Hilary Putnam

Hilary Whitehall Putnam (Chicago, 1926 – Arlington, Massachusetts, 2016) was one of the most influential analytic philosophers of the second half of the twentieth century. Besides philosophy, he worked as a mathematician and theoretical computer scientist, and his work ranges over the philosophy of mind, language, science and mathematics. He is usually counted among the leading defenders of scientific realism and of a late form of pragmatism, although his views shifted many times over his career.

He was the son of a journalist and translator linked to the American Communist Party. The family moved to France shortly after his birth and returned to the United States in 1933, settling in Philadelphia. He studied mathematics and philosophy at the University of Pennsylvania and earned his doctorate at UCLA in 1951 with a thesis on probability applied to finite sequences, supervised by Hans Reichenbach, a leading logical positivist whose views Putnam would later reject. After teaching at Northwestern, Princeton and MIT, he joined Harvard, where he spent most of his career. In 1962 he married the philosopher Ruth Anna Putnam.

His best-known contribution lies in the philosophy of mind. In several papers from the late 1960s he proposed the hypothesis of multiple realizability: the same mental state, such as being in pain, may be realized by very different physical systems, so equating each mental state with a specific brain state is too narrow. From this he defended functionalism, on which mental states are defined by their causal role rather than by their material make-up. In the philosophy of language, together with Saul Kripke and others, he helped develop the causal theory of reference and proposed semantic externalism, illustrated by the Twin Earth thought experiment, in which the meaning of a word depends partly on the physical and social environment of its speakers.

In the philosophy of mathematics, together with W. V. O. Quine, he formulated the indispensability argument, which holds that we should accept mathematical entities because scientific theories cannot do without them. He later argued that mathematics is not purely logical but quasi-empirical. In epistemology he challenged the brain-in-a-vat thought experiment, which seems to support scepticism, by questioning whether it is coherent at all. In metaphysics he first defended metaphysical realism, then became one of its sharpest critics, proposed what he called internal realism, and finally gave it up for a direct, pragmatist-inspired realism. Throughout these changes he kept a moderate scientific realism.

Outside philosophy, together with Martin Davis he developed the Davis–Putnam algorithm for the Boolean satisfiability problem and helped show that Hilbert's tenth problem is unsolvable. His public life was also controversial: in the 1960s and early 1970s he opposed the Vietnam War and was involved with the Progressive Labor Party, a link he later described as a mistake. He retired in 2000, and his large body of work remains a reference in contemporary analytic philosophy. His influence is most visible in the philosophy of mind, the theory of reference and the debate over realism.