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. This knowledge is represented in a formal language, CycL. The KB consists of terms and assertions which relate those terms. These assertions include both simple facts (i.e., ground assertions) and rules.
The Cyc KB is divided into many (currently thousands of) “contexts” (or “microtheories”), each of which is essentially a collection of assertions that share a common set of assumptions; some microtheories are focused on a particular domain of knowledge, some a particular interval in time, some a particular level of detail, 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. The Cyc Knowledge Base is so large that it can seem unwieldy and difficult to navigate at first glance, but the microtheories are themselves first class Cyc KB objects, and navigating their ontology is a powerful tool for getting to the right “region” of the Cyc KB quickly.
At the present time, the Cyc KB contains almost 2 million terms, including about 42,000 types of relations, and about 24,500,000 assertions relating these terms. New assertions are continually added to the KB through a combination of automated and manual means. Many more concepts can be expressed functionally, thereby enabling the automatic creation of millions of non-atomic terms, such as (LiquidFn Nitrogen) being used to describe liquid nitrogen. Additionally, Cyc adds a vast number of assertions to the KB by itself as a result of the inferencing process.