ThatFanficGuy,

I know this question is many months old, but maybe as the author of the post I can at least provide one version of an answer.

As far as I'm aware, challenges to free will from established science come in two forms. The first is the obvious one: if the state of the physical world is dependent only upon 1) the previous state of the physical world and 2) probabilistic effects of quantum mechanics, then all our actions and the brain states that lead to them are the result of these two factors, which we (in the usual, subjective sense) are powerless to change or influence in any way.

The second challenge is more empirical. Beginning with Libet in the 80s, many studies have shown that what we perceive as "our" conscious decisions are in fact made by our brains well before we are aware of them. See for a recent and competent example this paper from Chun et. al. in Nature Neuroscience and the references in it.

I wouldn't say by any means that science offers proof of the non-existence of free will. Proof of the non-existence of anything is a dicey business, and might not be possible even in principle in this case. What I would say is that established science -- taken as both the results of experiments and a more general perspective regarding the nature of reality that is influenced by but not strictly derivable from those results (ie, materialism) -- leaves very little room for free will under any particularly meaningful definition of the term (of course, if established science were to change, this could change as well). More details in my reply to Odder.

Adam