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).