 The system of semantic inheritance networks, developed by Horty, Thomason, and Touretzky (1990), is similar to Pollock’s system. Both represent cognitive states by means of directed graphs, with links representing defeasible inferences. The semantic inheritance network theory has a intentionally narrower scope: the initial nodes of the network represent particular individuals, and all non-initial nodes represent kinds, categories or properties. A link from an initial (individual) node to a category node represents simply predication: that Felix (initial node) is a cat (category node), for example. Links between category nodes represent defeasible or generic inclusion: that birds (normally or usually) are flying things. To be more precise, there are both positive (“is a”) and negative (“is not a”) links. The negative links are usually represented by means of a slash through the body of the arrow.