This article stresses the main lines of Thomas Aquinas's philosophy on the nature of the body-soul union. Following Aristotle, Aquinas sees the soul as a 'principle of life' which is intimately bound to a body. Together they form a non-contingent composition. In addition, the distinctive feature of the human soul is rationality, which implies that a human needs a mind to be what it is. However, this is not to say, as Descartes proposes, that the reason that I am a human is that I am fully self-conscious. On the contrary, I will show that self-consciousness is not necessarily a key to defining a human being. To that aim, and based on Aquinas's views, I draw a distinction between what I will call 'egos' and selves'.