 Putnam’s landmark article “The Meaning of ‘Meaning’” (1975: 215–271) introduced the Twin Earth thought experiment, which postulates a world just like our own except that H2O is replaced by a qualitatively similar substance XYZ with different chemical composition. Putnam argues that XYZ is not water and that speakers on Twin Earth use the word “water” to refer to XYZ rather than to water. Burge (1982) extends this conclusion from linguistic reference to mental content. He argues that Twin Earthlings instantiate mental states with different contents. For example, if Oscar on Earth thinks that water is thirst-quenching, then his duplicate on Twin Earth thinks a thought with a different content, which we might gloss as that twater is thirst-quenching. Burge concludes that mental content does not supervene upon internal neurophysiology. Mental content is individuated partly by factors outside the thinker’s skin, including causal relations to the environment. This position is externalism about mental content.