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.…

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