In several writings, John Martin Fischer has argued that those who deny a principle about abilities that he calls ‘the Fixity of the Past’ are committed to absurd conclusions concerning practical reasoning. I argue that Fischer’s ‘practical rationality’ argument does not succeed. First, Fischer’s argument may be vulnerable to the charge that it relies on an equivocation concerning the notion of an ‘accessible’ possible world. Secondly, even if Fischer’s argument can be absolved of that charge, I maintain that it can (...) be defeated by appeal to an independently plausible principle about practical reasoning that I call ‘the Knowledge Principle’. In addition, I point out that Fischer’s own presentation of his argument is flawed by the fact that the principle that he labels ‘the Fixity of the Past’ does not, in fact, succeed in representing the intuitive idea that it is intended to capture. Instead, the debate should be recast in terms of a different principle, which I call ‘the Principle of Past-Limited Abilities’. The principal contribution of my paper is thus twofold: to clarify the terms of the debate about the fixity of the past, and to undermine Fischer’s ‘practical rationality’ argument for the fixity of the past. (shrink)