*Philosophic and not scientific because there is no experiment to test the positive claim of Free Will existing.
As far as I know, you can't test the positive claim of Determinism either.
Determinism in this context is not the positive claim that causality exists. It is the negative claim that no Free Will exists that could alter the effect caused even with the same causal variables.
That said it is not rational to be biased toward either the positive or negative claim when both are equally untestable/unprovable.
Well, I actually wanted to mean what you said in your last sentence...
However, if we assume the human race being nothing more than a chemical reaction of the universe, we can compare ourselves to any repetitively constant output in a given experiment. Unless I'm just spouting ridiculousness.
Well, you can't know the exact position AND direction of a same small enough particle (I think it had to be subatomic). That said, knowing the EXACT ouput of this "chemical reaction of the universe" would be impossible, and since some of our reactions are directed (at least indirectly) by some of that small enough differences of a particle, it would be impossible to determine the output of a single human fetus even knowing all the possible data about it.
Apart from that, there's a theory in quantum physics, that everything can happen with a large enough sample. Literally. I mean, your rubber could turn into uranium and travel to Andromedae, then come back as gold (one in a bazzillion-trillion chances of happening). But if you take in account smaller things and smaller differences (like where's a particle inside a neuron), you got bigger chances of random stuff happening.
In the end, you can't know if that "random" output is actually random, or if it is just the data you miss from the input the one that make those things to happen. So it ends being a philosophical debate.