Most people think that free will is threatened by determinism. However, it really isn’t determinism in the physical sense that really drives the worry. Any account of the production of one’s choice will seem incompatible with freedom. Freedom — real choice — requires mystery. It requires ignorance.
This can be seen by cooking up any scenario you like for how a decision is made. It needn’t be deterministic; we can include chance processes. But once you have a story about how that decision was made, you won’t see your self in that process, and so you’ll think that it wasn’t you “making the choice.”
Arguably this will be true even if the decision process is non-reductive, and appeals to high-level features like the character traits or beliefs of a particular individual. It’s still either a mechanism or it’s chance. There’s no third option that we can make sense of.
We can also see this by considering the existence of a book that specifies all your future actions. Can you be “really free” if all your actions are already written in that book? Note that this does not require any appeal to determinism. The mere existence of a fact — the lack of ontological mystery — is enough to trigger our intuitions that we don’t have a real choice.
On this point, it’s worth considering what it would be like to be a person reading the book of one’s own life. If we take it to be an *accurate* account of your future life, in what sense could you still *decide* what you are going to do? It seems that decisions require ignorance. If you know what you’re going to do, there is no decision to be made. There is no choice.
And that is where determinism comes in. If you knew what you were determined to do, you could not engage in the psychological process of decision making. (Although perhaps you suspend your knowledge sufficiently to go through the motions of decision making — pass through the mental process even though you know the outcome, like counting on your fingers to find the sum of 2 and 3.)
So knowledge and choice are fundamentally incompatible. And that’s the *real* issue at heart of the free will debate.