"A man should make up his mind with emphasis as to what he rationally believes, and should never allow contrary irrational beliefs to pass unchallenged or obtain a hold over him, however brief."

("The Conquest of Happiness" 1943)

His parents having died when he was very young, Russell was brought up in the home of his grandfather, Lord John Russell, a liberal British statesman. He received a mathematical scholarship to Trinity College, Cambridge, but turned from mathematics to philosophy, gaining first class honours in 1894. His first years lecturing at Cambridge were devoted mainly to mathematical logic. Yet he was always interested in social issues and politics, and his later writings considered virtually all aspects of philosophy, including metaphysics, epistemology and ethics. He was twice jailed for his vocal opposition to war, and condemned for his atheism. His writings reached a broader audience than the academic world and he received the Nobel Prize for literature in 1952.

Russell's first works set out to reduce the entirety of mathematics to logic. His "Principia Mathematica", co-authored with Whitehead, was fundamental in the development of symbolic logic. He was a prolific writer and often changed his views. Yet his approach was ever consistent and his aim always to find reasons for accepted beliefs, whether these be in mathematics, natural science or common sense. Everything was to be questioned and his approach was always to start with the propositions least susceptible to doubt and and try to reconstruct the edifice of knowledge on their base, with as few assumptions as possible. In this sense, he was a modern representative of British empiricism, following in the footsteps of Hobbes, Locke and Hume, and of his godfather, John Stuart Mill.

His "Problems of Philosophy" introduces the distinction between knowledge by description (savoir) and knowledge by acquaintance, (connaître).