|Josh, in Detroit|
Wayne Grudem, in his magisterial Systematic Theology, writes:
"How many parts are there to man? Everyone agrees that we have physical bodies. Most people (both Christians and non-Christians) sense that they also have an immaterial soul - a "soul" that will live on after their bodies die.
But here the agreement ends. Some people believe that in addition to "body" and "soul" we have a third part, a "spirit" that mostly relates to God. The view that man is made of three parts (body, soul, and spirit) is called trichotomy. Though this has been a common view in popular evangelical Bible teaching, there are few scholarly defenses of it today." (Grudem, op. cit., 472)
Grudem argues that the correct view is that a person is a dichotomy; viz., body and soul/spirit, because "soul" and "spirit" are used interchangeably to refer to the same thing.
Grudem's defense of persons as essentially dichotomies (rather than trichotomies) is lengthy. His main points are:
1. Scriptures uses "soul" and "spirit" interchangeably. To give one example, in John 12:27 Jesus says, "Now my soul is troubled." But in a similar context, Jesus was "troubled in spirit" (John 13:21).
2. At death, Scripture says either that the "soul" departs or the "spirit" departs. "Scripture nowhere says that a person's "soul and spirit" departed or went to heaven or were yielded up to God. If soul and spirit were separate and distinct things, we would expect that such language would be affirmed somewhere, if only to assure the reader that no essential part of the person is left behind." (Ib., 474) For example, when Rachel died, we read that her "soul" was departing (Gen. 35:18). David prayed, "Into your hands I commit my "spirit"" (Ps. 31:5).
3. Man is said to be either "body and soul" or "body and spirit." "Jesus tells us not to fear those who "kill the body but cannot kill the soul," but that we should rather "fear those who can destroy both soul and body in hell" (Matt. 10:28). Here the word "soul" clearly must refer to the part of the person that exists after death... On the other hand, many is sometimes said to be "body and spirit."" See 1 Cor. 5:5. "Similarly, James says that "the body apart from the spirit is dead" (James 2:26), but mentions nothing about a special soul." (Ib., 475)
4. The "soul" can sin or the "spirit" can sin. Trichotomists usually think of the "spirit" as purer than the "soul." But Paul encourages the Corinthians to cleanse themselves "from every defilement of body and spirit (2 Cor. 7:1). Thus, "he clearly implies that there can be defilement (or sin) in our spirits." (Ib.)
5. Everything that the soul is said to do, the spirit is also said to do, and everything that the spirit is said to do the soul is also said to do. Grudem spends two pages defending this.
To study more on this:
N. T. Wright maintains a distinction between "soul" and "spirit." See "Mind, Spirit, Soul, and Body." But note: neither is Wright a trichotomist. (See here, e.g.)
Here Wright talks of a "limited dualism" - "The Good Bishop Weighs In."
J. P. Moreland is a dichotomist/dualist. See The Soul: How We Know It's Real and Why it Matters.