# Common Logic
**Common Logic** (CL) is a family of logic-based languages standardized as ISO/IEC 24707, designed for the representation and interchange of information among disparate computer systems.[1](https://www.iso.org/standard/66249.html) The standard defines an abstract syntax and an associated model-theoretic semantics for a specific extension of first-order logic, and every language in the family can be translated by a semantics-preserving transformation into a common XML-based syntax.[1](https://www.iso.org/standard/66249.html) Its purpose is interoperability: systems that use different knowledge-representation notations can exchange content through a shared logical core.[2](https://jfsowa.com/ikl/Hayes04.pdf)
| Key fact | Detail |
|---|---|
| Standard | ISO/IEC 24707, first published October 2007, revised as ISO/IEC 24707:2018; confirmed in 2023[1](https://www.iso.org/standard/66249.html) • [3](https://web.archive.org/web/20090301101023/http:/cl.tamu.edu/) |
| What it defines | An abstract syntax and model-theoretic semantics for an extension of first-order logic, plus a common XML-based interchange syntax[1](https://www.iso.org/standard/66249.html) |
| Concrete dialects | CLIF (LISP-like, based on KIF), CGIF (conceptual graphs), and XCL (XML), specified as annexes[4](https://www.iso.org/standard/39175.html) |
| Expressive power | Signature-free syntax allowing quantification over classes and relations while preserving a first-order model theory; theories can describe intensional entities such as classes or properties[4](https://www.iso.org/standard/39175.html) |
| Names | In CLIF, any character string can be a name, any name can play any syntactic role, and any name can be quantified[5](https://jfsowa.com/ikl/HayesST08.pdf) |
| Relation to the semantic web | More expressive than RDF, OWL Lite, and OWL DL; RDF, RDFS, and OWL all translate straightforwardly into CL[6](https://ceur-ws.org/Vol-352/paper3.pdf) • [7](https://ihmc.us/users/phayes/CL/SW2SCL.html) |
| Maintenance | Confirmed at stage 90.93 after a systematic review closed 2023-12-04; Edition 2 development continues[1](https://www.iso.org/standard/66249.html) • [8](https://github.com/gruninger/Common-Logic) |
## Origins and standardization
Common Logic arose from two separate projects that were developing parallel ANSI standards for [conceptual graphs](https://www.edgepedia.org/conceptual-graph) and the Knowledge Interchange Format (KIF). These two projects eventually merged into a single ISO project.[9](https://shura.shu.ac.uk/45/1/fulltext.pdf) The name "Common Logic" was chosen deliberately to reduce bias toward the two starting notations and to emphasize a common basis in first-order logic.[10](https://xml.coverpages.org/CommonLogicStandardAnnounce.html)
**Working group and editorship.** In 2000, Michael Grüninger convened a working group with the mandate of combining KIF and Conceptual Graphs into an international standard, and served as project editor from 2000 to 2003.[11](http://stl-fs.mie.utoronto.ca/projects/common.html) The ISO effort began in June 2003 with approval of a New Work Item assigned to WG2 (Metadata) under SC32 (Data Interchange) of ISO/IEC JTC1.[3](https://web.archive.org/web/20090301101023/http:/cl.tamu.edu/) • [12](http://www.omg.org/archives/businessrules/msg02012.html) In October 2003, Harry Delugach, a computer scientist at the [University of Alabama in Huntsville](https://www.edgepedia.org/university-of-alabama-in-huntsville), was designated editor for the standard, and the effort ended on October 1, 2007 with publication as ISO 24707.[3](https://web.archive.org/web/20090301101023/http:/cl.tamu.edu/)
The project grew out of the KIF and CG collaboration but was hoped to serve many other languages with declarative semantics, such as RDF, UML, DAML, and [Topic Maps](https://www.edgepedia.org/topic-maps); any language mappable to and from the CL abstract syntax inherits the same model-theoretic semantics and the interoperability that comes with it.[13](https://lists.w3.org/Archives/Public/www-webont-wg/2002Apr/0061.html)
## The three syntaxes: CLIF, CGIF, and XCL
The standard prescribes a common abstract semantics rather than a single syntax. Any dialect that provides the prescribed syntax and semantics is eligible for conformance, and the published standard specifies three concrete syntaxes as annexes:[6](https://ceur-ws.org/Vol-352/paper3.pdf)
- **CLIF** (Common Logic Interchange Format), based on KIF and resembling LISP syntax, in Annex A.[6](https://ceur-ws.org/Vol-352/paper3.pdf)
- **CGIF** (Conceptual Graph Interchange Format), a textual form of conceptual graphs designed to be parseable in one pass, in Annex B.[6](https://ceur-ws.org/Vol-352/paper3.pdf) CGIF was itself an early result of the Common Logic effort, a new textual form of the graph notation.[6](https://ceur-ws.org/Vol-352/paper3.pdf)
- **XCL** (XML for Common Logic), an XML dialect developed for the standard, in Annex C.[6](https://ceur-ws.org/Vol-352/paper3.pdf)
The plan changed along the way: an early proposal called for grammars for KIF, CGIF, and traditional predicate calculus (TPC) with a Unicode encoding, but the published standard settled on CLIF, CGIF, and XCL.[10](https://xml.coverpages.org/CommonLogicStandardAnnounce.html) • [4](https://www.iso.org/standard/39175.html) The abstract syntax deliberately leaves concrete notation and ordering undefined, so it does not favor prefix, infix, or graph notations.[10](https://xml.coverpages.org/CommonLogicStandardAnnounce.html)
## Semantics and expressive power
Common Logic's syntax is signature-free: it has no built-in distinction between names for individuals, names for predicates, and names for functions. This permits "higher-order" constructions such as quantification over classes or relations while preserving a first-order model theory, and the semantics allows theories to describe intensional entities such as classes or properties.[4](https://www.iso.org/standard/39175.html) The result is a deliberate relaxation of classical first-order logic, in some ways going beyond it.[2](https://jfsowa.com/ikl/Hayes04.pdf)
**Names and denotation.** The treatment of names is unusually liberal. In CLIF, any character string can be a name, any name can play any syntactic role, and any name can be quantified; anything that can be named can be the value of a function.[5](https://jfsowa.com/ikl/HayesST08.pdf) The language is fully type-free by default, with no syntactic or semantic typing, and CL dialects with their own restricted syntaxes interoperate through the common abstract syntax; CL can also map to and from controlled natural languages such as CLCE (Common Logic Controlled English).[9](https://shura.shu.ac.uk/45/1/fulltext.pdf)
The design goals recorded by Pat Hayes, a logician at the [Institute for Human and Machine Cognition](https://www.edgepedia.org/institute-for-human-and-machine-cognition) and a CL editor, were: a common interlingua for a variety of knowledge-representation notations; a syntax as unconstrained as possible; semantics as simple and conventional as possible; full first-order logic with equality at least; and web-savviness, meaning an XML framework for encoding and transmitting logic on open networks.[2](https://jfsowa.com/ikl/Hayes04.pdf)
## Dialects and conformance
Conformance in ISO/IEC 24707 is defined against the abstract standard, not against any one concrete notation. The abstract syntax was designed at a level of abstraction satisfiable by many concrete languages, such as KIF, Conceptual Graphs, and Principia-style mathematical syntax; compliance of a language is demonstrated by mapping it into the abstract standard.[14](https://www.ihmc.us/users/phayes/CL/CommonLogic-AbstractSyntax.html) A compliant language must specify its lexicon's sets of constants, ordinary variables, and sequence variables, and demonstrate that these sets are pairwise disjoint; a language is CL compliant if it is a CL language, or indirectly compliant relative to some CL language.[14](https://www.ihmc.us/users/phayes/CL/CommonLogic-AbstractSyntax.html)
The standard specifies semantics but leaves proof theory to users: languages in the family are logically comprehensive, providing for the expression of arbitrary first-order logical sentences, and computability issues such as efficiency and optimization are not addressed.[1](https://www.iso.org/standard/66249.html) Clarifying the conformance conditions was one of the issues taken up for the second edition.[15](http://ontolog.cim3.net/file/work/RulesReasoningLP/2014-01-09_Rule-Standards/CommonLogic-v2--MichaelGruninger_20140109.pdf)
## Comparison with OWL and the semantic web stack
Common Logic occupies a different point in the expressiveness/decidability trade-off from the W3C stack. Since RDF expresses only binary relations, CL is more expressive; CL is also more expressive than OWL Lite and OWL DL, which are decidable subsets of first-order logic.[6](https://ceur-ws.org/Vol-352/paper3.pdf) In expressive power, OWL Full matches CL, yet it lacks the advantage that comes from a hundred years of research into handling first-order logic.[6](https://ceur-ws.org/Vol-352/paper3.pdf) Because of CL's expressiveness, the meaning of any RDF, OWL Lite, or OWL DL representation can be preserved when translated into CL.[6](https://ceur-ws.org/Vol-352/paper3.pdf) RDF, RDFS, and OWL can all be translated straightforwardly into Common Logic, with full translations given.[7](https://ihmc.us/users/phayes/CL/SW2SCL.html)
The comparison extends to engineering practice. Relatively minor changes to CL axioms yield theories corresponding to various [description logics](https://www.edgepedia.org/description-logic), OWL-Full, RDF, and RDFS, using one common syntax and one fixed logic; semantic decisions can be recorded as CL axioms and checked mechanically.[5](https://jfsowa.com/ikl/HayesST08.pdf) CLIF texts also retain their full meaning when transported across a network, archived, or freely combined with other CLIF texts, which CL's editors contrast with OWL-DL.[5](https://jfsowa.com/ikl/HayesST08.pdf) The model theory itself has semantic-web descendants: Hayes and Chris Menzel defined a very general model theory for CL, which Hayes and Graham McBride used to define the semantics for RDF and OWL.[9](https://shura.shu.ac.uk/45/1/fulltext.pdf)
An alternative approach to the same interoperability problem is OMG's Distributed Ontology, Model, and Specification Language (DOL), which does not propose a subsuming language but instead provides a formal language for expressing mappings between ontologies formalized in different languages, on a sound formal semantic basis, making them amenable to checks such as consistency and conservativity.[16](https://www.omg.org/dol/)
## Implementations and use in practice
Tools listed in the second-edition work include the Heterogeneous Tool Set (HETS), the Samian Platform from Kojeware Corp, MACLEOD, OntologyPortal, and RECON, with COLORE and OntoHub as ontology repositories.[15](http://ontolog.cim3.net/file/work/RulesReasoningLP/2014-01-09_Rule-Standards/CommonLogic-v2--MichaelGruninger_20140109.pdf) As of September 2012, the COLORE repository contained over 600 CL ontologies, according to Grüninger.[17](http://ontolog.cim3.net/file/work/CommonLogic/2012-09-20_Revision-of-CL/revision-of-CL_chat-transcript_unedited_20120920a.txt) COLORE specifies all of its ontologies using Common Logic (ISO 24707), described as a standardized logical language for first-order ontologies and knowledge bases, with the objective of an open repository serving as a testbed for ontology evaluation and integration techniques.[18](https://github.com/gruninger/colore)
Common Logic also serves as a specification language for other standards. It is used to specify the axioms of the Process Specification Language (ISO 18629), OMG's Date-Time Vocabulary and Foundational UML, the W3C Semantic Web Services Framework, and OntoIOp (ISO WD 17347).[15](http://ontolog.cim3.net/file/work/RulesReasoningLP/2014-01-09_Rule-Standards/CommonLogic-v2--MichaelGruninger_20140109.pdf) The University of Toronto project page notes that numerous editors and theorem provers have been implemented to support CL in connection with the Process Specification Language.[11](http://stl-fs.mie.utoronto.ca/projects/common.html)
## Recent developments
The 2018 edition remains current. ISO/IEC 24707:2018 was confirmed at stage 90.93 following a systematic review that closed on 2023-12-04, meaning no revision was set in motion at that review.[1](https://www.iso.org/standard/66249.html) The US national adoption, INCITS/ISO/IEC 24707:2018, was reaffirmed in 2023, retaining the 2018 text's declarative semantics, logical comprehensiveness, and XML-based syntax-preserving translation requirements.[19](https://webstore.ansi.org/standards/incits/incitsisoiec2470720182023) Work on a second edition continues outside the formal publication track: a GitHub repository maintained by Grüninger documents the development of ISO 24707 Edition 2.[8](https://github.com/gruninger/Common-Logic)
The second edition's technical agenda, laid out in the 2014 working draft, includes definitional extensions, which become possible once names can be restricted out of the universe of discourse; domain restrictions, which enable the integration of ontologies even if they do not share the same universe of discourse; circular imports, allowed syntactically and well-defined semantically; cl-module semantics; segregated dialects and interoperability; conformance clarification; and namespacing.[15](http://ontolog.cim3.net/file/work/RulesReasoningLP/2014-01-09_Rule-Standards/CommonLogic-v2--MichaelGruninger_20140109.pdf) As of that working draft, the CLIF, CGIF, and XCL concrete syntaxes had not yet been updated to reflect the new abstract syntax and semantics.[15](http://ontolog.cim3.net/file/work/RulesReasoningLP/2014-01-09_Rule-Standards/CommonLogic-v2--MichaelGruninger_20140109.pdf)
## Points of disagreement
The liberal design has drawn technical criticism on specific points rather than in the aggregate. A NIST publication on module semantics states that modules play a crucial role in ISO/IEC 24707 because they enable users to reuse ontologies and axiom sets, but that the semantics of modules as presented in the standard is surprisingly hard to define.[20](https://www.nist.gov/publications/semantics-modules-common-logic) Module semantics is accordingly one of the items on the second edition's agenda.[15](http://ontolog.cim3.net/file/work/RulesReasoningLP/2014-01-09_Rule-Standards/CommonLogic-v2--MichaelGruninger_20140109.pdf) The choice of concrete dialects also shifted during standardization, from a planned KIF/CGIF/TPC trio to the published CLIF/CGIF/XCL trio.[10](https://xml.coverpages.org/CommonLogicStandardAnnounce.html) • [4](https://www.iso.org/standard/39175.html)
## References
1. [ISO/IEC 24707:2018 — Information technology — Common Logic (CL) — A framework for a family of logic-based languages, ISO](https://www.iso.org/standard/66249.html)
2. [Common Logic (Pat Hayes, 2004)](https://jfsowa.com/ikl/Hayes04.pdf)
3. [Common Logic Working Group Documents (archived official project site)](https://web.archive.org/web/20090301101023/http:/cl.tamu.edu/)
4. [ISO/IEC 24707:2007 — Information technology — Common Logic (CL), ISO](https://www.iso.org/standard/39175.html)
5. [Using Common Logic (Hayes, ST08)](https://jfsowa.com/ikl/HayesST08.pdf)
6. [Towards Conceptual Structures Interoperability Using Common Logic, CEUR-WS Vol-352](https://ceur-ws.org/Vol-352/paper3.pdf)
7. [Translating Semantic Web languages into Common Logic (IHMC)](https://ihmc.us/users/phayes/CL/SW2SCL.html)
8. [gruninger/Common-Logic, GitHub](https://github.com/gruninger/Common-Logic)
9. [A commentary on standardization in the Semantic Web, Common Logic and MultiAgent Systems, Sheffield Hallam University](https://shura.shu.ac.uk/45/1/fulltext.pdf)
10. [The Cover Pages: Common Logic Standard Proposed](https://xml.coverpages.org/CommonLogicStandardAnnounce.html)
11. [STL Lab — Common Logic, University of Toronto](http://stl-fs.mie.utoronto.ca/projects/common.html)
12. [The ISO Common-Logic Framework project, OMG archive](http://www.omg.org/archives/businessrules/msg02012.html)
13. [NEWS: Common Logic, W3C webont mailing list, April 2002](https://lists.w3.org/Archives/Public/www-webont-wg/2002Apr/0061.html)
14. [Common Logic: Abstract Syntax and Semantics (Pat Hayes, 2002 working document)](https://www.ihmc.us/users/phayes/CL/CommonLogic-AbstractSyntax.html)
15. [Towards the Second Edition of ISO 24707 Common Logic (Michael Grüninger, 2014)](http://ontolog.cim3.net/file/work/RulesReasoningLP/2014-01-09_Rule-Standards/CommonLogic-v2--MichaelGruninger_20140109.pdf)
16. [Distributed Ontology, Model, and Specification Language (DOL), OMG](https://www.omg.org/dol/)
17. [Revision of CL chat transcript (2012-09-20), Ontolog forum](http://ontolog.cim3.net/file/work/CommonLogic/2012-09-20_Revision-of-CL/revision-of-CL_chat-transcript_unedited_20120920a.txt)
18. [gruninger/colore — COLORE project, GitHub](https://github.com/gruninger/colore)
19. [INCITS/ISO/IEC 24707:2018 (2023), ANSI webstore](https://webstore.ansi.org/standards/incits/incitsisoiec2470720182023)
20. [The Semantics of Modules in Common Logic, NIST](https://www.nist.gov/publications/semantics-modules-common-logic)
---
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