This is a great question and one that is becoming increasingly important as neuroscientists and psychologists increasingly suggest that their research is showing that free will is an illusion, a claim I call 'willusionism', and as increasing evidence comes in that shows that these willusionist claims, which the media loves to report and exaggerate, can have (at least short-term) negative effects on people's behavior. I have lots to say on this topic, some of which I say in papers on my website if you want to hear more, but here's a brief take on the issues. Suppose you believe that free will requires the abilities to make choices based on conscious deliberation and reasoning and to control your actions in light of these choices (against internal desires to do otherwise and without external contraints preventing your action). Free will requires that your conscious self can make a difference in what happens. You may also happen to believe that the only way to have such free will is to have a non...

What if a burning bush appeared out of nowhere in Central Park in front of hundreds of people and said, "It is God speaking. I have come to tell you that you should believe that I exist. Also, love one another. And, by the way, the health care bill does not say anything about a 'death panel'"? Well, a scientifically minded person would want to consider various alternative explanations for this event. For instance, perhaps it's an elaborate hoax involving holograms or even mass hypnosis. But another hypothesis worth considering is that it was God, and God is able to perform miracles that cannot be explained by anything the sciences study (or could study). There might be ways to test whether this event was a hoax, but presumably a 'hoaxster' would try to cover his tracks. So, what if an agnostic scientist in the crowd said to the burning bush, "I would like to believe in God but I need evidence that the best explanation for this event is that you really exist and that this isn't just a hoax...

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...