What is the thesis of determinism? Though it is obvious that in principle there is more than one possible thesis that might be given this name, it seems to be the case that philosophers working on the free will problem have gradually gravitated towards a more-or-less standard definition, minor variations on which can now be found widely scattered through the free will literature. I call it the ‘entailment definition’ and it states, roughly, that determinism is the thesis that for any (...) given time, a complete statement of the facts about that time, together with a complete statement of the laws of nature, entails every truth as to what happens after that time. In this paper, I argue that acceptance of the entailment definition has been a mistake—and that we need a definition of determinism which, by contrast with the entailment definition, makes explicit mention of the notion of natural necessity. (shrink)