Saul Kripke passed away earlier this year. His work changed philosophy forever. Inspired by Leibniz’s possible worlds, he ushered in the return of metaphysics, while remaining very much with his feet on the ground, writes Timothy Williamson.

Saul Aaron Kripke was born on 13^th November 1940 and died on 15^th September 2022.

The stereotype of a philosopher is an old man with a long beard. Saul Kripke made world-leading contributions to philosophy and logic as a teenager. It was the starting-point for his most distinctive later work, which gave philosophers a new framework to think in.

Kripke’s first breakthrough came in modal logic, the branch of logic concerned with structural principles about necessity and contingency, possibility and impossibility. In English, such matters are expressed by everyday modal auxiliary verbs like ‘can’ and ‘must’; a language incapable of making such distinctions would be radically impoverished. The study of modal logic goes back at least to Aristotle. When we accept the inference from ‘It can’t happen’ to ‘It won’t happen’, but reject that from ‘It can happen’ to ‘It will happen’, we are already doing simple modal logic.