Bertrand Arthur William Russell (18 May 1872 - 2 February 1970) was a British philosopher, logician, mathematician, historian, social critic, and political activist. Over a career spanning more than seven decades, he made foundational contributions to mathematical logic, analytic philosophy, and the philosophy of language, while also writing widely on ethics, education, religion, and politics. He was awarded the Nobel Prize in Literature in 1950. Russell is widely regarded as one of the most influential intellectuals of the twentieth century.

Russell was born into an aristocratic family - his grandfather, Lord John Russell, had twice served as Prime Minister of the United Kingdom. He studied mathematics and philosophy at Trinity College, Cambridge, where he encountered the work of Giuseppe Peano and Gottlob Frege, whose attempts to ground mathematics in logic became central to his own intellectual project. With Alfred North Whitehead, Russell co-authored Principia Mathematica (1910-1913), a landmark attempt to derive all of mathematics from logical axioms. See Principia Mathematica for a detailed treatment of that work.

Russell held a fellowship at Trinity College but was dismissed in 1916 following his anti-war activism during World War I. He was briefly imprisoned in 1918 for a pacifist pamphlet. His later career included stints at various American universities, a long-running public presence as an essayist and broadcaster, and prominent involvement in anti-nuclear campaigning in the 1950s and 1960s, including co-authorship of the Russell-Einstein Manifesto (1955) and founding of the Pugwash Conferences on Science and World Affairs. He died in Wales in 1970 at the age of 97.

Russell's technical philosophical work falls into several areas. In logic and foundations, his discovery of what became known as Russell's Paradox (1901) exposed a contradiction in naive set theory and contributed to the crisis in the foundations of mathematics. His theory of types and later work on logical atomism were attempts to resolve these and related problems.

In the philosophy of language, Russell developed the Theory of Definite Descriptions (1905), which became one of the most discussed and influential contributions to analytic philosophy, offering an account of how referring expressions can be meaningful even when they lack a real-world referent.

In epistemology, Russell drew a distinction between knowledge by acquaintance and knowledge by description, and explored the relationship between sense-data, perception, and the external world across several works, including The Problems of Philosophy (1912) and Our Knowledge of the External World (1914).

Russell's views evolved considerably over his long life. His early Platonism gave way to neutral monism; his political positions shifted from liberal to pacifist to, at times, hawkish and back again. Interpreters disagree about the coherence and consistency of his philosophical development. See Viewpoint: Consistency of Russell's Philosophy for competing assessments.

Russell was a prominent atheist and critic of organized religion. His essay “Why I Am Not a Christian” (1927) and related writings argued against the existence of God, the moral authority of Christianity, and the social utility of religious institutions. He regarded religious belief as primarily fear-based and as historically corrosive to human freedom and inquiry. Religious thinkers and philosophers have offered sustained responses to these arguments. See Debate: Russell on Religion for a structured treatment of these exchanges.

On ethics, Russell held broadly empiricist and at times non-cognitivist positions, arguing that moral claims are not straightforwardly factual. He was a utilitarian in practical orientation, though his metaethical commitments shifted over time.

Russell's political commitments were varied and sometimes in tension. He advocated for women's suffrage, pacifism during World War I, and nuclear disarmament. At other points he argued for preemptive confrontation with the Soviet Union. He opposed Stalinism and Fascism alike. In education, he founded and briefly ran the progressive Beacon Hill School. He wrote extensively on marriage, sexuality, and social convention, often controversially for his era.

His appointment to a professorship at City College of New York was revoked in 1940 following legal action by a parent who objected to the sexual ethics expressed in his book Marriage and Morals (1929). A New York court ruled his appointment “an insult to the people of the City of New York,” a decision that drew significant controversy in academic freedom circles.

There is broad consensus among historians of philosophy that Russell's work in mathematical logic and the foundations of mathematics was foundational to twentieth-century analytic philosophy. The reception of his epistemology, metaphysics, and ethics is more contested. See Consensus: Analytic Philosophy for relevant scholarly positions.

• Viewpoint: Russell's Case Against Religion - Russell's arguments for atheism and against institutional religion, and responses from theist philosophers
• Viewpoint: Consistency of Russell's Philosophy - debate over whether Russell's shifting positions reflect coherent development or fundamental inconsistency
• Viewpoint: Russell's Political Legacy - assessments of his pacifism, anti-nuclear activism, and Cold War positions
• Viewpoint: Logicism and Its Limits - competing views on whether Principia Mathematica achieved its foundational aims

• Bertrand Russell - History
• Debate: Russell on Religion
• Principia Mathematica
• Russell's Paradox
• Theory of Definite Descriptions
• Analytic Philosophy
• Logicism
• Russell-Einstein Manifesto
• Consensus: Analytic Philosophy

1. Russell, Bertrand. The Autobiography of Bertrand Russell. 3 vols. London: Allen & Unwin, 1967-1969.
2. Whitehead, Alfred North, and Bertrand Russell. Principia Mathematica. 3 vols. Cambridge: Cambridge University Press, 1910-1913.
3. Russell, Bertrand. “On Denoting.” Mind 14, no. 56 (1905): 479-493.
4. Russell, Bertrand. The Problems of Philosophy. London: Williams and Norgate, 1912.
5. Russell, Bertrand. “Why I Am Not a Christian.” Lecture delivered to the National Secular Society, London, 6 March 1927.
6. Russell, Bertrand. Marriage and Morals. London: Allen & Unwin, 1929.
7. “Kay v. Board of Higher Education of City of New York,” 18 N.Y.S.2d 821 (Sup. Ct. 1940).
8. Griffin, Nicholas, ed. The Cambridge Companion to Bertrand Russell. Cambridge: Cambridge University Press, 2003.
9. Monk, Ray. Bertrand Russell: The Spirit of Solitude, 1872-1921. London: Jonathan Cape, 1996.
10. Monk, Ray. Bertrand Russell: The Ghost of Madness, 1921-1970. London: Jonathan Cape, 2000.