If there really is a complete unified theory that governs everything, it presumably also determines your actions. But it does so in a way that is impossible to calculate for an organism that is as complicated as a human being. The reason we say that humans have free will is because we can't predict what they will do.
Stephen HawkingIt often happens that I have an idea, but then I try to fill in the intermediate steps and find they don't work, so I have to give it up.
Stephen Hawking