 One of Locke’s main purposes in making this distinction is to deploy it in his criticisms of the Scholastic account of (a), the species of natural substances. However, when criticizing the Scholastic account of (b), the explanation of the qualities of bodies, Locke typically refrains from invoking the concept of ‘real essence’ and instead talks about the primary, secondary and tertiary qualities of bodies (see, for example II.viii.10–26). So, at a minimum, it seems that Locke invoked this distinction between real and nominal essences in order to prove a negative point: real essences are not relevant to role (a).