Saul Kripke revolutionised philosophy of language, logic, and metaphysics with his development of possible worlds semantics and his critique of descriptivist theories of reference. His 1970 lectures, published as Naming and Necessity (1980), are among the most influential works in 20th-century analytical philosophy.
Rigid designators and necessity
Kripke introduced the concept of rigid designators—terms that refer to the same object in all possible worlds. Proper names like “Aristotle” are rigid: they pick out the same person regardless of how history might have gone differently. Definite descriptions like “the teacher of Alexander the Great” are not rigid—someone else might have taught Alexander in a different possible world.
This distinction undermined Frege and Russell’s descriptivist theories, which held that proper names are essentially abbreviations for definite descriptions. Kripke showed that names and descriptions behave differently with respect to necessity and possibility.
Necessity a posteriori
Kripke also demonstrated that some truths are necessary but knowable only through experience (a posteriori). The classic example: “Water is H₂O” is necessarily true—water couldn’t fail to be H₂O—but we had to discover this empirically. This breaks the traditional connection between the necessary/contingent distinction (metaphysics) and the a priori/a posteriori distinction (epistemology).
Causal theory of reference
Kripke developed a causal theory of reference as an alternative to descriptivism. Names don’t get their reference from associated descriptions; instead, there’s a causal chain connecting current uses of a name back to an initial “baptism” or dubbing. This social-historical account complements Putnam’s externalism about meaning.
Influence
Together with Putnam’s Twin Earth argument, Kripke’s work fundamentally reoriented philosophy of language away from internalist, descriptivist theories toward externalist, causal-historical accounts of reference and meaning. His contributions to modal logic, philosophy of mind (the mind-body problem), and his interpretation of Wittgenstein on rule-following and private language continue to shape contemporary philosophy.