What is the biblical basis for the Dual Nature of Man as opposed to the Tripartite Nature? A common belief regarding the nature of mankind is that we have a body, a soul and a spirit. This is known as the Tripartite Nature of Man (or trichotomism).  Some, however, hold to only a Dual nature of man (also known as dichotomism).

What is the essential argument in favor of the Dual Nature of Man?  It would be interesting to see how the evidence for the Tripartite Nature of Man is explained as well, especially a verse like Hebrews 4:12.


  For the word of God is living and active, sharper than any two-edged sword, piercing to the division of soul and of spirit, of joints and of marrow, and discerning the thoughts and intentions of the heart.  Hebrews 4:12 ESV
Wayne Grudem's defense of dichotomism in his Systematic Theology, chapter 23 is particularly helpful in this regard.  He lays out several kinds of biblical evidence, cleanly organizing the arguments made by previous Reformed theologians.  His points are:


Scripture uses soul and spirit interchangeably


"Soul" departs and "spirit" departs
Man is "body and soul" as well as "body and spirit"
"Soul" can sin and "spirit" can sin

The actions of the soul are also the actions of the spirit, and vice versa


(All scripture cited is from the ESV unless otherwise noted)

Scripture uses soul and spirit interchangeably

Grudem points to a number of passages that he argues are examples of the interchangeable use of soul and spirit.  For example:


  John 12:27: "Now is my soul troubled"
  John 13:21: Jesus was troubled in his spirit


He sees Luke 1:46–47 as an example of Hebrew parallelism:


  "My soul magnifies the Lord, and my spirit rejoices in God my Savior"


And those who have died are described as both spirits (Hebrews 12:23, 1 Peter 3:19) and souls (Revelation 6:9, 20:4).

He makes a simmilar argument for three main areas: the departing of soul and spirit at death, the formula "body and soul/spirit," and how both soul and spirit can sin.

Departure at death

A number of passages use "soul" to describe what departs when man dies:


  Genesis 35:18: And as her soul was departing (for she was dying), she called his name Ben-oni
  1 Kings 17:21 (NKJV): “O Lord my God, I pray, let this child’s soul come back to him.”
  Isaiah 53:12: because he poured out his soul to death
  Luke 12:20: 'Fool! This night your soul is required of you’


But elsewhere, "spirit" is used similarly:


  Psalm 31:5: Into your hand I commit my spirit
  Ecclesiastes 12:7: the dust returns to the earth as it was, and the spirit returns to God who gave it.
  John 19:30: he bowed his head and gave up his spirit
  Acts 7:59: "Lord Jesus, receive my spirit."


In response to the argument that these passages refer to different entities, Grudem notes that the Bible never says that a person's "soul and spirit" departed; always one or the other.  Biblical writers don't seem concerned that one or the other might be left on earth with the body, suggesting that they are one and the same.

"Body and soul/spirit"

Grudem argues that biblical passages regularly refer to "body and soul" or "body and spirit" to describe the whole nature of man, not leaving out any parts.  For example, Matthew 10:28:


  And do not fear those who kill the body but cannot kill the soul. Rather fear him who can destroy both soul and body in hell.


The same thing occurs with spirit, for example, 1 Corinthians 5:5:


  you are to deliver this man to Satan for the destruction of the flesh, so that his spirit may be saved in the day of the Lord.


Other passages along these lines include James 2:26, 1 Corinthians 7:34, and 2 Corinthians 7:1.

Both soul and spirit can sin

The sinfulness of the spirit is often denied by trichonomists, but Grudem points to scriptural evidence for the sinfulness of both soul and spirit.  Regarding the soul, he points to 1 Peter 1:22 and Revelation 18:14, while passages indicating the sins of the spirit include 2 Corinthians 7:1:


  let us cleanse ourselves from every defilement of body and spirit


Similarly, Deuteronomy 2:30 refers to the hardened spirit of the king of Heshbon, and Psalm 78:8 refers to the faithlessness of the spirits of the people of Israel.  Other passages like Proverbs 16:2, Psalm 32:2, Psalm 51:10, and Proverbs 16:32 suggest that our spirits can be sinful.

Identical capabilities

Grudem finally argues that the soul and spirit are shown to have the same capabilities in the Bible.  Trichonomists attempt to draw distinctions between the spirit's actions (such as worship and prayer) and the soul's actions (such as thinking, feeling, and deciding), but this distinction is not maintained by Scripture.

Feeling is described of the spirit in John 13:21 and Acts 17:16, the latter of which says:


  Now while Paul was waiting for them at Athens, his spirit was provoked within him as he saw that the city was full of idols.


Thinking too is a function of the spirit in Mark 2:8, Romans 8:16, and especially 1 Corinthians 2:11:


  For who knows a person's thoughts except the spirit of that person, which is in him?


On the other side of things, the soul is often said to worship God.  Among many examples are Psalm 103:1, 146:1, and Luke 1:46, and the Bible teaches that souls pray and long for God (1 Samuel 1:15, Psalm 42:1–2, Psalm 119:20).

Grudem concludes:


  Scripture does not seem to support any distinction between soul and spirit.  There does not seem to be a satisfactory answer to the questions that we may address to a trichotomist, "What can the spirit do that the soul cannot do?  What can the soul do that the spirit cannot do?"


Counterarguments

Grudem addresses a variety of trichonomist arguments, including that of Hebrews 4:12.  Full explanation of his counterarguments is beyond the scope of this answer, but in brief:


1 Thessalonians 5:23, "may your whole spirit and soul and body be kept blameless": Grudem argues that this is best understood as a "piling up" of synonyms, similar to Matthew 22:37's heart, soul, and mind.  Treating that passage and Mark 12:30 the same way would seem to indicate five or six parts of man.
Hebrews 4:12, "piercing to the division of soul and of spirit": Soul is not separated from spirit here, says Grudem, but both are said to be penetrated and divided.  For more on this, including other approaches, see How do proponents of the dual nature of man address Hebrews 4:12, “dividing soul and spirit”?


Summary

Grudem admits that arguments for trichotomism have "some force" but finds that:


  the wide testimony of Scripture [shows] that the terms soul and spirit are frequently interchangeable and are in many cases synonymous.