 The notions of “same” or “similar” (tóng 同) and “different” (yì 異) lie at the heart of later Mohist thought. As we have seen, they are a core part of the explanation of how words refer to things and how we use language to communicate. They determine what counts as correct “describing,” “sorting,” and “matching” of names and things and thus what knowledge is. The issues of how to distinguish the “same” from “different” kinds of things and how to identify particular individuals as belonging to one kind or another are central issues in Mohist semantics and epistemology, and through them, in the Mohist theory of argumentation as well. 