  In characterizing the nature of difference, the Stranger compares it to knowledge (Sophist 257c–d) (on this analogy, cf. Lee 1972). Knowledge is a categorial kind, and many species of knowledge differ structurally from one another (cf. below § 7.2). Even so, the comparison is instructive because some branches of knowledge, such as applied mathematics, are distinguished from one another, not by intrinsic differences in the structure of the expertise, but simply by differences in the objects to which the same expertise applies (e.g., calculation in surveying and navigation). Like varieties of applied mathematics, whose content derives from the domain to which the knowledge applies, parts of difference acquire their content from the item whose name is negated.