Proper names are even more of a problem than mathematical expressions. These days proper names are generally understood to be rigid designators but, unlike mathematical terms, they have no structure that we can make use of. Here is a very standard example. Suppose “Hesperus” is used as a name for the evening star, and “Phosphorus” for the morning star. It should be understood that “the evening star” is conventional shorthand for a definite description, “the first heavenly body seen after sunset” and similarly for “the morning star”. Definite descriptions have structure, they pick out objects and in different possible worlds they may pick out different objects. But proper names are not like that. Once the designations of “Hesperus” and “Phosphorus” are fixed—as it happens they both name the planet Venus—that designation is fixed across possible worlds and so they are rigid designators. It follows that while the morning star is the evening star, that identity is not necessary because definite descriptions are not rigid, but Hesperus is Phosphorus and that identity is a necessary one. How, then, could the identity of Hesperus and Phosphorus not be a known truth, known without doing any astronomical research?