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The Cyc Knowledge Base
The Cyc knowledge base (KB) is a formalized representation of a vast quantity of fundamental human knowledge: facts, rules of thumb, and heuristics for reasoning about the objects and events of everyday life. The medium of representation is the formal language CycL, described below. The KB consists of terms--which constitute the vocabulary of CycL--and assertions which relate those terms. These assertions include both simple ground assertions and rules. Cyc is not a frame-based system: the Cyc team thinks of the KB instead as a sea of assertions, with each assertion being no more "about" one of the terms involved than another.
The Cyc KB is divided into many (currently hundreds of) "microtheories", each of which is essentially a bundle of assertions that share a common set of assumptions; some microtheories are focused on a particular domain of knowledge, a particular level of detail, a particular interval in time, etc. The microtheory mechanism allows Cyc to independently maintain assertions which are prima facie contradictory, and enhances the performance of the Cyc system by focusing the inferencing process.
At the present time, the Cyc KB contains tens of thousands of terms and several dozen hand-entered assertions about/involving each term. New assertions are continually added to the KB by human knowledge enterers. The aforementioned numbers do not include (i) non-atomic terms such as (#$LiquidFormOf #$Nitrogen), nor (ii) the vast number