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.
What if you found out that free will is an inherently flawed concept and therefore impossible to conclusively obtain
I also cannot imagine finding out that the hole in my ass is bigger than I am.
Then I have some bad news for you, about calculating the interior volume of a cylinder. You’ve got a lot of hole coiled up inside you