Hilary Putnam

• 1. Introduction

  ›

  Hilary Putnam was a philosopher who changed his mind for a living. He arrived at Harvard in 1965, organized campus protests, taught courses on Marxism, and eventually celebrated a belated bar mitzvah at the age of sixty-seven. The same man who invented one of the dominant theories of the mind then spent years systematically dismantling it. He argued for mathematical realism, then qualified it. He defended metaphysical realism, then became one of its sharpest critics. Noam Chomsky, who first met Putnam at a Philadelphia high school when they were teenagers, later called him someone with "enormous talents and creativity, one of the finest minds I've ever encountered." Martha Nussbaum compared him to Aristotle. Warren Goldfarb, a colleague at Harvard, said he was "essentially the quickest mind I've ever encountered."

  What drove a man to keep overturning his own conclusions? What does it mean to spend a career in productive disagreement with yourself? And what ideas did Putnam leave behind that still shape how philosophers think about the mind, language, mathematics, and reality?

• 2. A Life in Motion

  ›

  Putnam was born on the 31st of July, 1926, in Chicago, Illinois. Six months after his birth, his family moved to France, where his father Samuel had a contract to translate the surviving works of Francois Rabelais. Samuel Putnam was a scholar of Romance languages and a columnist who wrote for the Daily Worker, a publication of the American Communist Party, from 1936 to 1946. Because of that political commitment, Hilary grew up in a secular household, though his mother Riva was Jewish. In a 2015 autobiographical essay, Putnam recalled that his first childhood memories were from France, and that his first language was French.

  The family returned to the United States in 1933 and settled in Philadelphia. At Central High School, Putnam met Noam Chomsky, a year his junior, and the two remained friends and often intellectual opponents for the rest of Putnam's life. He went on to study philosophy at the University of Pennsylvania, where he earned his B.A. and joined the Philomathean Society, the country's oldest continually existing collegiate literary society. Graduate work followed at Harvard, then at UCLA, where he received his Ph.D. in 1951. His dissertation, on the meaning of the concept of probability applied to finite sequences, was supervised by Hans Reichenbach, a leading figure in logical positivism. One of Putnam's most durable positions would be his rejection of logical positivism as self-defeating.

  After teaching at Northwestern and Princeton, Putnam joined MIT in 1961. In 1963, he organized one of the institution's first faculty and student anti-war committees. He moved to Harvard in 1965 and became an official faculty advisor to the Students for a Democratic Society. He joined the Progressive Labor Party in 1968, and the Harvard administration attempted to censure him for activities it considered disruptive. He severed that relationship in 1972. At a meeting of former draft resistance activists at Boston's Arlington Street Church in 1997, he called his PLP involvement a mistake, saying he had initially been impressed by the party's commitment to alliance-building and its willingness to organize from within the armed forces.

  In 1962, Putnam married fellow philosopher Ruth Anna Jacobs, who took a teaching position at Wellesley College. The couple, both reacting against antisemitism they had experienced in their youth, decided to establish a traditional Jewish home. Having no experience with Jewish rituals, they sought out invitations to Seder at other families' homes. Over time, they studied Hebrew, self-identified as Jews, and actively practiced Judaism. In 1994, Hilary celebrated a belated bar mitzvah; Ruth Anna's bat mitzvah followed four years later.

• 3. The Mind and Its Many Homes

  ›

  Putnam's most influential work in philosophy of mind centered on a deceptively simple question: does every mental state correspond to one specific physical state in the brain? His answer, set out in several key papers published in the late 1960s, was no. The hypothesis he called multiple realizability held that pain, for instance, might correspond to entirely different physical states of the nervous system across different organisms, even when all those organisms experience the same mental state.

  To make the argument vivid, Putnam drew on examples from the animal kingdom. He asked whether it was likely that the brain structures of diverse animal types realize pain or other mental states in the same way. If an octopus and a human both feel pain but share no common brain structure, then mental states cannot simply be identified with physical states in the way type-identity theorists claimed. Putnam then extended the argument further, asking about alien beings, artificially intelligent robots, and silicon-based life forms. These entities, he argued, should not be considered incapable of experiencing pain merely because they lack human neurochemistry. He concluded that type-identity theorists were making what he called an "ambitious" and "highly implausible" conjecture that a single counterexample could dissolve.

  Putnam also built an a priori case for multiple realizability through what he called "functional isomorphism." His definition was precise: "Two systems are functionally isomorphic if 'there is a correspondence between the states of one and the states of the other that preserves functional relations.'" A computer built from silicon chips and one built from cogs and wheels can be functionally isomorphic while constitutionally different. The implication was that the same mental state could be realized by radically different physical substrates.

  From this foundation, Putnam developed machine-state functionalism, drawing an analogy between mental states and the states of a Turing machine. Each state of a Turing machine is defined purely by its relations to other states and to inputs and outputs; its physical constitution is irrelevant. Being in pain, on the functionalist account, is simply being in the state that disposes one to cry out, become distracted, and seek causes. Jerry Fodor collaborated with Putnam in developing these ideas. Functionalism went on to shape the foundations of modern cognitive science and became the dominant theory of mind in philosophy through much of the latter part of the 20th century.

  Yet Putnam did not stop there. In his 1988 book Representation and Reality, he turned against his own creation. Ian Hacking read the book as "Putnam's denunciation of his former philosophical psychology." Writing in Noues, Barbara Hannon described "the inventor of functionalism" as arguing against his own former computationalist views. By 2012, Putnam had come to accept a modified position he called "liberal functionalism," which holds that what matters for consciousness is "the right sort of functional capacities and not the particular matter that subserves those capacities."

• 4. Meaning Beyond the Skull

  ›

  In 1973, Putnam published Meaning and Reference, and two years later The Meaning of "Meaning." Together, these works introduced a thought experiment that upended how philosophers understood language. On Twin Earth, Putnam imagined a planet identical to Earth in every detail except one: where Earth has water made of H2O, Twin Earth has a liquid that looks, tastes, and behaves identically but has an entirely different chemical structure, which Putnam called XYZ.

  An earthling named Fredrick uses the word "water" to refer to H2O. His physically identical counterpart Frodrick on Twin Earth uses the same word to refer to XYZ. Since the two speakers are physically indistinguishable and yet their words have different meanings, meaning cannot be determined solely by what is in their heads. Putnam summed up this conclusion in a slogan that became one of philosophy's most quoted lines: "meaning just ain't in the head." The philosopher Donald Davidson, despite many disagreements with Putnam, wrote that semantic externalism amounted to an "anti-subjectivist revolution" in philosophy. Since Descartes, philosophers had tried to prove knowledge from subjective experience; thanks to Putnam, Saul Kripke, Tyler Burge, and others, Davidson argued, philosophy could take the objective realm for granted and begin questioning the supposed certainties of inner experience.

  Along with Kripke, Keith Donnellan, and others, Putnam contributed to the causal theory of reference. For natural kind terms like "tiger," "water," and "tree," he argued, the objects referred to are the primary elements of meaning. There is, Putnam suggested, a linguistic division of labor analogous to Adam Smith's economic division of labor. The reference of "lion" is fixed by zoologists, "elm tree" by botanists, "table salt" by chemists. These references function as what Kripke called rigid designators and radiate outward to the broader linguistic community.

  Putnam further specified a four-part vector for the meaning of any term: the object itself, a set of typical descriptions he called "the stereotype," semantic indicators placing the object in a general category, and syntactic indicators describing its grammatical role. This meaning-vector structure made it possible to track whether the meaning of a term had genuinely shifted. According to Putnam, a meaning change occurs only when the reference changes, not merely the stereotype. Because no algorithm can determine which element has shifted in a given case, all other expressions in the language must be consulted, and since there is no limit to how many those might be, Putnam arrived at semantic holism. Critics including Michael Dummett, Jerry Fodor, and Ernest Lepore argued that if semantic holism is true, learning a language or communicating at all becomes nearly impossible to explain.

• 5. Mathematics as Empirical Inquiry

  ›

  In his 1971 book Philosophy of Logic, Putnam presented what came to be called the locus classicus of the Quine-Putnam indispensability argument. The argument, which Putnam attributed to Willard Van Orman Quine, ran as follows: quantification over mathematical entities is indispensable to science, both formal and physical; therefore we should accept such quantification; and this commits us to accepting the existence of the mathematical entities in question.

  Putnam later built his own version of the argument, grounded in the no-miracles argument from philosophy of science. He wrote that "realism is the only philosophy that doesn't make the success of the science a miracle" and proposed an analogous claim for mathematics. Unlike Quine's version, which argued for the existence of abstract mathematical objects, Putnam's own argument was for a realist interpretation of mathematics that he believed could be provided by treating mathematics as modal logic, without committing to abstract objects at all.

  Putnam also held that mathematics, like physics, uses both strict logical proofs and what he called "quasi-empirical" methods. His example was Fermat's Last Theorem, which states that for no integer are there positive integer values that satisfy the relevant equation. Before Andrew Wiles proved it for all cases in 1995, researchers had verified it for many individual values of n. Those partial proofs built a quasi-empirical consensus that inspired further work even before a complete proof existed. Such knowledge, Putnam acknowledged, is more conjectural than a strictly proved theorem, but it functions as genuine mathematical knowledge all the same.

  The Stanford Encyclopedia of Philosophy has described the Quine-Putnam indispensability argument as considered by many in the field to be the best argument for mathematical realism. Prominent counterarguments come from Hartry Field, who contests that mathematics is actually indispensable to science, and from Penelope Maddy and Elliott Sober, who dispute whether indispensability would commit us to mathematical realism even if it were established.

• 6. Algorithms and Unsolvability

  ›

  Working alongside Martin Davis in 1960, Putnam developed the Davis-Putnam algorithm for the Boolean satisfiability problem, a question at the core of computer science and logic. The algorithm determines whether there exists an assignment of true or false values to variables that makes a given Boolean expression evaluate to true. In 1962, Putnam, Davis, George Logemann, and Donald W. Loveland refined the method into what became known as the DPLL algorithm, named for all four contributors. It remains efficient and still underlies most complete SAT solvers in use today.

  Putnam also contributed to the resolution of Hilbert's tenth problem, one of the twenty-three problems David Hilbert posed to mathematicians at the turn of the 20th century. The problem asks whether there exists an algorithm to determine if a given Diophantine equation has integer solutions. Yuri Matiyasevich settled the question in 1970 with a proof that relied heavily on earlier work by Putnam, Julia Robinson, and Martin Davis. The result, now called Matiyasevich's theorem or the MRDP theorem after all four contributors, established that no such algorithm exists.

  In computability theory, Putnam investigated the ramified analytical hierarchy and its relationship to the constructible hierarchy and Turing degrees. He showed that many levels of the constructible hierarchy add no new subsets of the integers. Later, working with his student George Boolos, he identified the first such non-adding level as the ordinal of ramified analysis, the smallest such that a certain model of full second-order comprehension holds. With his student Richard Boyd and Gustav Hensel, he also extended the Davis-Mostowski-Kleene hyperarithmetical hierarchy of arithmetical degrees.

• 7. The Brain in the Vat

  ›

  Putnam turned the most famous skeptical scenario in modern philosophy back on itself. Descartes had worried that an evil demon might be deceiving him about everything he experienced. Putnam's version was more modern: a disembodied brain floating in a vat, its nerve endings wired to a computer that simulates a complete reality.

  Putnam's argument against this scenario followed from his causal theory of reference. Words refer to the kinds of things their user, or the user's ancestors, actually encountered. If Mary is a brain in a vat whose entire experience comes through wiring and gadgetry created by a mad scientist, her idea of a brain does not refer to a real brain, since neither she nor her linguistic community has ever encountered one. Her idea of a vat does not refer to a real vat either. When she says "I'm a brain in a vat," she is actually saying something closer to "I'm a brain-image in a vat-image." The statement becomes incoherent. And if she is not a brain in a vat, the statement is still incoherent, because she cannot mean by it what she would need to mean. The skeptical scenario cannot be coherently entertained.

  Putnam clarified that his real target was never skepticism itself but metaphysical realism. Metaphysical realism assumes a gap between how one conceives the world and the way the world actually is, and this gap is what makes skeptical scenarios seem possible. By showing such scenarios to be incoherent, Putnam aimed to show that the gap itself is illusory. One cannot have a "God's-eye" view of reality and is always limited to one's own conceptual schemes.

  Crispin Wright has criticized Putnam's formulation as too narrow. Wright points out that the possibility of being a recently disembodied brain is not undermined by semantic externalism. A person who has lived their entire life outside the vat, speaking English and interacting with the world, would still carry genuine referential content when waking up inside the vat. Their words would still refer to trees and grass as they had before envatment.

• 8. Realism, Pragmatism, Legacy

  ›

  In the late 1970s and 1980s, Putnam abandoned the position he had long defended under the name metaphysical realism and replaced it with what he called internal realism. Metaphysical realism holds that the categories and structures of the external world are independent of the human mind. Internal realism holds that although the world may be causally independent of us, its division into kinds, individuals, and categories is a function of the human mind. The view drew on Immanuel Kant's idea that our knowledge of the world depends on the categories of thought.

  In Reason, Truth, and History, Putnam identified truth with "idealized rational acceptability": a belief is true if it would be accepted by anyone under ideal epistemic conditions. He also accepted what he called conceptual relativity, the view that it may be a matter of choice or convention whether, for example, mereological sums exist or whether spacetime points are individuals or mere limits. Nelson Goodman had formulated a similar notion in his 1956 book Fact, Fiction and Forecast, writing that "all possible worlds lie within the actual one."

  Putnam later abandoned internal realism as well. His reply to Simon Blackburn in the volume Reading Putnam renounced it on the grounds that it assumed a "cognitive interface" between mind and world. Under the growing influence of William James and John Dewey, Putnam moved toward direct realism, then by 2012 to a position he called transactionalism, which holds that perceptual experiences are world-involving transactions that are also functionally describable.

  In the mid-1970s, Putnam grew dissatisfied with analytic philosophy's focus on metaphysics over ethics and everyday life. He came to believe, drawing on James and Dewey, that there is no strict fact-value dichotomy: ethical judgments have factual foundations, and scientific judgments have normative elements. His later work addressed democracy, social justice, and religion, and engaged with thinkers from continental philosophy, including Juergen Habermas.

  Putnam retired from teaching in June 2000 as Cogan University Professor Emeritus, though as of 2009 he continued to give a seminar almost every year at Tel Aviv University. He held the Spinoza Chair of Philosophy at the University of Amsterdam in 2001. He received the Rolf Schock Prize in 2011 and the Nicholas Rescher Prize for Systematic Philosophy in 2015. His corpus includes five volumes of collected works, seven books, and more than two hundred articles. He died at his home in Arlington, Massachusetts, on the 13th of March, 2016. A book co-authored with Ruth Anna Putnam on William James and John Dewey, Pragmatism as a Way of Life: The Lasting Legacy of William James and John Dewey, was published in 2017, the year after his death.