Why is the human mind able to perceive and understand the truth about reality; that is, why does it seem to be the mind's specific function to know the world? Sean Kelsey argues that both the question itself and the way Aristotle answers it are key to understanding his work De Anima, a systematic philosophical account of the soul and its powers. In this original reading of a familiar but highly compressed text, Kelsey shows how this question underpins Aristotle's inquiry into the nature of soul, sensibility, and intelligence. He argues that, for Aristotle, the reason why it is in human nature to know beings is that 'the soul in a way is all beings'. This new perspective on the De Anima throws fresh and interesting light on familiar Aristotelian doctrines: for example, that sensibility is a kind of ratio (logos), or that the intellect is simple, separate, and unmixed.