1. A method for using indexes to search a knowledge base in at least one computer system, the method executed by the computer system and comprising: accessing a knowledge base comprising a semantic network of relationships among concepts, each concept having a set of values associated with it, wherein each concept is an instance of a category and each value is an instance of an attribute and wherein categories and attributes form a knowledge base schema; receiving, from a user searching for one or more target concepts, a query represented as criteria and criteria values that specify constraints on the categories and the attributes; executing a search of the concepts and the values of the knowledge base to retrieve the one or more target concepts using indexes selected from a group consisting of: a) one or more transitive indexes that index transitive relationships from concepts in a first category through at least one second category to concepts in a third category and b) one or more transitive closure indexes that index transitive closure relationships amongst concepts within a category; and retrieving a result subset of the target concepts and the values that satisfies the criteria and criteria values.