 Since ‘Mark Twain’ and ‘Samuel Clemens’ pick out the same entity in every possible world where they pick out anything, this identity statement is a necessary truth. (Note that the statement is conditionalized on Mark Twain’s existence, which makes it possible to avoid the question whether (1) is true in worlds where the two names pick out nothing.) But it is far from immediately obvious that (1) expresses something that can be known a priori. At least on the face of it, we may think that someone who knows her neighbor by the name of ‘Samuel Clemens,’ who has read several stories by an author named ‘Mark Twain’ and who fails to realize that her neighbor and the author are identical may not know that which is expressed by (1). Moreover, it may seem that her ignorance is irremediable by reasoning alone, that she requires empirical evidence to come to know that which is stated by (1).