Hilary Putnam

Putnam introduced the Twin Earth thought experiment in 1975 to challenge the idea that meanings are only in the head. The example imagines a substance that looks like water but has a different chemical structure from H2O.

Semantic externalism says that meaning depends partly on the external world and a speaker's linguistic community. Putnam's slogan, often paraphrased as meaning is not just in the head, comes from this view.

No, Putnam also worked on philosophy of mind, mathematics, science, and pragmatism. His career at Harvard made him one of the most wide-ranging American philosophers of the late 20th century.

Putnam helped develop functionalism, the idea that mental states can be understood by their roles rather than by their physical material alone. He later criticized and revised many of his own earlier positions.