 Thus, for Locke, the real essence is an unknown atomic constitution. The types that we categorise them in depend on the properties we happen to be able to perceive and kinds or sorts are defined in terms of these observable properties. Definition in these terms he calls ‘nominal essence’. Our concepts of natural substances presuppose that the nominal essence hides a real essence—that is, that all water, gold, horses etc are in some way similar at a microscopic level. But he both denies that these real essences play any role in the formation of our concepts and is deeply sceptical of our ever being able to discover what they are. His attitude towards the Aristotelian view is expressed later in the paragraph just quoted: