According to Aristotle, the three main varieties of soul – nutritive, perceptual, and rational – are hierarchically ordered. I develop and defend an interpretation of the soul’s unity that centers on Aristotle’s attempt to explain this hierarchy’s organizing cause. Aristotle draws an analogy between this series of souls and the series of figures. I first elucidate the fundamental feature both series share: each series’ prior members are present in capacity in its posterior members. I do so by examining several other cases – mathematical, biological, and physical – where Aristotle appeals to presence in capacity. I then argue that an organism’s living body is continuous by nature. That is, an organism’s soul is the principle, cause, and end of a single, articulate activity of living and each of an organism’s vital bodily movements are aspects or partial manifestations of this unitary, natural activity. This account of natural continuity is, I contend, the key to understanding what it is for one soul to be present in capacity in another. And this account of presence in capacity is, I contend, the key to understanding what it is for a soul that comprises parts to be a unity.