C. I. Lewis

Clarence Irving Lewis was an American logician and epistemologist who taught for most of his career at Harvard. Dissatisfied with material implication, he developed strict implication and the modal systems S1 to S5, which became the starting point for modern modal logic. In epistemology he defended a 'conceptual pragmatism' in which the given in experience is interpreted through concepts we choose on pragmatic grounds.

Key ideas

• Material implication fails to capture 'entails'; strict implication requires that the antecedent cannot be true with the consequent false.
• Modal notions can be axiomatised in a series of systems of increasing strength, S1 to S5.
• A priori truths are definitional and conceptual, but the choice of conceptual scheme is pragmatic.
• Empirical knowledge rests on immediate sensory givens interpreted by concepts.

Major works

• A Survey of Symbolic Logic (1918)
• Mind and the World-Order (1929)
• Symbolic Logic (1932, with C. H. Langford)
• An Analysis of Knowledge and Valuation (1946)

In the conversation

• Question My debaters say ‘if the moon is cheese, then 2 + 2 = 4’ is true. Are they right? Sam Ellery · 4 replies
• Question How did Kripke’s possible worlds turn S4 and S5 from arbitrary axioms into something meaningful? Pieter de Jong · 4 replies