Well, it isn’t provable - in the same category as the a priori proposition that our entire experience of everything is actually a dream. Like determinism, it’s logically possible, and impossible to disprove. But the evidence doesn’t really support either ‘dream’ or ‘determinism’ for me.
1. Free will is affirmed by the human experience, across all cultures (phenomenality of agency) - Human behaviour and descriptions of our experience affirm that we experience (have introspective evidence of) making choices and changing our mind. It is evidenced by the preponderant existence of human phenomema directed directly at influencing the exercise of choice and responsibility for using it: rules, teaching, punishment, morality, ethics, justice, arguments, blame, plans… None of these ‘determine’ because all can be ignored or followed, which further affirms free will. We conceive a plan, we implement, we change our plans. We regret decisions - we change our behaviour. We teach our children responsibility. All this is pointless if we can only make one choice. (For occurrences of human belief in determinism, see ‘contradiction of behaviour’ below number 8.)
2. Affirmed by rational process: Effective agency is presupposed by all scientific inquiry (rational enquiry) and so cannot rationally be doubted. (I didn’t make that up.)
3. It is confirmed by neuroscience. The famous 1961 experiment that was used to disprove free will to many - affirms that some decisions are made automatically, giving the illusion of choice. But this experiment only included System 1 thinking, quick automated responses, not System 2 thinking which is considered. Moreover, we now also affirm that system 1 thinking is based on complex processing and is conscious (eg, Thinking Fast and Slow, Heineman, Nobel Prize ). Interestingly, System 1 thinking - useful for efficient, quick responses - begins as system 2 thinking until automated or intuited.
4. Free will is a necessary condition for moral responsibility. If we can ‘only make one choice’ then we are not responsible for our choices. This is a quite solid piece in philosophy. Moral responsibility is distinguished from causal responsibility. Making judgments about whether a person is morally responsible for her behaviour, and holding others and ourselves responsible for actions and the consequences of actions, is a fundamental and familiar part of our moral practices and our interpersonal relationships.
5. Affirmed by all legal systems - All legal systems are based on personal accountability for decisions/actions. ‘The universe made me do it’ is not deemed an adequate defence.
6. Weakness of deterministic arguments - Almost all the arguments against free will are based on an a priori assumption that free will is incompatible with causal determinism. The assumption is used to dismiss evidence in favour of the assumption. It can’t be true, otherwise my presupposition would be wrong. The two major arguments, Consequence and Origination, are circular reasoning. Here’s a classic example of Origination -
1. An agent acts with free will only if she is the originator (or ultimate source) of her actions.
2. If determinism is true, then everything any agent does is ultimately caused by events and circumstances outside her control.
3. If everything an agent does is ultimately caused by events and circumstances beyond her control, then the agent is not the originator (or ultimate source) of her actions.
4. Therefore, if determinism is true, then no agent is the originator (or ultimate source) of her actions.
5. Therefore, if determinism is true, no agent has free will.
The logic is true, but it has a gaping hole in step 3. “Ultimately caused” inserts an unproven assumption. It might equally read, ‘If something an agent does is only influenced (rather than ‘caused’) by events and circumstances, then the agent is the originator (or ultimate source) of her actions.’ Try using this as a defence in court: ‘My wife started it and made me cross so when I killed her she is the ultimate source of my action.’ Yeah, right.
7. It is affirmed by consciousness and phenomenality: Determinism makes us observers, not actors. This means there is no personal responsibility for action, as the action was inevitable.
8. People who believe in determinism contradict it in their behaviour: ‘Blame’ is evidence that you believe the person had a choice. This means that if you get angry about someone’s action and hold them responsible, or demand justice or are disappointed that you don’t receive justice - your actions demonstrate that the way you live in the world contradicts to your stated belief. Drunk people at parties insist that there is no free will, no morality and everything is determined. Until you pour your drink all over them. Enter contradiction, stage left.
So - you can argue against all of these to hold your presupposition true. I could even make those arguments for you. But you as a person will still act and plan and decide, and get cross with people as if they are responsible for their actions rather than that they, ‘could only make one choice.’ Enter contradiction, stage left.