Although any opposition is necessarily presented as between two components, this does not necessarily imply that man is only composed of two parts: body and spirit. Tradition unanimously affirms that man is in fact composed of three ‘elements’: body, soul, and spirit.
The tendency to forget this threefold division is due to two things: first, the fact that in the New Testament, particularly in the writings of St. Paul, the emphasis is on the opposition, which presents itself as two contraries even though this is not really the case. Second, in the modern mind the distinction between the first two terms—soul and spirit—is completely lost, especially after the rise of the Cartesian mindset where all is either spirit or matter. Soul and spirit are, for most Christians, vaguely synonymous, and when they come to teachings about how the word of God ‘penetrates even to dividing soul and spirit,’ they nod and move forward without ever knowing what it is they just read. Elsewhere in this manual, where we discussed the ‘tripartite constitution of man’, we carefully differentiated the two terms at length, and here we can simply recall that the spirit is the ‘divine seed’ in man, the Person within the person, immortal and unchangeable. The soul, on the other hand, is of the created order and is wedded to the body, and so in this context it is reasonable to group them together as ‘body-and-soul,’ one being the form and the other the matter and joining together to make the individual being possible.
 Hebrews 4:12.