This paper studies the distinction between the essence of the soul and its powers in Thomas Aquinas as it develops from a theological to a psychological claim. A reconstruction of this development will show that the distinction is initially foreign to Aristotle’s philosophical psychology and leads Aquinas to break away from Aristotle’s conception of form. Ultimately, the distinction is postulated for its theoretical benefit, as it allows Aquinas to explain how the intellect can be a non-bodily power of a bodily form – something other authors of the Aristotelian tradition can difficultly account for.