I am somewhat confused over whether Aquinas can accurately be called a dualist. I had one professor who adamantly denied that he is, instead calling him a psychosomaticist. Another professor of mine called him a hylomorphic dualist. Are these simply two names for the same concept, or is there a real disagreement between my two professors?
I must admit that I have never heard Aquinas called a psychosomaticist. It makes him sound like a hypochondriac. The difference between your two professors probably just comes down to a matter of emphasis. Both terms seem to mean the same thing on a literal level: psuche (soul) + somatos (body) is the same as hule (matter) + morphe (form). For Aquinas and Aristotle, the soul is the form of the body which is its matter. Now this analysis applies to all living things, not just humans. Plants have souls which is the form of the bodies (matter). And in all living things except humans, a soul does not exist except that it be enforming some matter; souls exist through the composite substances of which they are a part. Moreover, all matter has some form, and what kind of body a thing is depends on the kind of soul it has. Thus, matter/form composites have an integral and substantial unity, which is probably what your one professor meant to convey by calling Aquinas a psychosomaticist.
But the human soul, because it has an activity in which the body does not share (i.e. intellection), exists of itself and not through the body. The body, however, receives its existence from the soul. Thus, the human soul can exist without the body (though this is not normal for it), but the human body cannot exist (as the kind of body it is) without a human soul. This latter asymetry is probably what your one professor has in mind when he says that Aquinas is a hylomophic dualist (since dualism carries the connotation of an opposition between mind and matter).
I prefer to say that Aquinas endorses Aristotle’s hylomorphism, since this is the traditional term and it avoids calling him a dualist (too many negative connotations). One has to add the caveat about intellection, though, so that one understands that not all psychic activities take place through the body, and that the human soul survives the death of the body. (For more on this topic, see the essay “Body and Soul” on this website.)