he most famous illustration of how the name of a substance is supposed to function in this way is provided not by Kripke, but by Putnam, another leading proponent of the ‘theory of direct reference’. Putnam asks us to imagine a Twin Earth – just like our Earth – which contains doppelgängers of us humans. The only difference between the two Earths is that on Twin Earth the clear, thirst-quenching, etc liquid that fills the oceans, lakes and rivers is not the chemical substance H2O, but another substance – XYZ. Suppose it’s 1750, before the chemical composition of water was discovered. On both Earths, the inhabitants call their liquid ‘water’. And, because it’s 1750, they associate the same mental checklist with that term: both think of ‘water’ as the substance that’s clear, thirst-quenching, boils at 100°C and so on. Now suppose a glass of XYZ is brought from Twin Earth to Earth and presented to Locke. Locke would believe it’s water, because it would tick his mental checklist. But would it be water? Not according to Putnam. Intuitively, that’s merely water-like stuff in the glass, not water. Putnam concludes that, while the term ‘water’ is associated with the same descriptions on Earth and Twin Earth, it has different meanings and picks out different chemical kinds. It is, and was, a necessary condition of something being water that it be H2O, despite this condition not being known back in 1750.