Thought experiment 6: Twin Earth

Hilary Putnam , born in Chicago in 1926 , had an unusually eclectic range of philosophical interests, from mind to language to metaphysics.

He is particularly famous for inventive thought experiments one of which is about Twin Earth’ He said that when Sarah on Twin Earth uses the word “water”, even if she has the same psychological states, beliefs and desires as Twin Sarah , she means something different.

This is “semantic externalism” - where meanings’ just ain’t in the head!.

17th- and 20th-century users mean the same by water’ and both think about water.

Meanwhile, the people on Twin Earth are not thinking of water, but twin water.

What all this shows is that there are constraints on what we can talk and think about, says Sawyer .