The concept of free will, as imagined by AI
There's a common argument that the laws of physics means that free will doesn't exist. That argument is complete nonsense.
Before addressing that argument directly, I'm going to discuss what free will actually is. Free will means that my actions are a consequence of my beliefs and my preferences. Simply put, it's the difference between walking through a door on your own and being pushed through a door by an external force.
I think that this is very clearly a useful distinction to make. There's definitely some gray area where it's not clear if an action is a result of free will or not (for example, when a person with OCD acts on a compulsion). But I think most actions pretty clearly fall into one of those two categories. And that's what I mean when I say that free will exists. It's a useful category.
Some people say that because the laws of physics are deterministic, free will doesn't exist. We are all just billiard balls bouncing around on paths that can't be deviated from. If you set up exactly the same starting condition, then exactly the same things will happen. In my view, this doesn't contradict the existence of free will. In fact, free will requires determinism. My beliefs and preferences are physical things. They're patterns in my brain. I don't know the details, but those physical parts of me interact with my nerves and muscles and other physical parts of me to cause me to act. If my actions were not determined by my physical self, well then, those actions wouldn't really be my choices.
(Tangentially, some people say that determinism contradicts free will, but free will exists because quantum physics isn't deterministic. First of all, whether or not quantum physics is truly non-deterministic is still up for debate. But more importantly, that doesn't actually support free will. If my actions are a result of random chance, then they're not a result of my beliefs and preferences.)
An argument for why determinism negates free will is that of counterfactuals. That is, the idea that you could have acted differently given the same conditions. I agree that, if determinism is true, given exactly the same initial conditions, a person will make exactly the same decisions. But I don't think that's really what people mean when they say you could have acted differently. The thing is, a person's beliefs and preferences change all the time, without changing who they are. For example, if I'm hungry, I want to eat. Then I eat, I'm no longer hungry and I don't want to eat anymore. My preference has changed, but I'm still the same person. So when I say that I could have acted differently given the same conditions, I mean the same external conditions, but my internal state could vary slightly.
There's also an argument that even if your actions are caused by your beliefs and preferences, those actions still aren't a result of free will, because you didn't choose to have those beliefs and preferences. My response is that that doesn't matter. My beliefs and preferences are mine, regardless of where they came from. The molecules that are now in my arm came from food I ate. Those molecules weren't part of my body before but now they are. Likewise, my beliefs and preferences have been influenced by things outside of me, but they're still a part of me now. And it seems to me there's a worthwhile distinction to be made between someone pushing me through a door, and someone convincing me to walk through a door.
I think the real reason people think determinism prohibits free will is an implicit (or explicit) belief in a non-physcial self. The laws of physics as we understand them don't permit physical things to be influenced by non-physical things. If you think your beliefs, your preferences, your self exists as an incorporeal soul, then yeah, physical determinism means that incorporeal soul cannot be responsible for your corporeal actions. My response to that is simply that there is no such non-physical soul. We are physical beings, through and through. Our bodies, our brains, our minds, our thoughts, our feelings, are all physical things, and they are not in any way lessened by that.
Now, a discussion about free will isn't complete without talking morality, if only because everyone else always talks about how the two are related. People say, for example, that free will is required to pass moral judgements, or it's immoral to punish someone if they don't have free will. My thoughts about morality aren't entirely settled, but I'll say this much. If a river floods and destroys some buildings, and then the people who lived there build a levee to reduce the risk of future flooding, that doesn't require those people to make a moral judgement of the river, nor is it a punishment on the river. It's just people acting to prevent future harm. If a person has an epileptic seizure while driving a car, we take away their driver's license. That's not a moral judgement, we don't think a person with epilepsy is evil. It's just acting to prevent future harm. If a person murders another person, we want to act to prevent future harm. Knowing the cause of that killer's behavior is important in figuring out what the best way to prevent future harm is.
Furthermore, if a person acts with free will, then that person can respond to incentives and be negotiated with. You can say, "If you kill someone, then we'll imprison you" and if that person believes you and their preference for not being imprisoned outweighs their preference to kill someone, then they'll act on their beliefs and preferences and not kill someone. There's a lot more that can be said about that, but that deserves it's own post.
Just as my beliefs and preferences have been influenced by things outside of me, my thoughts about free will have been influenced by reading things other people have written about it. The most notable is Eliezer Yudkowsky. The Stanford Encyclopedia of Philosophy is also great if you want a detailed history of every argument about free will.