 Very simply stated, a real essence, for Locke, is what makes something what it is, and in the case of physical substances, it is the underlying cause of the object’s observable qualities (unless otherwise noted, we shall confine our discussion in this article to the real essences of material substances). A nominal essence, on the other hand, is an abstract idea that we make when we identify similar qualities shared by objects; the nominal essence is the idea of those shared similarities. The distinction between real and nominal essences plays an important semantic role in Book III and an important epistemological role in Book IV of the Essay. The semantic role will be discussed more below, but in a nutshell, Locke argues in Book III that words have meaning only insofar as we can attach ideas to them. Since nominal essences are the abstract ideas we use to name and distinguish the species or genera of things, the meanings of our species or genera terms just are abstract ideas. For example, a common idea of penguins is of short, black and white flightless waterfowl. That abstract idea is the meaning of the word ‘penguin’. By contrast, Locke goes on to argue, we have no ideas of real essences and so our species or genus terms refer to the nominal essences, not to the real essences (II.xxxi.6; citations to Locke’s Essay are given as book, chapter, section; so Book II, chapter xxxi, section 6 is given as “II.xxxi.6”).