Many of us are tempted by the thought that the future is open, whereas the past is not. The future might unfold one way, or it might unfold another; but the past, having occurred, is now settled. In previous work we presented an account of what openness consists in: roughly, that the openness of the future is a matter of it being metaphysically indeterminate how things will turn out to be. We were previously concerned merely with presenting the view and exploring its consequences; we did not attempt to argue for it over rival accounts. That is what we will aim to do in this paper.