Hilary Putnam is a key figure in the philosophy of mind during the 20th century. After receiving his BA at Penn (where he was an undergraduate with Noam Chomsky) and PhD at UCLA (under Hans Reichenbach), he taught at Princeton, MIT, and Harvard, where he is now Cogan University Professor emeritus.

Putnam has earned a reputation for changing his mind frequently during the course of his career, and he has written on so many diverse topics that it is often difficult to sort out his views.

Table of contents
1 Philosophy of Mind
2 Philosophy of Language
3 Philosophy of Mathematics
4 Metaphilosophy

Philosophy of Mind

Putnam is probably most famous for his contributions to the philosophy of mind. He was an early and influential advocate of functionalism (roughly, the view that the human mind is analogous to a digital computer) but in "Representation and Reality" he recanted his earlier views and set out to explain why functionalism, in his revised view, would not work.

Philosophy of Language

One of Putnam's most significant results in the philosophy of language is his doctrine that "meaning just ain't in the head", which is most famously illustrated by his Twin Earth thought experiment. Concisely, he argues that if you see a liquid and call it "water" and some alien twin of yours (identical down to the last detail) on an alien planet sees what appears to be an identical liquid and calls it water AND if it turns out it was really XYZ and not H2O, then you and your double actually mean something different by water even though water has the same function for both of you (note that this relates to his abandoning functionalism as a feasible philosophy of mind).

Putnam and Saul Kripke both made important contributions to what is known as the Causal Theory of Reference. (add more here)

Putnam has also made an argument that is regarded by some as a refutation of skepticism known as the Brain in a Vat argument (I'm not sure if he was the first to use that terminology, though). Basically, he argues that supposing you actually were merely a brain in a vat, then you could never know it anyway.

Philosophy of Mathematics

Putnam and Benacerraf are co-editors of Philosophy of Mathematics: Collected Essays, which contains a useful introduction to the subject.

Putnam also contributed to the resolution of Hilbert's tenth problem in mathematics.

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Putnam, while not quite as famous as Richard Rorty, has nevertheless contributed to metaphilosophical questions, narrowly averting the total relativism associated with Rorty and postmodernism.

add more here on - pragmatism - metaphysical realism - internal realism