A study of human nature involves, first and foremost, a study of the human soul. The fact that we have a soul is not even a point of controversy, given the way Aquinas defines his terms. What is controversial is the nature of soul. The first and perhaps hardest article of the Treatise asks whether the soul is a body. Aquinas answers in the negative, but this does not rule out the soul's being something material, in our modern sense (§1.1). Aquinas is concerned with refuting the ancient natural philosophers, who thought that all things were bodies (§1.2). In opposition to their reductive account, Aquinas insists on the explanatory priority of actuality (§1.3). But his dispute with the ancients in fact rests on a deep metaphysical disagreement about the nature of matter, a disagreement that points toward the reductive nature of Aquinas's own account (§1.4).

What is a human being?

Aristotle remarks in Metaphysics VII 17 that the question What is a human being? is inherently obscure because it doesn't give us any help in breaking down the problem.