 The potential for a stronger realism in Locke has been exploited by Putnam and Kripke in their development of a modern, essentialist conception of natural kind terms. Locke was pessimistic in two connected respects. First he was sceptical about the possibility of science discovering the nature of the real essences—the structures of atoms or molecules—that underlie kinds of substance picked out by our ordinary sortals. Second, he doubted whether the objects picked out by our actual substance concepts really shared a real essence in the way we assume that they do. He was not confident, that is, that everything we call gold, or iron, or a monkey, was actually interestingly similar at a non-superficial level. Both these forms of pessimism proved largely unfounded. The consequence of this is the possibility of bringing together real essence and the sortal concepts originally picked out by a nominal essence. We now know water to be H2O and iron to be the element of atomic weight 56. Our substance concepts were, often at least, tracking real essences in the world. This involves reinterpreting the rationale of our substance concepts. Locke thought that our concept of gold was properly expressed as ‘anything gold in colour, malleable, soluble in aqua regia’. He thought that we add the optimistic assumption that there will be similarity at the microscopic level. What many philosophers, under Putnam’s influence, now think he should have said is that gold is that kind of thing which is individuated by a particular kind of minute structure that underlies the stuff that is gold in colour, malleable, soluble in aqua regia. In other words, our confidence that these kinds have a genuine real essence is built into the concept. If this is correct, then our ordinary ‘natural kind’ concepts, like water and gold, have built into them intimations of a deeper unity than that supplied by superficial features, even if it be allied to a complete agnosticism about what that unity might consist in or whether we might ever uncover it.