The standard view of metaphysical necessity is that it is truth in all possible worlds, and therefore that the correct modal logic for metaphysical necessity is S5, in models of which all worlds are accessible from each other. I argue that S5 cannot be the correct logic for metaphysical necessity because accessibility is not symmetric: there are possible worlds that are accessible from ours, but from which our world is inaccessible. There are (or could be) some individuals who, if they had not existed, could not have existed. Once such individuals are lost, their possibility is gone forever
