated but to become sober and gain release.]


text:
is buddhism outside the idealism and materialism?
is buddhism is not certainly the idealism or materialisn?Buddhism philosophy is about the consequence mechanism about five elements* and those elements aren't viewed as the material,antimatter or spirit.But the motion and change of them is showing that Buddha or at least the nearest original of his book want to say that the motion and objective reality(material or time) is real,and it sounds like materialism.But the motion is come from,where?isn't it an conscious,thinking of mind construct from data,to create the motion,the time concept and feeling about it?Some time buddhism mention about mind,but doesn't it say the mind is a part of motion of elements?* fundamental elements (vietnamese: ngũ uẩn)

"...But the motion and change of them is showing that Buddha or at least the nearest original of his book want to say that the motion and objective reality(material or time) is real..." - please provide a reference to this statement and preferably to where question-body is copy-pasted from.

there's no copied pasted from any where,i had referred a Vietnamese word dumb

Could you add a reference to the book or text?

no that's my thinking stream,you have to research much to understand what i said.

Buddhism points out that both are concepts, reality is free of concepts (unless we impose them).
In my experience concepts like idealism etc. are deeply ingrained, unlike "my nose is itchy", so it's useful to remember they're just in my head.

Material(rupa) is ultimate reality in Buddhism.
The mind(citta) is an ultimate reality too. Cittas come one by one very fast and each lasts for an extremely short period of time. The more we are mindful, the more we can see these cittas. If we aren't mindful, like most people(unfortunately), we can't see the individual cittas that make up our stream of consciousness or our mind stream very well because they blur together.
It's not that "Conceptual Reality" is wrong, The entire Tipitaka  is conceptual. Among the many purposes of the Buddha's teaching, is making us understand  the difference  between "Ultimate  Reality" and "Conceptual  Reality" because we usually just blur the two realities together.

I hope I understood the question. Anyway, spiritual materialism is about how one treats the spiritual world the same way they treat the material world?

Yes without going to the extremes existence and non-existence or eternalism and nihilism Buddha taught the middle path which is Dependent Origination.


text:
Does Krishnamurti claim of No Path, No Progress, No Goal fit with Buddhist thought?
Quoted below is an article posted on BuddhaNet Magazine relating Krishnamurti's philosophy with Buddhism. The writer relates Krishnamurti's view that there is "no "path", no procedures, no organization, and no rules" with Buddhism referring to The Sutra of the Heart.
The writer claimed that this Sutra teaches that "There is ... no path, no wisdom, no attainment..." I haven't read this Sutra, but I have a hard time following anything which states that there is no "Wisdom"! If what the person says is true then it's wisdom and if false then there is wisdom!
Learning from other schools of thought, the critical response for anyone who says " listen to me there is no wisdom" is that what he/she says is meaningless because it fit perfectly with the liar paradox.
At the end of the article, the writer quoting to a Buddist teacher said that the Buddha "compromised" himself by teaching on the two different levels...  and that Krishnamurti's view is identical with the higher level of teaching.

....
To Krishnamurti there is no "path", no procedures, no organization, and no rules that should be laid down by men for other men to follow
  on the road to enlightenment. As part of the path, Buddhists must
  observe a very typical, man-made, structure which begins at the top
  with The Three Precious Ones: the Buddha, the Dharma, and the Sangha.
  Each of these pillars has subsets of rules associated with it: The
  Five Skandhas, The Eight Siddhis, etc. Some would have us believe that
  learning all these articles of faith are necessary for enlightenment.
Much Buddhist literature suggests that in following Buddhism there is
  a great object that one must attain and that one progresses towards
  this goal as one takes each step along the path. To Krishnamurti
  setting a psychological goal and working for progress in any direction
  will only lead to more confusion and suffering. Any attempts at
  psychological self-betterment will amount to no more than just one
  more futile duplication of many similar past efforts, all of which had
  previously failed.
The typical pattern of human behavior that we always seem to fall
  into, perhaps by virtue of conditioning, is the "work for a reward"
  stereotype. One finds a religion and sees something desirable in it
  which becomes an object of attainment. The next step is to devise a
  plan to acquire the object, and finally, with great deliberation we
  set about to carry out that plan with hard, unrelenting work.
Krishnamurti tells us that the "work for a reward" operandi has been
  tried countless times by homo sapiens, but it has never led us to
  anything new or different in the area of spiritual enlightenment. What
  do we make of all this? Buddhist leaders round the world tell us that
  there are Buddhist goals and a path of hard work and attainment for
  reaching these goals.
Here again Krishnamurti seems to be more in agreement with the very
  core of Buddhist teachings than the Buddhists themselves. The Sutra of
  the Heart of Transcendent Knowledge sounds more like Krishnamurti than
  does many of the Buddhist teachers: "There is ... no path, no wisdom,
  no attainment, and no nonattainment ..." Here Krishnamurti is telling
  us to live up to the precepts of this great Buddhist Sutra. He is not
  telling us to follow a path, but to under stand that there is no path.
  He tells this just as bluntly and simply as the Sutra does. There is
  no apparent sympathy or embellishments for the benefit of those who
  either fail to understand or for those who have beliefs in goals to
  which they must continue to cling.

http://www.buddhanet.net/khrisna.htm

It seems to me that the topic of the question isn't really Krishnamurti's claim, instead the topic is what BuddhaNet says about his claim. And, you're not even really interested in what BuddhaNet says about his claim, instead what you're questioning is what BuddhaNet says about the Heart Sutra. And, you say you haven't read the Heart Sutra. But I think what your question is asking is, "BuddhaNet says that the Heart Sutra says that 'there is no wisdom' ... but if the Heart Sutra says 'there is no wisdom; then surely the Heart Sutra can't be Buddhist?"

@ Christ thanks for summarizing my question, but I'm also interested to know how Krishnamurti's view is seen by Buddhists. if you think BuddhaNet writer is misquoting Krishnamurti or even more so misrepresenting Buddism I would be glad to hear you thought. IMO the first paragraph negating the triple gem is also disturbing, but the writer absorb this a higher teaching?

@Epic Heart Sutta isn't mean no path in initial state it is when there's no self, when one being pure after followed path and all things. Heart Suttra - "The Heart of the Perfection of Wisdom".And if Buddha's teaching is lower level then why he enlightened and not krisnamurti?  I don't think his philosophy is useful even if higher level cause Buddha spend his whole life with it. Thanks

See also Have any notable Buddhists commented on Krishnamurti's teachings? (there were two Krishnamurtis); I don't know whether Krishnamurti was or is considered (or considered himself) "Buddhist".

People have written about the Heart Sutra too (on Wikipedia and on this site).

Jiddu K.? No one understands Heart Sutra, HS has been mis-translated, nor the HS really addressing Avalokitesvara... The Buddha doesn't teach what called Middle Way - nevertheless this is the Theravadin's favourite etymology, this saying is like saying the Buddha taught in compromise - can truth be compromised?

The idea there is no path is itself a path, which obviously has a goal. If there was no goal, why did KM teach?
The Buddha's path, properly comprehended, is the abandonment of craving & letting go. This is similar to what KM was attempting to teach but, unlike KM, the Buddha comprehended there is actually a path (patipada) or method (gamini).

See also Why is the Buddha described as trackless? (also here). And the Brahmana Sutta (SN 51.15) can talk about desire having an "end", and thus the path having an end.

Brahmana Sutta is spoken by Ananda and best ignored, Regards