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22 January 2025

Thought experiment 6: Twin Earth

Different understandings of semantics expose the limitations of language.

By David Edmonds

W hat is a hilary? In the joke dictionary The Philosophical Lexicon, words have been invented from names of philosophers. It’s very funny, though non-philosophers will just have to take this on trust. One definition is for Putnam, Hilary Putnam. A hilary is “a very brief but significant period in the intellectual career of a distinguished philosopher. ‘Oh, that’s what I thought three or four hilaries ago.’” For Hilary Putnam was famous for changing his mind. While other philosophers greet every objection to their views as a personal insult, Putnam was more interested in the truth, relished criticism, and often abandoned old positions. He even changed his mind about politics and religion, veering away from communism, and turning to Judaism after a secular upbringing. (He had a bar mitzvah at the age of 68, 55 years later than is customary.)

Putnam, born in Chicago in 1926, had an unusually eclectic range of philosophical interests, from mind to language to metaphysics, all of which he approached with his characteristic nuance. “Any philosophy that can be put in a nutshell belongs in one,” he wrote. He is particularly famous for inventive thought experiments – one of which is about “Twin Earth”. Imagine that there is a planet just like this one but in some faraway galaxy. With a single exception, everything on Twin Earth is identical to things on Earth. On Twin Earth, somebody identical to you is reading a New Statesman article about a Twin Earth duplicate called Hilary Putnam.

The difference is this. On Twin Earth there is a liquid that the inhabitants call “water”. It functions just like water on Earth. People drink it and bathe in it, fish swim in it, and it drops to the ground from clouds. But on Twin Earth this liquid has a different chemical composition. It is not H2O. Let us call it XYZ.

Putnam’s question is this. When Twin Sarah on Twin Earth uses the word “water”, does she mean the same as Sarah on Earth when she uses the word “water”? Putnam’s answer was “no”. According to him, when Sarah on Earth uses the word “water”, even if she has the same psychological states, beliefs and desires as Twin Sarah, she means something different. Sarah on Earth is referring to H2O. Twin Sarah is referring to the substance made up of XYZ. “Semantic internalism” is the view that what you mean by your words depends on how you use them as an individual. As Sarah Sawyer, professor of philosophy at the University of Sussex, explains: “According to semantic internalism, if you and I use the word ‘cat’ differently (because, for example, you think all cats have fur while I know there are some hairless cats), then we mean different things by our respective uses of the word ‘cat’. So if a hairless cat runs past, and I say, ‘There goes a cat,’ and you reply, ‘That’s not a cat,’ we’re not actually disagreeing; we’re just talking about different things, and both right in our own way.”

However, if Putnam is correct, then word meaning doesn’t work like this, because it depends on what’s actually out there in the world, like water and cats, rather than on our beliefs about what’s out there. This is “semantic externalism”. Or, as Putnam put it, “Cut the pie any way you like, ‘meanings’ just ain’t in the head!”

Agreement is rare in philosophy, but Sawyer says, “We’re all externalists now.” She identifies Twin Earth as a pivotal movement, “where philosophy as a discipline just shifts”. This is not to say that a competent user of the word “water” has to know that out there in the world this liquid, water, is made up of hydrogen and oxygen molecules or that cats really can be hairless.

Putnam also said that meaning is possible because of “the division of linguistic labour”. I may not be able to distinguish all the multiple varieties of mushroom, but if I refer to giant puffballs, porcinis or chanterelles, the word manages to refer to the correct fungi because there are mycophiles who can tell them apart.

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That introduces a difficulty, however. Putnam wants to claim that 17th-century and 20th-century users of the term “water” mean the same since they both refer to H2O. But the composition of water was only discovered by Henry Cavendish in the 18th century. So Putnam concluded that sometimes what we mean is determined by what we refer to and sometimes by deference to experts. True to form, Putnam later had a (slight) change of heart. He began by believing that his arguments applied only to meaning. He came to believe that they must also apply to thoughts and, though it sounds odd, that thoughts couldn’t be in the head either. Not only do 17th- and 20th-century users mean the same by “water”, they also both think about water. Meanwhile, the people on Twin Earth are not thinking of water, but twin water.

What all this shows, says Sawyer, is that there are constraints on what we can talk and think about: “Language and our minds are shaped by the world and others around us in deeper ways than we might think.”

[See also: Ludwig Wittgenstein: a mind on fire]

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This article appears in the 22 Jan 2025 issue of the New Statesman, Messiah Complex