What kind of scientific evidence, if any, could prove that free-will does not exist?

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Let me turn the tables on you and ask, "What is free will?" When people use this concept, they may have any of several different ideas in mind. Some people think of free will as freedom from external factors such as bribery or threats, some think of it as freedom from acting in accordance with one's own baser urges, some think of it as lack of determination (by the laws of nature/brain processes etc.) I think different sciences are relevant to each of these questions and that we can have evidence supporting or disconfirming claims about free will.

Another way for you to think about your question: if there is no free will, what would you have lost?

This is an interesting question, in fact, so interesting that I am writing a whole book about it (Rediscovering Free Will). As Miriam says, much depends on how you define free will. Let's not begin with the problematic assumption that free will requires a non-natural power to transcend the causal interactions in the natural world, though I think we can begin with the idea that free will involves our powers to control our actions in light of our deliberations about what to do, such that we can be properly held responsible for our actions. In that case, we should not begin with the assumption made by some scientists writing about free will: that increasingly complete scientific (naturalistic) explanations of human decision-making thereby rule out any role for free will. Rather, it may be that neuroscientific and psychological explanations of human decision-making can help to explain--rather than explain away--our capacities to deliberate about our reasons and to control our actions accordingly. Here is a very brief summary of what I think science might say about human free will:

1. Scientificevidence for determinism would not prove that free will is anillusion. This is because determinism does not properly entail most ofthe things people take to be intuitively threatening to free will, including the things that scientists have recently been talking about when they say they are showing free will is an illusion, such as the idea that our "conscious will" plays no role in our actions (e.g., Libet, Wegner, etc.). Determinism, properly construed, is consistent with our conscious deliberations and intentions influencing our actions. And these purported scientific threats are also consistent with indeterminism. (By the way, neither neuroscience nor psychology is going to show determinism is true--i.e., that given certain causal antecedents, certain effects necessarily follow. Only fundamental physics has a hope of showing this to be true, and the current interpretation of quantum physics suggests determinism is false.)

2. The scientific evidence for epiphenomenalism--i.e., the causal irrelevance of conscious mental states--is not there. The claims that non-conscious processes are sufficient to cause our actions, while our conscious awareness of our intentions comes too late to play a causal role, are not supported by the evidence, especially if one considers conscious deliberation, planning, and intention formation that occurs well before action, which is the sort that seems most relevant for free will. It's more important that my thoughts today about what I want to do tomorrow (or with my life!) affect what I end up doing than that my thought about which finger to move in a second affects which finger I move.

3. However, this doesn't mean we are out of the woods yet. There is some worrying evidence from psychology that we often do not know why we do what we do and are influenced by factors we would not want to influence us if we knew about them (advertisers certainly know this!). To the extent that is true, it seems we have diminished free will, because we are unable to act on the reasons we have accepted in prior deliberation--or even that we would accept if we did deliberate about it. I don't think the evidence goes as far as some suggest, but I think it may suggest we have less free will than we tend to think (I take free will to be a set of capacities we possess and exercise to varying degrees, rather than an all-or-nothing thing).

So, in my view, once we work out a clear conception of free will--preferably one that is amenable to empirical investigation--then the sciences of the mind have the potential to inform us about how it works (e.g., how the brain subserves it) but also to show how it is limited.

Hope this helps!

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