"Kant is not saying - about freedom or any other subject - anything of the form: "Not-p but we must assume that p." That's close to self-contradictory, like Moore's paradox: "p, but I don't believe that p"."

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Not only in order to act morally, but even to formulate theoretical questions, devise experiments, choose which ones to perform and what conclusions to draw from then - we must presuppose that we are free. That's the sense in which it is true that for Kant "we must assume we are free."

— Allen W. Wood