On Considering a Possible World as Actual

In Naming and Necessity, Saul Kripke presented some striking examples that convinced many philosophers that there are truths that are both necessary and a posteriori, and also truths that are both contingent and a priori. The classic examples of the former are identity statements containing proper names (Hesperus = Phosphorus) and statements about the nature of natural kinds (Gold has atomic number 79). Realistic examples of the second kind of statement are harder to come by perhaps there are none but once one sees the idea it is easy to construct artificial examples. The most famous example is based on Gareth Evans’s descriptive name “Julius.” “Julius” is, by stipulation, a proper name of the person (whoever he might be) who invented the zip. So the statement “Julius invented the zip” can be known a priori to be true, but since the description is used to fix the reference rather than to give the meaning , of the name, the fact that Julius invented the zip is a contingent fact. Someone other than Julius (the person we in fact named) might have invented the zip, and if some else had invented it instead, it would not have been true that Julius invented the zip.