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 personally think the debate over the existence of Free Will is simply an extraordinary debate over semantics.
If you look at a human being from its basic biological and cellular makeup, a human being is a walking bundle of competing desires that appears to present itself as a single cohesive corporate entity.
The people who are against the concept of free will say that because you have innate desires for food sex and entertainment, that you have no choice to not act upon those in a desires and therefore any delusion that you carry about the choices that you make being done of an entirely unencumbered and Free Will are false.
Then there are people who say that Free Will doesn’t exist for religious purposes, that God is an all-knowing creature who knows the beginning and the end and everything in between and so you cannot make a choice that he or she or it does not already know that you will make and therefore your choices are not free.
The people who say Free Will does exist on a biological level will point to people who choose to self-immolate or to starve themselves to death in protest of a spiritual or psychological issue, valuing the ideals that life has imprinted upon them over the biological necessities of continuing to live.
The people who say Free Will does exist on a spiritual level say many things, such as we carry a spark of the Divine in us and therefore we are as little gods ourselves, capable of creating and destroying in roughly the same proportional magnitude as the greater gods above are, or they say that since we have the ability to make choices and we are judged by those choices than our choices must be free otherwise judgment is meaningless.
I personally tend to lean into the Free Will side, while understanding simultaneously that sometimes there are exigencies that induce us to choose one option over another on a more likely than not basis, or to phrase it another way, our will is as free as we choose it to be.
I think you’ve mischaracterized the arguments of the “no free will” camp, or at least omitted a major argument. You may find this interesting https://youtu.be/eELfSwqJNKU
That’s not the argument against free will. The argument is just that there’s a physical process to every thought in your head. When you think of a tree, inside your brain a specific pattern of neurons and chemical messengers activate which is what creates the thought of a tree.
When you’re consciously deciding whether to eat a donut or a salad, a specific pattern of neurons and chemical messengers are the mechanism by which that decision process is occurring. The pattern of neurons and chemical messengers happening in your brain is the physical mechanism that is performing the decision making process.
There are no thoughts outside of the ones generated by your neurons and chemical messengers. The pattern of neurons and chemical messengers IS the thought that you’re thinking. Your brain (and the thoughts that occur within it) is a physical object that obeys the laws of nature, the same as all physical objects do.