The issue faced here is that free will is being defined in a certain way that guarantees the conclusion of free will either being random or an illusion. To summarise; you’ve defined free will as having higher complexity in predicting what will happen; that a truly free agent applies decision-maker X in their brain randomly and comes out with a decision and then, as we cannot find this, concluded that decision-maker X couldn’t exist and so neither can free will.

When we define free will in terms of moral responsibility we can come to other conclusions.

My second assumption, closely related to the first, is that nature is fundamentally causal.

So I can assume we’re dealing with compatibilist free will; deterministic universe and free will. So I’m going to defend Dennett’s idea of free will against this to lend another side to this argument.

More significant than the missile’s brute obedience to physics is the fact that its behavior calls into question what it is that constitutes a “choice”. If there are at least two heat sources in front of it on which it might potentially home, then, in at least some sense, the missile’s behavior is the result of a “choice” between alternative imaginable paths.

Obviously the missile cannot do anything other than it does; it does not select a heat source based on anything other than whether the heat source matches a signature. Do I think the missile has free will? Absolutely not. Do I think it is any closer to free will than a ball rolling down a hill? Again, no, I believe it is identical on free will decisions. What is different in your choices of examples is the complexity and unpredictability of the action - you’re implying that free will is simply an awfully complex illusion we can perform.

What we call freedom is nothing more or less than the general belief that each of our predictions really could represent some future state of affairs.1 It is a byproduct of the impressively complex, evolutionarily advantageous, but ultimately deterministic, way our nervous systems respond to a varied but not wholly unpredictable environment.

I think that falls in line with what I’m claiming your argument set up. With the definitions you’ve given us you really couldn’t conclude any other way.

Consider Sphexishness:

Some Sphex wasps drop a paralyzed insect near the opening of the nest. Before taking provisions into the nest, the Sphex first inspects the nest, leaving the prey outside. During the inspection, an experimenter can move the prey a few inches away from the opening. When the Sphex emerges from the nest ready to drag in the prey, it finds the prey missing. The Sphex quickly locates the moved prey, but now its behavioral "program" has been reset. After dragging the prey back to the opening of the nest, once again the Sphex is compelled to inspect the nest, so the prey is again dropped and left outside during another stereotypical inspection of the nest. This iteration can be repeated again and again, with the Sphex never seeming to notice what is going on, never able to escape from its programmed sequence of behaviors. Dennett's argument quotes an account of Sphex behavior from Dean Wooldridge's Machinery of the Brain (1963). Douglas Hofstadter and Daniel Dennett have used this mechanistic behavior as an example of how seemingly thoughtful behavior can actually be quite mindless, the opposite of free will (or, as Hofstadter described it, sphexishness).

Pretty cool little example of impressively complex behaviour that looks like free will.

What about a bird?

the bird’s behavior differs from the ball’s in at least two ways, one from our point of view and one from its own. From our perspective, the bird is unpredictable; the ball is not. The bird appears to have free will, not because it is unconstrained by physics, but because we can imagine it taking any of any number of plausible paths.

I dislike relying on quantum argument because my understanding is limited. As an aside, by this definition all photons have free will as we can imagine it taking any of any number of plausible paths. So, according to your definition of free will given, I would deny that the bird has free will because it requires me to accept that many things which do not have free will do.

Concerning ourselves with non-quantum objections, you have to deal with why free will is important to us rather than whether it fits with your definitions of not being constrained and agents being the source of their actions. Not-constrained I’m taking as if I feel hungry and someone stops me from eating, my free will is threatened. Also that I have free will because of a source-based theory; I am the originator of my actions and that is why they are significant.

An agent being the source of their own actions is only significant because it implies a responsibility onto them. Free will is important to us because it is tied to moral responsibility. If I act of my own free will then I am responsible for what I have done and any definition that avoids this does not tell us anything useful. To paraphrase Strawson - we already know what free will means to us because of how we react morally to actions done freely by agents. That cannot be taken away by whether or not a philosopher denies it exists.

In place of an unknown (and perhaps even unknowable) physical solution to the problem of behavioral unpredictability, we are postulating an explanation which is little better than magic.

If you believe that choices are merely being unpredictable, as a bird's actions are to us, then free will choices must necessarily be magic. If you believe that free will is a form of assigning blame and praise for actions then you needn't.

Free Will is not saying that we have a RNG in our heads that computes actions; if so our actions are arbitrary and meaningless, we could decide anything and moral responsibility would be meaningless. If it’s an illusion then we aren’t morally responsible either.

When you avoid the moral implications of free will, something people deal with and are prepared to talk about on a regular basis, you are avoiding a key aspect of the argument. Asking people to define free will is difficult; no person on the street could do that. If you ask someone if they were responsible for something they can tell you immediately if that is so.

What matters when we talk about free will is that the action is a suitable indicator of our morality. When I say that they had free will what I’m saying is that “Well I think someone could have done other than what they did, so I think morally they’re praiseworthy/blameable for what happened”. If someone does something immoral, but was forced into it, then I don’t feel that was morally significant of them as a person. If I feel that they were able to choose anything and decided to do something morally reprehensible, then I feel it is significant. When someone trips and kicks a puppy I don’t think they’re acting immorally; they couldn’t have done anything different and it doesn’t communicate their morality.

Consider free will as our control over our own actions in respect to our morality. When I act or don’t act in a certain way I cannot do differently than I do, but when it goes well or badly I can adjust my morality so that in the future I act differently. I can place myself in situations where I could not act differently than I did and so act morally.

I used this intuitive argument that free will is morality when talking to mk about it, so see what you make of it.

If I agree to meet you at the bus stop and oversleep, you might feel that I've wronged you because I am responsible for me getting up on time and being there. If I told you that I wasn't there on time because I was mugged on the way to meet you, you (hopefully!) wouldn't think I'd wronged you because it was outside of my control. This encompasses the first definition, that it is my action independent of others. What about if I agree to meet you at the door of my house and I wake up on time, but because of my severe OCD I am late to meeting you. You may feel wronged that I took so long, but upon realisation that I have to check all the light switches are off before I can come to meet you, you may feel that this was outside of my realm of control. In this example I didn't have any other choice to checking all my light switches, it feels intuitively that if someone is to be held morally responsible they ought to have had an option to refrain from the action.

In the first case I lacked free will because of outside interference and so I was not held to be acting under free will. In the second I lacked free will because of neurological interference and so was not acting under free will.

The difference in whether I had free will was whether I would be found morally responsible for an action. Whether I was morally responsible is something almost anyone can form a decision on. Free will is not to be defined by philosophers, but explained. So, you might reasonably say that I am defending that free will is whether the agent could have done other than they did, or whether they could have attempted to do other than they did. That implies that decisions are made purely in the moment, that there is a second of which before that moment the agent could have gone either way and after that the agent could not stop their action. I think free actions are decided by our thoughts prior to that event. If we are to believe that MLK did really say “Here I stand. I can do no other” when he refused to recant his writings, I don’t think he was saying “I am not acting of free will” or that he was speaking nonsense when he said “I can do no other”, nor do I think he was saying “I am not responsible for this”. He was saying that his morality was that he could only do that and it would be impossible to do otherwise, fully accepting responsibility. So I think free will is making decisions based on our based actions and our thoughts now and using those decisions to decide what we do next. It is control over our life in a way that is congruent with how we use free will in society and does not confine us to any of the classical fears about not being ‘truly’ free.