 As with any substantial form, the primary function of a soul is to account for the substance’s nature as a certain kind of thing, possessed of the unity and persistence that characterizes substances. This actualizing role is what Aquinas calls the soul’s essence (ST 1a 77.1c). The ongoing existence of living things also requires that they carry out distinctive operations: taking nourishment and reproducing, moving and perceiving (in the case of animals), and reasoning (in the human case). Corresponding to these operations are the powers of the soul. Medieval authors had long puzzled over the relationship between the soul and its powers. Aquinas takes a strong position that would be the subject of much later dispute: that the soul’s powers, being potentialities, are really distinct from the soul’s essence, which is an actuality (ST 1a 77, Quest. on the Soul 12).