 One of the central issues in Book III has to do with classification. On what basis do we divide things into kinds and organize those kinds into a system of species and genera? In the Aristotelian and Scholastic tradition that Locke rejects, necessary properties are those that an individual must have in order to exist and continue to exist. These contrast with accidental properties. Accidental properties are those that an individual can gain and lose and yet continue in existence. If a set of necessary properties is shared by a number of individuals, that set of properties constitutes the essence of a natural kind. The borders between kinds are supposed to be sharp and determinate. The aim of Aristotelian science is to discover the essences of natural kinds. Kinds can then be organized hierarchically into a classificatory system of species and genera. This classification of the world by natural kinds will be unique and privileged because it alone corresponds to the structure of the world. This doctrine of essences and kinds is often called Aristotelian essentialism. Locke rejects a variety of aspects of this doctrine. He rejects the notion that an individual has an essence apart from being treated as belonging to a kind. He also rejects the claim that there is a single classification of things in nature that the natural philosopher should seek to discover. He holds that there are many possible ways to classify the world each of which might be particularly useful depending on one’s purposes.