Clarence Irving Lewis (April 12, 1883 - February 3, 1964) was a pragmatist philosopher. Born Stoneham, Massachusetts, Lewis was educated at Harvard University, studying logic under Josiah Royce, and taught there from 1920 until his retirement in 1953. He died in Cambridge, Massachusetts.

Alongside his work in epistemology and ethics, Lewis was a supporter of bayesian probability and a pioneer of modal logic. His study of the Principia Mathematica of Bertrand Russell and Alfred North Whitehead led him to develop his own alternative system of logic.

His publications include:

  • A Survey of Symbolic Logic (1918)
  • Mind and World Order: Outline of a Theory of Knowledge (1929)
  • Symbolic Logic (1932), with C H Langford
  • An Analysis of Knowledge and Valuation (1947)
  • The Ground and Nature of Right (1955)
  • Our Social Inheritance (1957)