I definitely lack the words to be super eloquent on this front, but I boil it down to a computer comparison. No matter how complex a computer we make it will take inputs and compute outputs. I now of no evidence of a level of complexity at which this is no longer true. I think the idea of free will is enticing, I'd love to look at a situation in which I prevailed and say "yes, I overcame everything to make that happen." But I think the only supported conclusion is that I made the decision and action I made due to a complex, lifelong history of things that happened internally (biology) and externally to me.
Comparatively, I think free will is essentially asking that a computer take inputs and it's programming and shrug that all off to come up with a completely novel output.
It is true that a computer will take inputs and compute outputs, and that is all it ever can do. However, this statement does not quite end up explaining Space Invaders. There's a couple of steps in between that might have been glossed over slightly. [1]
A computer is entirely capable of generating novel (enough) output. Elsewhere I point to the trivial-seeming python function math.random(). Good luck predicting what that will do!
In reality, it turns out that math.random() and the theory behind PRNGs is not so trivial at all, and is actually quite interesting to dive in to. [2]
Your argument boils down to assuming there is true randomness in the universe. You need hardware level rng with a natural source of entropy to generate non-predictable random numbers, but even then you're just at the point where maybe we could predict the randomness if we could go low enough.
We can't, so at the moment it looks as though the universe might have true randomness (I'm very sceptical of this).
But even then, so what? Does that mean free will is just decisions with some true universal randomness thrown in? How does that give you free will if it is just random? And then we're back to, what does free will even mean.
Well, funnily enough rand() isn’t all that random, but that’s beyond the point. I don’t think randomness is a good argument for free will, that’s just nature. Does a flipping coin have free will? An electron before it decoheres?
Well, randomness is definitely novel output though.
PRNGs are not-quite-random, they're actually chaotic functions.
Which is actually a bit closer to how I think free will functions if you look at it empirically from the outside. (At least: if you were to use your <free will> to try to generate 'random' numbers; which will also not be perfectly random!)
Of course, it does sort of depend on your definition of <free will>.
Oh shoot, I see why I made the error, but on checking more sources to back me up, it turns out only some PRNGs are considered chaotic, and the most used ones are not necessarily viewed with that lens in literature. Eh, it would have been easier. In future I'll have to switch to a different set of algorithms to make my argument,.
For me the crux of it is where's the boundary of 'self'. Regardless of how it came to be, we can define a bounded self (whether bio or software) that runs freely taking energy (but not decision input) from outside. Then we can say that that self produces outputs that have internal sources as being 'free'.
To me it basically comes down to infinite butterfly effects where the knowledge of past/current states of the environment can't be effectively used to predict the output of the self.
Another fun way to imagine it is to say 'I' is the complex gut microbiome that's interchanged and evolving and it's the 'source' of decisions more than anything else. It would also solve the problem of how the human population keeps increasing but souls are discrete and countable--even with reincarnation the number of creatures varies greatly. With microbiomes they can be cultivated and/or subdivided, and mixed. Of course this doesn't work out, but it's easy to see there are things that we haven't considered yet to be ruled out.
But that assumes that with the same inputs, the outputs are always the same. As far as I know this hasn't been proven for quantum mechanics. I'm just a layman but in my understanding you can only predict a probability of the result.
That's why with exactly the same inputs, the output can actually differ. Whether or not that's free will is another discussion, but the "we are a very complex yet in theory predicable computer" argument doesn't hold.
If you're willing to accept inputs that are only almost exactly the same, there's already a class of deterministic systems that can provide you with wildly different outputs.
Moreover, in such systems, the output might be impossible to predict based on the input (other than by running the system).
I'm just saying, you don't need to reach for quantum mechanics here, living organisms (and computers) are quite capable of being unpredictable even without it.
Comparatively, I think free will is essentially asking that a computer take inputs and it's programming and shrug that all off to come up with a completely novel output.