Saul Kripke

Kripke was born on Long Island and grew up in Omaha, Nebraska, son of a rabbi and a children's author. He taught himself Hebrew at six, had read Shakespeare by nine, learned calculus and logic in his teens, and at seventeen proved a completeness theorem for modal logic, published in the Journal of Symbolic Logic in 1959. Harvard's mathematics department reportedly offered him a post; he replied that his mother said he should finish high school first. He graduated from Harvard in mathematics in 1962, never attended graduate school and held no doctorate.

A series of papers in the 1960s established 'possible-worlds semantics' for modal logic: a proposition is necessary if and only if true in all accessible possible worlds, with different accessibility relations yielding different modal systems. The framework turned modal logic from a syntactic game into a theory with a definite semantics and spread through linguistics, computer science and every part of philosophy. He taught at Harvard and Rockefeller University, moved to Princeton in 1977, and after retiring in 1998 joined the City University of New York.

In January 1970 he gave three lectures at Princeton without notes; the transcripts were published in 1972 and became the book Naming and Necessity in 1980. He argued that proper names are 'rigid designators' whose reference is fixed not by descriptions but by a causal chain from an original baptism; hence identity statements such as 'Hesperus is Phosphorus' and 'water is H2O' are necessary yet discovered empirically, so there are 'necessary a posteriori truths', while statements like 'the standard metre is one metre long' are a priori yet contingent. Necessity and apriority are distinct notions, one metaphysical, one epistemic, which Kant had run together. He thereby defended essentialism and used rigid designation to argue that mental states cannot be identical with brain states.

'Outline of a Theory of Truth' (1975) offered a treatment of the liar paradox. Wittgenstein on Rules and Private Language (1982) posed the 'quus' paradox and provoked a vast literature on meaning and rules. He published very little; masses of lecture recordings and manuscripts are held at the Kripke Center at CUNY, and the first volume of his collected papers appeared only in 2011. Widely regarded as one of the most important analytic philosophers of the later twentieth century, he received the Schock Prize in 2001. He died in 2022 at eighty-one.