   Another related puzzle is the puzzle of how identity statements can both be true and informative. It seems clear that Ava could not doubt that ‘Hesperus is Hesperus’ expresses a truth. But it seems clear that she can—in fact she does—doubt that ‘Hesperus is Phosphorus’ expresses a truth. In fact, when she learns that ‘Hesperus is Phosphorus’ does express a truth, she is surprised.  How can it be? (See Richard 1990 and Salmon 1986.) 