An “illusion” is something which is wrongly perceived by the senses. We certainly believe and act as though we are making’ free-will’ decisions. Indeed, it is essential for survival that all living creatures do make decisions.
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ALL self-aware creatures, including humans, instinctively “conclude” that they exist – or that “I am”.
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You have yet to explain how humans make a libertarian act of free-will when they have been shaped by natural selection over millions of years of evolution and programmed by genes and sub-conscious environmental pressures. Just baldly asserting that “I think we can make choices that are acts of will” doesn’t do it.
I think the assertion has 'the high ground' though, as highlighted above. Two people are out walking, one says "I see a tree", the other "that's just an illusion". Unless the other person has good reason for saying it (e.g. they are in the desert), provisionally the first person has the stronger case.
Do we have evidence that the brain can't generate "free-will" decisions? I'd say 'no', and I'll quote Libet for his thoughts below. Evolution might favour the development of a brain capable of "free-will" decisions, or as Dennett calls it, making conscious choices to "avoid the future".
Does physics necessarily rule out the brain making "free-will" decisions? Again, I'd say 'no'. There is only conjecture about how the laws of physics might play into that. Maybe there is an 'immaterial soul', maybe 'quantum indeterminacy' - I know nothing about either topic. But I don't see why the laws of physics necessarily inhibits the brain from evolving a "free-will" engine. It hasn't stopped the development of self-awareness, consciousness and "the illusion" of free-will.
From the Wiki article on Libet:
Libet's experiments suggest to some[10] that unconscious processes in the brain are the true initiator of volitional acts, and free will therefore plays no part in their initiation...
Libet himself regards his experimental results to be entirely compatible with the notion of free will.[9] He finds that conscious volition is exercised in the form of 'the power of veto' (sometimes called "free won't"); the idea that conscious acquiescence is required to allow the unconscious buildup of the readiness potential to be actualized as a movement. While consciousness plays no part in the instigation of volitional acts, Libet suggested that it may still have a part to play in suppressing or withholding certain acts instigated by the unconscious.
Thus our "conscious mind" is constantly being feed a number of possible actions from our subconscious -- influenced by genes, memories and previous experiences -- and it is the role of our self-aware conscious mind to select one action and veto the others -- Libet's "free won't". That's also consistent with Dennett's version of "avoiding the future" compatibilism.
Perhaps science will come up with good reasons why free-will isn't possible. I don't think it has yet, though. And since we certainly appear to have free-will, that seems to be provisionally the stronger case.