I do not believe in free will. I think about where free will should come from if it existed and always come to the conclusion that without a supernatural device like the soul there is no way free will would emerge out of a brain that is governed by the laws of physics.
Free will would require that we can control every aspect of our brain that makes up our consciousness. That would specifically require that we are able to control how the neurons and molecules in our brain behave which should, based on my understanding of physics, not be possible. Even if it were possible, there would have to be something that tells us, how exactly we must control those neurons in order to think freely which would in turn require some sort of consciousness itself.
In other words, if we had a free will, we would need control of our consciousness and in turn of the mechanisms underlying our brain. Furthermore, we would need to know how they have to be manipulated to remove the randomness of molecular motion and to generate our free thoughts which would require (free) thinking itself. Therefore, at some point we would have to introduce something which cannot be explained by physics resulting in something called a soul for example.
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This Wikipedia article nicely illustrates a neuroscientific argument against free will. There is obviously subconscious brain activity prior to the conscious decision to move one’s hand. That means the decision is made even before we think we decided freely to move our hand. Moreover, one can influence somebody’s decision making process by stimulating the brain and the test subject still reports that he has freely decided what to do.