‘Water is H2O’ is one of the most frequently cited sentences in analytic philosophy, thanks to the seminal work of Saul Kripke and Hilary Putnam in the 1970s on the semantics of natural kind terms. Both of these philosophers owe an intellectual debt to the empiricist
metaphysics of John Locke's Essay Concerning Human Understanding, while disagreeing profoundly with Locke about the reality of natural kinds. Locke employs an intriguing example involving water to support his view that kinds (or ‘species’), such as water and gold, are the
workmanship of the human mind. This is the point of his story about a winter visitor to England from Jamaica, who is astonished to find that the water in his basin has turned solid overnight, and proceeds to call it ‘hardened water’. Locke criticizes this judgement, maintaining
that it is more consonant with common sense to regard water and ice as different kinds of substance. Putnam, by implication, disagrees. Deploying his imaginary example of Twin Earth—a distant planet where a watery-looking substance, XYZ, rather than H2O, fills the oceans and
rivers—he maintains that common sense supports the judgement that XYZ and H2O, despite their superficial similarity, are not the same kind of substance, precisely because their molecular compositions are different. Here it will be argued that both views are mistaken, but that,
in this dispute, Locke has more right on his side than his modern opponents do.