5 ideas
The soul is not a separate substance lodged in the body but the form, or first actuality, of a natural body that has life potentially. Asking whether soul and body are one is as pointless as asking whether the wax and the shape stamped in it are one: the soul is what the body is organized to do.
Living things are classified by their capacities, and the later ones presuppose the earlier: plants have nutrition and reproduction, and animals add perception. Since whatever perceives feels pleasure and pain, every perceiving animal also has desire, while only some animals also move themselves from place to place. Humans add intellect, and the series is nested the way a square contains a triangle.
In perceiving, the sense organ takes on the form of the object without taking on its matter, as wax takes the imprint of a signet ring without the gold. The sense becomes like the object in actuality while remaining physically distinct from it.
A capacity can be possessed but idle (first actuality, like knowledge held by a sleeping expert) or actively exercised (second actuality, like contemplating that knowledge). The soul is the first kind, which explains why a sleeping creature is still fully alive and ensouled.
Intellect has no nature of its own except the capacity to receive intelligible forms, so in thinking it becomes identical with what it thinks. This is why it must be unmixed with body. An active intellect is needed as well, which makes forms thinkable in the way light makes colors visible.