According to Descartes, man is essentially a thinking thing. It follows, then, that the soul, the thinking thing, is one substance and the body, which is not the thinking thing, is another separate substance. As two distinct substances which are ultimately independent of each other, the immaterial soul interacts with the material body in order to perform the operations proper to man. But how?
(Que the centuries long debate over the mind/body problem.)
So now we've arrived at a moment where the most popular answer is to deny the existence of an immaterial soul. The mind/body problem is thus solved by denying the 'mind' (or soul) part. We now do not have to wonder how the soul and body interact because there is now only one thing.
There is a sense in which this is close to the truth and a sense in which it is monstrously off the mark. Inasmuch as the materialist wants to conceive of the human being as essentially one thing, he maintains an accurate description of reality. However, inasmuch as he wants to conceive of the human being as essentially material, his description of reality is grossly mistaken.
We can say the same for Descartes, who is right to recognize that man has an immaterial soul but is incorrect to understand this soul as a distinct substance. So, we can maintain that a man constitutes a complete substance, not his soul individually or his body individually. We also maintain that the body, or the material, requires an immaterial soul in order to actually be a human being.
Thus we are left with one final question: Who affirms that man is a unity of matter (body) and form (soul)?