 Regarding knowledge, in Book IV Locke argues that knowledge is real only if our ideas correspond to their archetypes in the appropriate way. Scientific knowledge occurs when our nominal essence ideas match the real essences. According to Locke, in mathematics and morality the nominal and real essences are the same. That is, the definitions of mathematical or moral concepts constitute both the nominal and real essences of those concepts. For that reason it is possible to have a human science of mathematics and morality. In the case of substances, however, the real and nominal essences are always different, and so there is no possible human science of substances (IV.vi.12–16).