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 mean, if I said to you, ‘Calculate 13x16’ (or some other sum you don’t know off the top of your head) you could either do it or not do it. That would be a willed choice, whether or not you knew the answer.
My thoughts would be presented based the fact that you’ve asked me to calculate something. At that point, past experiences would guide my path forward. If I felt like doing math, I may do it, if I had poor childhood experiences in math class, I probably wouldn’t. At the end of the day, it’s all based on history or current questions/feelings. In every scenario my thoughts are presented to me. To prove it, ask yourself what your next thought will be. If you’re honest with yourself, you’ll see you can’t answer that question and when you try and force a thought direction, that direction itself is based on your knowledge from the past and that thought was also presented to you.
It’s wild because it absolutely feels like we have free will, but it sure doesn’t look like it. 🤷♂️
This is the problem of original intentionality. There are studies on it, for instance they found that with an mri they can detect when you have come to a decision before your conscious mind realizes you have. Some processes in our brain are outside of our control, because the brain is not just the neocortex but also includes tens of other structures that evolved separately with specific hard-coded purposes, but that doesn’t mean they are not working as a team. I think in any case you are still reaponsible for the decisions you take.