http://faculty.washington.edu/smcohen/320/psyche.htm
Scroll about 2/3 of the way down to "Degrees of Soul."
Aristotle says the soul is the "form" of the living body. By "form" he does not mean "shape," but "that which makes it what it is as a living thing." It approximately means the "shaper" of the body.
Aristotle was a natural philosopher, not a theologian. From a natural point of view, I suppose you could say the closest thing we've seen to a "soul" is the DNA molecule, since it is the "shaper" of the living body. This is not, of course, a distinctively religious view.
Here is a good article on Aristotle.
Aristotle also believed the sole died with the body.
http://www.nd.edu/Departments/Maritain/etext/aatcc12.htm