Eric Olson has made an important addition to the discussion started by Parfit of the argument from the possibility of fission to the unimportance of personal identity. Olson’s discussion is challenging. I want, more briefly, to highlight what is the most important consequence of it. This is that it is metaphysically impossible, impossible in the strongest sense, that any version of Parfit’s argument from fission can yield his conclusion. Olson argues specifically that this is impossible if what he calls a ‘capacious ontology’ is assumed. I argue that it is a consequence of Parfit’s reasoning that this is so even without the assumption of a capacious ontology.