Ergo: An Open Access Journal of Philosophy. forthcomingThe principle of the fixity of the past says that nobody is able to act in a way such that the past would have been different. The aim of this paper is to cast epistemic doubt on this principle. I do so by advancing two kinds of debunking arguments—arguments that purport to undermine our rational confidence in a given belief by pointing out its epistemically infelicitous origins—against it that, taken together, form a debunking dilemma. If our intuition that the past is fixed traces back to belief in (and/or purported experience of) temporal passage, then the fixity of the past is more likely than hitherto appreciated to be false. But if it doesn’t, then the fixity of the past is less likely than so far assumed to be true. Either way, Bayesian reasoning will demand a reduction of our current credence in this principle. Importantly, however, my argument stops short of mandating an outright rejection of the fixity of the past.