soul must be a substance in the sense of the form of a natural body having life potentially within it. But substance is actuality, and thus soul is the actuality of a body as above characterized.
This passage delivers Aristotle’s canonical hylomorphic definition of soul as the first actuality of an organized natural body, the locus classicus of the entire soul-body hylomorphism doctrine.
, On the Soul (De Anima), -350thesis