But in the opinions of some, the laws of physics stop at the neck, beyond which things are neither causal nor non-causal nor random nor non-random but 'free'. Whatever that might mean. (Apparently it's some sort of unmoving mover which moves itself and interacts with the universe without being interacted upon by the universe.)
This is probably the most succinct statement that sums up the problem with "free will". As soon as you start to try to define "free will" in terms of the natural sciences you run into insurmountable contradictions that suggest that it must be an illusion.
Edit: I wonder what proportion of posters here believe both that "free will" is not an illusion and that "strong AI" is possible.
Moreover, even if human brains were somehow special relative to animal brains, there's no reason to think they operate in a way that that's separate from all the other matter in the universe. And as the GP said - interactions between matter are either causal or random. There's no "free will" interaction. Such a thing would be - almost by definition - supernatural.