Comment by ScottAaronson

I’m not sure I fully understand your argument, but if someone showed that they could perfectly predict the actions of the people around them, say hours or days into future, placing the predictions into a sealed envelope or a cryptographic commitment and revealing them later, then to whatever extent fraud and the like had been ruled out as explanations (in the ordinary experimental ways, and possibly with the help of James Randi :-) ), I’d have to say that the person had successfully unmasked free will as an illusion.