tl;dr: There is no free will. But this is not a bad thing. It renders retribution meaningless and thus frees up energy to focus on what we can change in the future.
Epistemic state: I've heard about the topic from various sources, but only Sam Harris made me really think about it. I'm still confused about how relevant the illusion actually is in everyday life and what its relation to the Self is, but at least the latter is probably a topic for another post.
Have you ever thought of yourself as consciously and freely choosing something? For example, if you are able to raise your hand, you could just choose to do so in this moment, right? It seems as if you could influence the choice as a "free chooser", independent from any prior cause. Sure, many choices are restricted in various kinds of ways, but at least if you are unrestricted by any external factors, there seems to be at least some influence from such a "free chooser". I'll take this as the definition: Free will is the ability to influence, without prior causes, a choice between multiple options. (There is another meaning of the term, centering around constraints on choices, that I'll hopefully talk about some other time.)
The problem is, though, that such an influence does not exist. Last week, when I wrote about determinism, I stated
Everything of interest in our everyday experience, even things too small to see with the naked eye, consists of enough elementary particles to behave almost deterministically.
This also holds true for you. Your functioning is basically deterministic. Your reactions are almost exclusively dependent on your current state and your current perceptions. That is, there is no point in time at which we could "freely" choose between options. If we function completely deterministically, then free will can't exist, because there can't be a point in time at which multiple futures are possible. The conclusion must be: There is no free will. In other words, we can't freely select the next action we take, but our actions are determined by whatever happened before, in the very same way that 1+1 can't be anything else than 2.
I first want to spend a paragraph on the word "almost" in the quotation above. I was referring to quantum randomness. This randomness could possibly have some influence on our actions, such that our behavior isn't actually completely deterministic. The crux is though that quantum randomness can't bring any new information into the universe that could be used to transport freely made choices. If there was some pattern in the randomness, it wouldn't be totally random anymore and physicists would be able to detect that. So, quantum randomness could make our behavior a bit less deterministic, but it still leaves absolutely no space for free will. It's also worth mentioning that the departure of our brain's thinking processes from perfect determinism is probably negligibly small, such that a deterministic view is an extremely good approximation. For these reasons, I will ignore quantum randomness for the rest of the post.
Last week, I also stated
We are probabilistic reasoners, weighing chances of what might happen next, and choose our next actions based on our best guesses. In our direct experience, the world is clearly not deterministic.
So, free will is, if anything, an illusion. Laplace played with this idea already 200 years ago (and others even before that). It is the mere impression of choice that arises from the uncertainty of what will happen next, as far as our mental models are concerned. It is an evolutionarily useful program to navigate through an environment that is way too complex to fully understand. Nevertheless, since the brain operates deterministically, the illusion of free will would vanish if we just were smart enough to understand the brain precisely.
But it gets worse. As Sam Harris points out, if you closely investigate your subjective experience, you'll notice that even there is nothing that the concept "free will" could refer to. He proposes the following experiment: Choose a movie. Like, any movie, but observe how exactly you choose it. Is there any moment in which you actually do a choice? Try it! You might have thought for example "I have three movies in my head now, and now I choose the second one." How did you choose to say this? And how did you select the movies you came up with from the vast number of movies you have ever heard of? Do it again! Everything you actually experience is thoughts coming up and passing. And so is the decision to act on them, it's just another thought. So, free will is not even a proper illusion. It's an illusion of an illusion!
Stop. If whatever we do is basically predetermined, why then should we still spend any effort in life if it is all in vain anyway? Yes, it's almost predetermined where you'll end up in your life. And where you will go in your life is significantly influenced by what you'll be predetermined to choose to do. Hence, your choices are a crucial part of the causal chain. Your choices matter. It's just that choices aren't made "freely". Determinism is not fatalism.
Responsibility
If there is free will, that means that one could have done otherwise in the past. This gives rise to the belief that someone who did wrong can and should be punished for what they did in the past. Let's take an example: You live together with a person, who was negligent enough to push a plant from a table, resulting in shards and a lot of dirt being spread over the carpet. Despite of your reminders, the person keeps not cleaning it up. If you believe free will exists, you might say something like "You caused the mess. It's your fault. Therefore it's your responsibility to clean it up again." The person could have not dropped the plant, right? If we accept free will, punishment for past deeds seems like an appropriate behavior, letting them pay for what they've done.
But what happens if there is no free will? Then you see that the plant was dropped because the current perception and previous experiences deterministically led to the choices which ended up dropping the plant. Then you see that all desires and choices are part of the causal structure.
Wait, doesn't that mean that the person is not responsible for their actions? Following this logic, anyone could behave as badly as they wanted and just say, "I was deterministically caused to do so. I couldn't possibly do otherwise," and in this way excuse themselves from punishment, right? Not quite. Let's take a closer look at the concept of responsibility.
If free will doesn't exist, then a concept of punishment for past deeds isn't sensical either, because, in fact, they could not have done differently. Instead, look at responsibility as the ability to respond, that is, to observe current state of the world (aka. is), to imagine the desired state of the world (aka. ought), and then to take whatever action fits best to get the current state closer to the desired state. If you are "responsible for" something, it means that you should act such that the is of the something gets closer to its ought. In other words, responsibility is a property of the person that might be more or less present. But it is not a moral imperative to suffer if you failed (or to be rewarded if you succeeded).
Let's come back to our example. Let's assume you and your roommate agreed that both of you are responsible for the apartment. If the apartment is a mess, then you are both looking for good ways to clean it up. This might seem counterintuitive, because why should you spend the effort now if someone else caused the mess? But remember, the other could not have done otherwise. There is no reason to hate them for this. If you accept this and observe your roommate being negligent, a useful action instead might be to ensure them being responsible for the apartment in the future. Suddenly, the approach is very different. It's not about making them suffer for creating the mess by forcing them to clean up, but about making sure that the other can and does take responsibility to keep the apartment clean. Instead of making the other feel ashamed or treated unfairly (which might not necessarily lead to a better apartment cleanliness in the future), you help to clear the root cause of not being sufficiently responsible, increasing the chance of actually having a cleaner apartment in the future. Accepting the non-existence of free will means to fully engage in bringing the is closer to the ought.
(For this conclusion, it is actually already sufficient to observe that reality (approximately) obeys the Markov property, i.e., that the future is independent of the past, given the present. Thus, to reflect this fact in a moral system, our actions should not be any more or less influenced by the past than the past influences the present. Anyway, back to the topic.)
So, when taken seriously that there is no free will, this doesn't mean that people will stop to act or even start to act badly. This happens if people think that it means they can't want things. Or if they think that letting go of responsibility does not have consequences. When I wrote "as badly as they wanted" above, this implicitly implied free will. If there is no free will, people will want whatever their present state and past experience causes them to want. If there is no free will, it doesn't mean that you can't choose things. It just means that the actions are deterministically chosen (like in AIs). It doesn't mean you won't get angry. It means that the anger is directed towards behavior and not the person itself. What does change is the approach to resolve harmful behavior. Let go of the past and change the future instead!
What if ...
In the end, I'd like to entertain the thought what would happen, if there actually was an objective free will. Imagine neuroscientists suddenly found a mechanism in the brain that would amplify outcomes of quantum measurements by some weird particles that don't follow quantum mechanics, but instead transfer information when repeatedly measured. That is, such a particle would be a one-way portal from a conscious decision maker. I'm not quite sure which consequences on current physics the finding of such a particle would have, but I guess it would lead to a lot of contradictions. Another idea would be that we could consciously change the quantum states of arbitrary particles. In other words, any particle could have this weird behavior described above, but they just usually don't. Then you could change, create, or destroy arbitrary things around you (or arbitrarily far away in space and time?). Wouldn't that be fun? Another, rather sad consequence would be that 1+1 might still be 2 for some people, but maybe 3 for others. Or 42. That is, clear reasoning based on the same evidence could come to totally different conclusions, dependent on the additional information channeled into the universe.
Edit 21-01-23: Rewrote the first paragraph to be more clear about my definition of free will.
Edit 10-02-23: Added a paragraph on fatalism.