Hilary Whitehall Putnam (/ˈpʌtnəm/; July 31, 1926 – March 13, 2016) was an American philosopher, mathematician, computer scientist, and figure in analytic philosophy in the second half of the 20th century. He contributed to the studies of philosophy of mind, philosophy of language, philosophy of mathematics, and philosophy of science. Outside philosophy, Putnam contributed to mathematics and computer science. Together with Martin Davis he developed the Davis–Putnam algorithm for the Boolean satisfiability problem and he helped demonstrate the unsolvability of Hilbert's tenth problem.

Putnam applied equal scrutiny to his own philosophical positions as to those of others, subjecting each position to rigorous analysis until he exposed its flaws. As a result, he acquired a reputation for frequently changing his positions. In philosophy of mind, Putnam argued against the type-identity of mental and physical states based on his hypothesis of the multiple realizability of the mental, and for the concept of functionalism, an influential theory regarding the mind–body problem. Putnam also originated the computational theory of mind. In philosophy of language, along with Saul Kripke and others, he developed the causal theory of reference, and formulated an original theory of meaning, introducing the notion of semantic externalism based on a thought experiment called Twin Earth.

In philosophy of mathematics, Putnam and W. V. O. Quine developed the Quine–Putnam indispensability argument, an argument for the reality of mathematical entities, later espousing the view that mathematics is not purely logical, but "quasi-empirical". In epistemology, Putnam criticized the "brain in a vat" thought experiment, which appears to provide a powerful argument for epistemological skepticism, by challenging its coherence. In metaphysics, he originally espoused a position called metaphysical realism, but eventually became one of its most outspoken critics, first adopting a view he called "internal realism", which he later abandoned. Despite these changes of view, throughout his career Putnam remained committed to scientific realism, roughly the view that mature scientific theories are approximately true descriptions of ways things are.

In his later work, Putnam became increasingly interested in American pragmatism, Jewish philosophy, and ethics, engaging with a wider array of philosophical traditions. He also displayed an interest in metaphilosophy, seeking to "renew philosophy" from what he identified as narrow and inflated concerns. He was at times a politically controversial figure, especially for his involvement with the Progressive Labor Party in the late 1960s and early 1970s.

Hilary Whitehall Putnam was born on July 31, 1926, in Chicago, Illinois. His father, Samuel Putnam, was a scholar of Romance languages, columnist, and translator who wrote for the Daily Worker, a publication of the American Communist Party, from 1936 to 1946. Because of his father's commitment to communism, Putnam had a secular upbringing, although his mother, Riva, was Jewish. In early 1927, six months after Hilary's birth, the family moved to France, where Samuel was under contract to translate the surviving works of François Rabelais. In a 2015 autobiographical essay, Putnam said that his first childhood memories were from his life in France, and his first language was French.

Putnam completed the first two years of his primary education in France before he and his parents returned to the U.S. in 1933, settling in Philadelphia. There, he attended Central High School, where he met Noam Chomsky, who was a year behind him. The two remained friends—and often intellectual opponents—for the rest of Putnam's life. Putnam studied philosophy at the University of Pennsylvania, receiving his B.A. degree and becoming a member of the Philomathean Society, the country's oldest continually existing collegiate literary society. He did graduate work in philosophy at Harvard University and later at UCLA's philosophy department, where he received his Ph.D. in 1951 for his dissertation, The Meaning of the Concept of Probability in Application to Finite Sequences. Putnam's dissertation supervisor Hans Reichenbach was a leading figure in logical positivism, the dominant school of philosophy of the day; one of Putnam's most consistent positions was his rejection of logical positivism as self-defeating. Over the course of his life, Putnam was his own philosophical adversary, changing his positions on philosophical questions and critiquing his previous views.

After obtaining his PhD, Putnam taught at Northwestern University (1951–52), Princeton University (1953–61), and MIT (1961–65). For the rest of his career, Putnam taught at Harvard's philosophy department, becoming Cogan University Professor. In 1962, he married fellow philosopher Ruth Anna Putnam (born Ruth Anna Jacobs), who took a teaching position in philosophy at Wellesley College. Rebelling against the antisemitism they experienced during their youth, the Putnams decided to establish a traditional Jewish home for their children. Since they had no experience with the rituals of Judaism, they sought out invitations to other Jewish homes for Seder. They began to study Jewish rituals and Hebrew, became more interested in Judaism, self-identified as Jews, and actively practiced Judaism. In 1994, Hilary celebrated a belated bar mitzvah service; Ruth Anna's bat mitzvah was celebrated four years later.

In the 1960s and early 1970s, Putnam was an active supporter of the American Civil Rights Movement and he was also an active opponent of the Vietnam War. In 1963, he organized one of MIT's first faculty and student anti-war committees. After moving to Harvard in 1965, he organized campus protests and began teaching courses on Marxism. Putnam became an official faculty advisor to the Students for a Democratic Society and in 1968 a member of the Progressive Labor Party (PLP). He was elected a Fellow of the American Academy of Arts and Sciences in 1965. After 1968, his political activities centered on the PLP. The Harvard administration considered these activities disruptive and attempted to censure Putnam. Putnam permanently severed his relationship with the PLP in 1972. In 1997, at a meeting of former draft resistance activists at Boston's Arlington Street Church, he called his involvement with the PLP a mistake. He said he had been impressed at first with the PLP's commitment to alliance-building and its willingness to attempt to organize from within the armed forces.