Rationality and Synchronic Identity

Abstract

Many requirements of rationality rely for their application on facts about identity at a time. Take the requirement not to have contradictory beliefs. It is irrational if a single agent believes P and believes ¬P, but it is not irrational if one agent believes P and another believes ¬P. There are puzzle cases, however, in which it is unclear whether we have a single agent, or instead two or more. I consider and reject possible criteria of identity at a time before proposing a pluralist alternative on which there are vastly more agents than we might have thought. This pluralist thesis is analogous to mereological universalism, on which there are all sorts of strange disconnected objects we don’t usually take note of. I conclude by giving a pragmatic account of which of these rational agents it makes sense to attend to by appealing the purposes we have in employing rationally evaluative language.