 That this possibility is left open by the arguments so far considered is suggested by at least two points. The first concerns the analogy drawn above between transworld identity and identity through time. Even if we can understand the claim that there is some past time at which Angela Merkel is a baby without being able to specify informative (non-trivial) necessary and sufficient conditions for the identity of the adult Angela Merkel with some previously existing infant, it does not follow that there are no such necessary and sufficient conditions. And many philosophers have supposed that there are such conditions for personal identity over time. Secondly, the fact that one may be able to ensure, by stipulation, that one is talking about a possible world in which Bertrand Russell (and not someone else) is a playwright (if there is such a world) does not imply that, when making this stipulation, one is not implicitly stipulating that this individual satisfies, in that world, conditions non-trivially necessary and sufficient for being Russell, even if one is not in a position to say what these conditions are.