American logician and philosopher. After teaching briefly in California, Lewis taught at Harvard from 1920 until his retirement. Although he wrote extensively on most central philosophical topics, he is remembered principally as a critic of the extensional nature of modern logic, and as the founding father of modal logic. His two independent proofs showing that from a contradiction anything follows, still constitute the main problem for developing a relevance logic, using a notion of entailment stronger than that of strict implication. His works include A Survey of Symbolic Logic (1918) and Mind and the World Order (1929).