KEY!!! I.76.1

But if anyone says that the intellectual soul is not the form of the body he must first explain how it is that this action of understanding is the action of this particular man; for each one is conscious that it is himself who understands. Now anaction may be attributed to anyone in three ways, as is clear from the Philosopher (Phys. v, 1); for a thing is said to move oract, either by virtue of its whole self, for instance, as a physician heals; or by virtue of a part, as a man sees by his eye; or through an accidental quality, as when we say that something that is white builds, because it is accidental to the builder to be white. So when we say that Socrates or Plato understands, it is clear that this is not attributed to him accidentally; since it is ascribed to him as man, which is predicated of him essentially. We must therefore say either that Socrates understands by virtue of his whole self, as Plato maintained, holding that man is an intellectual soul; or that intelligence is a part ofSocrates. The first cannot stand, as was shown above (Question 75, Article 4), for this reason, that it is one and the sameman who is conscious both that he understands, and that he senses. But one cannot sense without a body: therefore the body must be some part of man. It follows therefore that the intellect by which Socrates understands is a part of Socrates, so that in some way it is united to the body of Socrates.