I think there's two conceptions of free will, one is as you described -- a body being able to act in and of itself without any influence of an external force (b

I think there's two conceptions of free will, one is as you described -- a body being able to act in and of itself without any influence of an external force (beyond sensation, physical constraints, etc). That sort of punts on the question of physical determinism, where it doesn't matter if it was possible for you to have done anything other than what you did. You could be a pure clock work automaton making deterministic responses to external stimuli and still have that sort of "free will", but I don't think that would be satisfying to a lot of people. The second kind of free will, I think would be the ability to have a free choice -- that there is some immaterial entity which can act in somehow a non-deterministic way to choose its behavior, that the course of your entire life wasn't set at the big bang -- that not only can you act independently, but that your choices _matter_ and will result in a future that was not predetermined. I think that is what a lot of people would think of as free will, and it's difficult to define or logically support IMO.