 There’s lots of room for argument about how, exactly, we should understand our commonsense beliefs about ourselves as persons with free will. (Are we born with free will? If not, when do we acquire it, and in virtue of what abilities or powers do we have it? What is the difference between acting intentionally and acting with free will?) Luckily we don’t have to answer these questions in order to say what is at issue between the compatibilist and the incompatibilist. Let’s define the free will thesis as the thesis that at least one non-godlike (human-like) creature has free will, leaving it open what that amounts to. The free will thesis is a minimal claim about free will; it would be true if one person in the universe acted with free will (acted freely, acted while possessing free will) on one occasion. We won’t assume that the free will thesis is true or even possibly true, but let a free will world be any possible world where the free will thesis is true. Since non-determinism is the negation of determinism, and since determinism is a contingent thesis, we can divide the set of possible worlds into two non-overlapping subsets: deterministic worlds and non-deterministic worlds.