 The idea that all and only the a priori truths are necessary was thrown into serious doubt by the work of philosophers including Hilary Putnam (1972) and Saul Kripke (1980). Kripke distinguishes between two different kinds of singular terms, rigid and non-rigid ones. A so-called rigid designator is an expression that singles out the same thing in all possible worlds. Kripke argues that ordinary proper names like ‘Al Gore’ are rigid. We can use this name to describe how things actually are, e.g., by saying ‘Al Gore became vice president in 1993.’ In such cases, the name picks out Al Gore. But we can equally use the name to describe how things stand in other possible worlds, e.g., by saying, ‘If Bill Clinton had chosen a different running mate, Al Gore would not have become vice president.’ In this case, we are talking about a non-actualized possibility, and we use the name ‘Al Gore’ to describe this possibility. Moreover, we use the name to say something about how things stand with Al Gore in that possibility. In general, when we use the name to describe any possible world, we use it to talk about the same person, Al Gore. Other examples of rigid designators include indexical expressions like the first-person pronoun ‘I,’ or the expression ‘now.’ When you use the term ‘I’ to describe any possible world, you are always picking out the same thing: yourself. Natural kind terms like ‘water’ and ‘gold’ can also be regarded as rigid terms, as they single out the same kinds in every possible world. Non-rigid singular term, by contrast, pick out different entities in different possible scenarios. The paradigmatic examples of non-rigid terms are descriptions that are satisfied by different objects in different possible worlds. For example, ‘the most annoying person in the history of the world’ may pick out Fred in the actual world, while picking out Cleopatra in some other possible worlds.