A collection of collections, and a specialization of type of relationship. Each instance of predicate category is a collection of Predicates. The predicates in a given predicate category represented in the Knowledge Base are typically those sharing some common feature(s) considered significant enough that the collection of all such predicates is useful to reify. Although what constitutes a significant common feature is difficult to specify precisely, there are many clear-cut cases. Instances of predicate category include temporal part slot, interval-based quantity measure, predicate that is functional in its second argument, inter-actor predicate, transitive binary predicate, and propositional attitude predicate. See also the comment on predicate.