We seem to be able to decide our behavior for ourselves – what we do is up to us. But if everything that we do can be explained by physics, does this leave room for freedom? Are all of our actions pre-determined? Are we slaves to fate? Is freedom compatible with determinism, or does science teach us that we’re nothing but complex machines, following out a complicated program that a good enough physicist could have predicted centuries ago? And what are the implications for morality and responsibility? John and Ken exercise their will with Manuel Vargas from the University of San Francisco, co-author of Four Views on Free Will.