How did Saul Kripke challenge Kant’s connection between necessity and a priori knowledge?

Saul Kripke challenged the traditional view, inherited from Immanuel Kant, that necessity and a priori knowledge are coextensive—meaning that all necessary truths are knowable a priori and all a posteriori truths are contingent.12

Kripke argued that this conventional identification relies on a conflation of metaphysical necessity with the epistemological notion of a priori knowledge and the linguistic notion of analyticity. In his influential work Naming and Necessity, he demonstrated that these categories do not perfectly overlap:12

• Necessary a posteriori propositions: Kripke argued that some truths are necessary but can only be known through empirical investigation. For example, the statement "Water is H₂O" is necessarily true because it concerns the essence of a natural kind, yet it is a posteriori because it required scientific discovery.12
• Contingent a priori propositions: Conversely, he suggested that some propositions are contingently true but knowable a priori, challenging the idea that all a priori knowledge must be necessary.2

By decoupling these concepts, Kripke overturned the long-standing assumption that necessity is strictly tied to a priori justification.12