 How does Putnam prove we can know we are not brains in a vat? To understand Putnam’s argument, we need to first recall the ‘Twin-Earth’ considerations used to support Semantic Externalism: on Twin-Earth things are exactly as they are here on Earth except for one difference—whereas for Earthly humans water has the chemical composition H2O, for our döppelgangers on Twin-Earth, twumans, water is instead composed of some substance unknown to us on Earth, XYZ. Now when you and your twuman counterpart say (or think) “‘Water’ refers to water” both of you utter (or think) truths. But which truth you both think or utter differs. For humans “‘Water’ refers to water” expresses the truth that the term ‘water’ in English refers to that substance whose chemical composition is H2O. For our twuman Twin-Earth counterparts, however, their sentence “‘Water’ refers to water” expresses the truth that their term ‘water’ in Twenglish refers to that substance whose chemical composition is XYZ.