In order to understand the philosophy of mind of Thomas Aquinas, we must first understand the revolution which our understanding of the metaphysical relation of the mind to the body underwent with the rise of Cartesian dualism. Before Descartes expounded his well-known dualism, the Thomistic-Aristotelian understanding of the human person had been reigning supreme for hundreds of years. For Aristotle, as for Aquinas the "mind" did not refer to the sum total of our consciousness, as it would for Descartes. The mind, instead, referred merely to the rational component of the intellect.
To be sure, for both thinkers, possession of a mind was what made humans distinct from animals. But for Aristotle and Aquinas, since the mind referred only to the intellect, this only meant that animals lacked rationality. They still, nonetheless, possessed sensation and perception. For Descartes, on the other hand, for whom the human mind was the sum total of all of our consciousness, to lack a mind meant to lack any consciousness whatsoever. Therefore, animals were like mechanical robots.
It follows, furthermore, that Descartes' understanding of the metaphysical relation of the mind to the body did violence to an understanding of the human person as an embodied being. Sensation and perception became only contingently related to the body. They were, ultimately, pure intellectual experiences.
But for the Aristotelian, the mind does not consist only of the intellect. It also consists of a will. To be human, for the Aristotelian and the Thomist, is to possess both a mind and a will. It is this capacitiy for symbolic, linguistic, rational thought, as well as the possession of a will, which constitute the distinctly human person.
Kenny, Anthony (1993). Aquinas on Mind. Routledge Publishing, New York, New York.