  For domains in which complex definitions are a natural way to organize information, knowledge engineering services based on definitions of concepts have been extremely successful. Like variable-free versions of first-order logic (see, for instance,  Quine 1960,  these systems are centered on concepts or first-order predicates, and provide a number of mechanisms for their definition. The fundamental algorithm associated with these taxonomic logics is a classifier which inputs a system of definitions and outputs the entailment relations between defined and primitive concepts. For background on these systems, see  Woods &amp; Schmolze 1992  and  Brachman et al. 1991.