There's a tension in the philosophical quest for the truth of free will versus determinism—which is explained (suspiciously) at both Wikipedia and the Internet encyclopedia of Philosophy in identical words, namely
the thesis that the course of the future is entirely determined by the conjunction of the past and the laws of nature.
The problem is just this: suppose I hospitably offer you tea or coffee.
After an instant's consideration you select one.
Under the presumption of causal determinism, you couldn't have made the other choice; the operations of your mind are completely the outcome of the chemistry of your brain operating on the structures created by your entire history of living.
You have, we suppose, some sense that you could have chosen the other, but you didn't. That sense, it would seem, is an illusion. You couldn't have.
About this there are three broad positions. Somehow the word "Incompatibilism" has been attached to the view that free will causal determinism is correct and that therefore the sensation of free will is an illusion.
The word "Compatibilism" labels the view that even though causal determinism is probably true, there's some sense in which free will exists. Mainly Compatibilists say the issue is not whether your choice between tea and coffee was determined by your brain or not. What matters is that it was determined by you—that is, it was not compelled by external forces, e.g. by my saying, I'd really rather you chose the tea.
Me, I'm with the Incompatibilists in so far as agreeing that your choice (my choices, anyone's choices) are determined by history and the operation of natural law. But I also say, it doesn't matter.
That's because of an aspect of determinism that is so often forgotten or neglected especially in casual arguments. To quote the IEP,
A system's being determined is different from its being predictable. It is possible for determinism to be true and for no one to be able to predict the future.
This is especially true given the unimpeachable arguments of Chaos Theory showing that simple mathematical functions which are implicit in many natural phenomena are "extraordinarily sensitive to initial conditions," such that their output is fundamentally unpredictable. The IEP blithely says
If determinism were true, then a being with a complete knowledge of P and L and with sufficient intellective capacities should be able to infallibly predict the way that the future will turn out.
but Chaos Theory says it is not possible even in principle for such a perfect observer to exist.
My point (and I'm sure you're glad to reach it) is just this: when the outcome of a deterministic process cannot be predicted, it is functionally identical to the same outcome arising from a nondeterministic process.
The process by which you reach a non-coerced choice between coffee and tea may be (I think probably is) fully deterministic—but absolutely unpredictable by any observer, even including you. Until you announce the choice, I or anyone who cares about the result has no choice but to wait to see what it will be. Was it fully determined by history? Doesn't matter! If you can't predict it, its deterministic nature has no functional or practical meaning.
Do you or I have free will? Probably not; but there is absolutely no choice but for all of us to continue acting as if we did.
