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.
I forget which philosopher said this but he said something along the lines of if you have the desire and the capacity for an action you do, then deterministic or not, you chose that action. If the tide pulls me where I was already swimming, I still chose to swim there, even if some other force took me half of the way.
But where does your desire and capacity to do that thing come from? It arises from the physical arrangement of neurons/hormones/etc. in your brain and body
Why are you separating “you” from the choices your physical self is making?
I’m not. Did you reply to the correct comment?