Twin Earth, Part 1

Hilary Putnam

There’s a debate in the philosophy of language about the meaning of “meaning.”

Specifically, it is about whether meaning is “intensional” or “extensional.” Intensional meaning is meaning as we normally understand it. If I say “beaver,” the meaning of that word is some kind of concept or sense, perhaps based on meeting certain criteria. It’s some idea in my mind. Extensional meaning is meaning based solely on reference. Thinking extensionally, the meaning of “beaver” is the collection of all things that are beavers.

The debate gets complicated and technical. Traditionally, meaning has been thought to be intensional (“internalism”). Influential arguments by Saul Kripke and Hilary Putnam, among others, have tilted the balance, such that contemporary philosophers of language tend to lean more toward “externalism”: an extensional view of meaning. In this post, I’ll quickly summarize Putnam’s argument. In a subsequent post, I’ll spell out possible objections to it.

Imagine a Twin Earth on a galaxy far, far away. Twin Earth is exactly like Earth. Everything that has ever happened on Earth has also happened on Twin Earth, down to the last detail. There is only one exception: on Twin Earth, water isn’t made of H2O. It’s made of a totally different compound, called XYZ. XYZ looks, smells, and tastes exactly like H2O. Although the chemical makeups of XYZ and H2O are distinct, there is no difference that is discernible without sophisticated equipment.

The year, on both Earth and Twin Earth, is 1750. Let’s pick out some person: Agatha. When Earth Agatha looks at “water” – that is, H2O – she has some idea in her mind about what it is. When Twin Earth Agatha looks at “water” – XYZ – she has the exact same idea about what it is. Remember that Earth and Twin Earth are identical in all respects except the chemical makeup of water. The year is 1750, which means the identity between water and H2O (or between water and XYZ on Twin Earth) hasn’t been discovered yet. No one has any notion that water is really some molecular arrangement. So the two Agathas don’t even have a different idea about the chemical makeup of water. Their mental contents are 100% identical.

Yet the waters are different. Whether or not anyone knows it in 1750, water refers to H2O on Earth and to XYZ on Twin Earth. The meaning of “water” is actually different in the two worlds, though no one is aware of it. Since the mental contents of the two Agathas are identical, but the meanings of “water” different, the meaning of “water” can’t solely be in their minds. The meaning of “water” must be, at least in part, extensional. Meanings “just ain’t in the head,” as Putnam quipped – they must also be out in physical reality. Part of the meaning of “water” is in the actual water.

This argument was hugely influential. Along with Kripke’s arguments, it is one of the decisive reasons for the surge in extensional approaches to meaning. In part 2, we’ll look at some objections to it.

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