In his classic Naming and Necessity, Kripke examined the semantic mechanism of proper names and rejected the theories of philosophers like Frege, Russell, and Searle that are based on definite descriptions. According to Kripke, who attempted to ground his view with modal, semantic, and epistemic arguments, proper names refer to a specific object as rigid designators that possess fixed references. It is not definite descriptions that enable proper names to refer to the same object in all possible worlds. For it is possible that definite descriptions refer to different objects in different possible worlds. Therefore, there must be another ground that gives a proper name its identity across possible worlds. After the initial naming, a proper name acquires its fixity in reference through the usages of a community that uses and transmits that name within a causal chain. Kripke did not restrict the idea of the rigid designator, also called the causal theory of reference, to proper names, and thought that natural kind terms like gold, water, heat, and tiger are also rigid designators just like proper names. These thoughts led to the conclusion that identity statements containing either proper names or natural kind terms are necessary. Accordingly, in the case where a specific person or a specific natural kind has two names, an identity statement where these two names appear becomes necessary. For instance, statements such as “Water is H2O” or “Heat is the motion of molecules” are necessarily true statements. However, such namings are contingent actions made within sensory experience, and the truth of such statements is known a posteriori. In this case, a situation emerges where a group of propositions, which includes many definitions in the sciences, are necessary a posteriori. Whereas, in the two-century period up to Kripke, the concept of “necessary” had been seen as correlative with the concepts of “a priori” and “analytic”.

On the other hand, in Naming and Necessity, Kripke showed, starting from the “standard meter” issue, that contingent a priori statements could also exist, and thereby made the nature and limits of the a priori / a posteriori epistemological distinction, which had been entrenched since Kant, a matter of debate. A similar situation was seen earlier in Quine’s attack against the analytic/synthetic distinction that had also been established since Kant. Synthetic a priori judgments, one of the most important elements of Kant’s philosophical system, had already been eliminated. With Kripke’s idea that there are necessary a posteriori and contingent a priori statements, the connection between the concepts of necessity and a priori, which have a very central position as concepts in Kant’s philosophy, has also become questionable.