CycL


CycL in computer science and artificial intelligence is an ontology language used by Doug Lenat's Cyc artificial intelligence project. Ramanathan V. Guha was instrumental in the design of early versions of the language. There is a close variant of CycL known as MELD.
The original version of CycL was a frame language, but the modern version is not. Rather, it is a declarative language based on classical first-order logic, with extensions for modal operators and higher order quantification.
CycL is used to represent the knowledge stored in the Cyc Knowledge Base, available from Cycorp. The source code written in CycL released with the OpenCyc system is licensed as open source, to increase its usefulness in supporting the semantic web.

Basic ideas

CycL has some basic ideas:
The concept names in Cyc are known as constants. Constants start with "#$" and are case-sensitive. There are constants for:
The most important predicates are #$isa and #$genls. The first one describes that one item is an instance of some collection, the second one that one collection is a subcollection of another one. Facts about concepts are asserted using certain CycL sentences. Predicates are written before their arguments, in parentheses:
For example:
"Bill Clinton belongs to the collection of U.S. presidents" and
"All trees are plants".
"Paris is the capital of France."

Rules

Sentences can also contain variables, strings starting with "?". One important rule asserted about the #$isa predicate reads

)
)
with the interpretation "if OBJ is an instance of the collection SUBSET and SUBSET is a subcollection of SUPERSET, then OBJ is an instance of the collection SUPERSET".
Another more complicated example is one that expresses a rule about a group or category rather than any particular individual, is:

which means that for every instance of the collection #$ChordataPhylum, there exists a female animal which is its mother.

Microtheories

The knowledge base is divided into microtheories, collections of concepts and facts typically pertaining to one particular realm of knowledge. Unlike the knowledge base as a whole, each microtheory is required to be free from contradictions. Each microtheory has a name which is a regular constant; microtheory constants contain the string "Mt" by convention. An example is #$MathMt, the microtheory containing mathematical knowledge. The microtheories can inherit from each other and are organized in a hierarchy:
one specialization of #$MathMt is #$GeometryGMt, the microtheory about geometry.