The third view, advanced by Aristotle, attempted to correct what he thought to be the errors in the two extreme views. In his conception of human nature, man is neither just a body or a collocation of atoms nor a union of two quite distinct and separable substances, one material and the other spiritual – one a body and the other a rational soul or mind.
In Aristotle’s view, man is a single substance and, in that respect, is like every other individual thing in the physical cosmos. However, unlike every other corporeal substance, man, as a single substance, is composite of matter and spirit, of material and immaterial aspects – the immaterial aspect consisting in the intellectual power that distinguishes man from other animals.
According to this third view, man is neither entirely a material thing, composed of elementary particles of matter or quanta of energy, nor is he compounded of two substances as alien to one another as body and soul or matter and mind. He is a living organism like any other animal, but he is distinguished from all other animals by virtue of having a mind or intellect – the powers and operation of which cannot be explained by the action of the brain.
Brain action is a necessary, but not a sufficient, condition for the occurrence of mental operations or processes. There is, in short, something immaterial about man, something spiritual in the sense that it is not reducible to bodily parts or movements and not explicable entirely by reference to them.
Mortimer J. Adler – The Angels and Us p.148