“He would say that our understanding of something depends on a community,” Warren Goldfarb, a friend of Professor Putnam’s and a former chairman of the Harvard philosophy department, said in an interview on Monday. “There are times it can be said you use and understand a term even when it has no distinction for you. I understand the word ‘larch’ or ‘elm’; I couldn’t tell the trees apart, but you couldn’t say I didn’t understand them.”

In a 1975 paper called “The Meaning of ‘Meaning,’” Professor Putnam further illustrated his argument with a famous thought experiment called Twin Earth. He imagined a planet alongside our own that was a facsimile in almost every way, including holding a replica of each person. The only difference on Twin Earth was its water. Though it looks like H2O, tastes like H2O, fills the lakes, rivers and oceans and performs the same functions as H2O, Twin Earth’s water had a different chemical makeup, abbreviated as XYZ.

Therefore, if an earthling named, say, Oscar, were to travel to Twin Earth and visit his doppelgänger, Twin Oscar, when they referred to water, they would actually be talking about two different things, even though they appeared to be the same. Because Oscar and Twin Oscar are identical in every way, including their thoughts at a given time, Professor Putnam argued, meaning cannot simply be a function of what is formulated in someone’s head.

Another notable thought experiment devised by Professor Putnam, known as “brain in a vat,” was in the field of epistemology. The experiment was intended to disprove a fundamental contention of metaphysical realism — that objects and relationships in the world exist independently of how we perceive them; in other words, that the world we see and hear is not the one that actually is, and that therefore, our brains are perception machines untethered to reality.

If that were the case, Professor Putnam argued, then a human brain would be no different from a brain in a vat placed there by a mad scientist. Human brains, however, employ words based on the things they refer to, which requires some kind of contact with those things. So the brain in a vat — call him Oscar — could not formulate the sentence “I am a brain in a vat,” because Oscar has no experience of a real brain or a real vat. Rather, he would actually be saying something like “I’m the image of a brain in the image of a vat.”

Professor Putnam’s death provoked striking encomiums among his colleagues. The philosopher Martha C. Nussbaum wrote in The Huffington Post that Professor Putnam was “one of the greatest philosophers this nation has ever produced” and compared him to Aristotle in the range of his “creative and foundational contributions.”