Aristotle's soul is a first principle (an 'efficient cause') of every vital change in an animal, in the way that a craft is a cause of its product's coming-to-be. We argue that the soul's causal efficacy cannot therefore be reduced to the formal constitution of vital phenomena, or to discrete interventions into independently constituted processes, but involves the exercise of vital powers. This reading does better justice to Aristotle's conception of craft as a rational productive disposition; and it captures the soul's continuous causal role as that which brings about all forms of vital change and underwrites their unity.