How do proponents of the dual nature of man address Hebrews 4:12, "dividing soul and spirit"? Those holding to a tripartite nature of man, or trichotomism, sometimes point to Hebrews 4:12 as evidence for their view that soul and spirit are distinct in man:


  For the word of God is alive and active. Sharper than any double-edged sword, it penetrates even to dividing soul and spirit, joints and marrow; it judges the thoughts and attitudes of the heart. (NIV)


How do proponents of a dual nature of man (dichotomism), believing that the soul and spirit are two words referring to the same entity, understand this passage?
Proponents of dichotomism argue in two primary ways:

That the division is between soul and spirit, but that these are simply two aspects of the same thing
That the division occurs within the soul, and within the spirit, not between them

Between soul and spirit
In his remarks on this passage (Commentary on Hebrews, §222), Thomas Aquinas affirms the dichotomism taught by the church, but nonetheless does not see soul and spirit as synonyms here:

the soul is that to which pertain the powers by which the soul acts in conjunction with the body; whereas the spirit is that to which pertain the powers by which it acts without the body

He connects this understanding the passage as follows:

The Word of God effects and distinguishes between all those divisions and species [...] [The phrase] can be explained according to a Gloss in two ways: so that the soul refers to carnal sins which involve bodily pleasures, such as lust and gluttony; but the spirit refers to spiritual sins, which involve an act of the mind, such as pride, vain glory, and the like. Or by soul is understood evil thoughts, and by spirit good thoughts. Then the sense is this: reaching, i.e., discerning, unto the division of the soul and the spirit, i.e., between carnal and spiritual sins, or between good and evil thoughts.

Within soul and spirit
Other commentators argue that the penetration of the Word of God occurs within both soul and spirit, not to separate them from each other.  John Calvin writes:

The word soul means often the same with spirit; but when they occur together, the first includes all the affections, and the second means what they call the intellectual faculty. [...] Now, to come to the passage before us, it is said that God’s word pierces, or reaches to the dividing of soul and spirit, that is, it examines the whole soul of man; for it searches his thoughts and scrutinizes his will with all its desires. (Commentary on Hebrews)

Berkhof argues similarly (ST, 2.1.2.A.2), and Wayne Grudem sees the words as essentially synonomous:

The author is not saying that the Word of God can divide &quot;soul from spirit,&quot; but he is using a number of terms [...] that speak of the deep inward parts of our being that are not hidden from the penetrating power of the Word of God.  If we wish to call these our &quot;soul,&quot; then Scripture pierces into the midst of it and divides it and discovers its inmost intentions.  If we wish to call this inmost nonphysical side of our being our &quot;spirit,&quot; then Scripture penetrates into the midst of it and divides it and knows its deepest intentions and thoughts. [...] In any case, soul and spirit are not thought of as separate parts; they are simply additional terms for our inmost being. (ST, 479)