Three possibilities come to mind:
Is there an evolutionary purpose?
Does it arise as a consequence of our mental activities, a sort of side effect of our thinking?
Is it given a priori (something we have to think in order to think at all)?
EDIT: Thanks for all the responses! Just one thing I saw come up a few times I’d like to address: a lot of people are asking ‘Why assume this?’ The answer is: it’s purely rhetorical! That said, I’m happy with a well thought-out ‘I dispute the premiss’ answer.
Because it’s not an illusion.
Determinism seems reasonable only because people have an inaccurately simplistic conception of causation, such that they believe that consciousness and choice violate it, rather than being a part of it.
Causation isn’t a simple linear thing - it’s an enormously complex web in which any number of things can be causes and/or effects of any number of things.
Free will (properly understood) is just one part of that enormously complex web.
How is our experience of decision making different to one where we reach an inevitable outcome based on a complex set of parameters?
Because there are points at which, exactly as seems to be the case, we consciouly choose to follow one particular path in spite of the fact that we could just as easily have chosen another.
Even in that scenario, the “conscious choice” happened via some particular arrangement of neurons/chemical messengers/etc. Your argument is a “god of the gaps” argument- science doesn’t know everything about how the brain works, therefore some supernatural process called “free will” is the cause of the stuff science can’t explain yet.
(No knock on you, you’re having a good faith debate :)
I agree that it “seems to be the case” that we consciously choose, but I don’t understand where you found justification to state that there really are such points. How do you dismiss the idea that our conscious choice is not simply an application of the myriad parameters?