Soul: Whatever a living body has that its corpse lacks.
Answer 1: this is some complexity or arrangement of parts, appropriately energized. The proof is that if we put together the right stuff in the right way and hooked up to the right energy source, it will, ipso facto, be alive.
Problem: some things only arise from a complexity of arrangement because they cause it. A melody only arises from the music box because it was a principle that determined how the music box was built in the first place; webs only catch fly-sized things because they were spun with > fly gaps.
When Aristotle says “if the eye were an animal, vision would be its soul” he means to speak about vision in the same way that melody is responsible for the way the music box is built. Notice we’re not talking about selection of the eye but its embryogenesis. Selection involves a lucky fit with an environment: embryogenesis does not.
If this is what soul is, then we come to understand the soul of things by looking to what operation sets them apart from other things. Man’s soul is said to be rational so far as, unlike either animals or machines, he deliberates over means and relates to objects as true.