RDF Terminology and Concepts

Revised:
25 July 2001
Editor:
Graham Klyne
Contributors:
Graham Klyne
Brian McBride
Bill dehOra
Dan Brickley
Pat Hayes
 

This is currently a live working document, being a collection of suggestions from participants in the W3C RDF Interest and RDF Core Working Groups.

Terminology

RDFM&S, M&S:
The RDF Model and Syntax Specification.
Metadata:
Data describing Web resources [RDFM&S].
Web Resource:
Anything that is identified by a URI [RFC2396].
Entity:
Anything which exists or has existed. Note that RFC2396 uses this term in a more restricted sense, to mean some data represents some aspect of a Web Resource.
RDF Resource:
[See RDF M&S section 5] Note that an RDF resource is not necessarily a web resource, though any web resource can be an RDF resource.
Consider: http://foo.com/#a and http://foo.com/#b may name distinct RDF resources, but if used to access web resources they both refer to the common web resource http://foo.com/
[[[This distinction between "Web resource" and "RDF Resource" is not a desired outcome, but an interpretation of different uses of the term "resource" in different documents.]]]
Resource:
May refer to an RDF resource or a Web Resource. Some resources may be both. In discussion of RDF, this term is often used to mean RDF Resource.
RDF Resource Identifier, Resource Identifier:
A URI plus optional anchor ID.[RDFM&S]
RDF Resource Identifiers are understood to name RDF Resources.
RDF Statement, Statement:
[See RDFM&S section 5]
RDF Description:
[See RDFM&S] Construct containing representations of a number of RDF statements about a specific RDF resource, and possibly some additional statements.
Referent:
The entity or concept that an RDF Resource describes. [RDFM&S]
Distributive Referent:
A Referent that describes each of the Resources in a container, not including the container. The Referent is said to be made on the container. [RDFM&S]
Description [of]:
(As opposed to RDF Description) Language or data structure providing information about some entity or concept.
Stand for:
The use of one entity or concept in a description to refer to some other entity or concept. For example, "X stands for Y in Z" meaning that occurrences of "X" in "Z" are to be understood as references to "Y".
Reification (of a statement):
[See RDFM&S section 5] A resource that stands for the statement together with the four statements that describe the statement. More than one reification may exist for a given statement.
(There is some debate whether multiple reifications of a statement are necessarily equivalent.)
Reified Statement:
[See RDFM&S section 5] A resource that stands for a statement in a Reification. This resource has four properties describing the statement, and maybe others.
Reification Quad:
The four statements that make up a Reification.
Representation:
A data structure (abstract or concrete) that captures some essential properties of some entity or concept.
Representing [x]:
Being a representation of [x] (see above)
Context:
An environment within which some statements are taken to be true.
Quoting:
A reference to a statement without necessarily making any assertion about its truth or falsity.
Stating:
The expression of an RDF statement [or set of statements] in some context of discourse that is taken to be an assertion of the truth of the statement[s] in that context.
Model:
[See RDFM&S section 5].
This term is used in three distinct ways:
(a) The RDF Model, meaning the underlying structure and interpretation of RDF data
(b) An RDF Model, meaning a collection of RDF statements
(c) Logical Model, being a formal logicians' term with quite specific meaning. (see http://www-rci.rutgers.edu/~cfs/305_html/Deduction/FormalSystemDefs.html).
(This term has caused some confusion, since it has a quite specific meaning to logicians, which is not the same as some would regard as its "natural" meaning.)
RDF Graph:
A set of RDF Statements.
Reification of an RDF Graph:
A (bag/collection?) containing the reifications of the statements in an RDF Graph
Reified Graph:
The (bag/collection?) in the reification of an RDF graph.
Higher Order Statement:
A Statement whose Referent is another Statement.[RDFM&S]
Higher Order Context:
A Context whose Referent is another Context.
Denote
Denote. (v) The fundamental semantic relationship between the syntactic and semantic domains; the relationship between an expression and the entity it is interpreted to mean or refer to, expressed (somewhat misleadingly) as an activity of the expression oriented towards the thing. For example, a person can be said be denoted by their name.

Hence, denotation (n), the thing or things denoted by a name or expression.

Exactly what counts as a suitable denotation for some kinds of expression has been the subject of much debate, eg assertional sentences may be said to denote truth-values, or propositions, or functions from possible worlds, etc.. Typically, a given semantic theory for a human language takes a particular stance on such issues, providing a precise analysis of some range of intuitive meanings while excluding others from consideration. For formal languages, the range of denotations is usually specified mathematically. In formal semantics, an interpretation of a language is specified by rules which determine the denotations of complex expressions in terms of the denotations of their subexpressions, often called a truth-recursion.

Note. The relationship between a sign and what it denotes - the denotation relationship - is not considered to have any particular causal or physical significance, in general. Philosophers have noted that if denotation were a physical relationship then it would travel faster than light every time an astronomer mentions a star. Similarly, there is no way, in general, to compute the denotation of a name from the name itself. Both of these observations follow from the fact that the denotation of any expression is only defined relative to an interpretation of the language or notation in which the expression occurs.

Concepts

Reification

John McCarthy at http://www-formal.stanford.edu/jmc/generality/node6.html:

Reasoning about knowledge, belief or goals requires extensions of the domain of objects reasoned about. For example, a program that does backward chaining on goals used them directly as sentences, e.g. on(Block1,Block2), i.e. the symbol on is used as a predicate constant of the language. However, a program that wants to say directly that on(Block1,Block2) should be postponed until on(Block2,Block3) has been achieved, needs a sentence like precedes(on(Block2,Block3),on(Block1,Block2)), and if this is to be a sentence of first-order logic, then the symbol on must be taken as a function symbol, and on(Block1,Block2) regarded as an object in the first order language.

This process of making objects out of sentences and other entities is called reification. It is necessary for expressive power but again leads to complications in reasoning. It is discussed in (McCarthy 1979).

Tim Berners-Lee has acknowledged that the last paragraph is the intent of reification in RDF (http://lists.w3.org/Archives/Public/www-rdf-logic/2001Jan/0048.html).

[[[TBD]]]

RDF issues

Dan Brickley maintains a page of RDF specification issues at http://www.w3.org/2000/03/rdf-tracking/.

Brian McBride also has a list of issues at http://www-uk.hpl.hp.com/people/bwm/rdf/issues.htm. Against each listed issue are pointers to messages on the RDF IG mailing list, and/or other relevant threads of commentary.

Additional resources

Some related resources / context:

http://www.w3.org/Help/siteindex W3C site index / technology keywords

http://www.w3.org/People/Berners-Lee/Weaving/glossary.html Glossary from 'Weaving the Web'.

http://www.w3.org/WCA/ Web Characterisation Initiative (historical interest)

http://www.w3.org/1999/05/WCA-terms/ Web Characterization Terminology & Definitions Sheet W3C Working Draft 24-May-1999 HTTP-NG Activity Statement (historical interest)

http://www.w3.org/Protocols/HTTP-NG/Activity.html

http://www.w3.org/TR/WD-HTTP-NG-interfaces/ HTTP-NG Web Interfaces (an attempt to formalise our a notion of URIs, resources etc in terms of a distributed object type hierarchy).

http://www.w3.org/Addressing/ Naming and Addressing: URIs, URLs, ...

http://www.ietf.org/rfc/rfc2396.txt -- URIs

http://www.w3.org/Protocols/rfc2616/rfc2616.html -- for HTTP 1.1's notion of URI, 'resource', entity etc...

Acknowledgements

Insightful comments were provided by Pierre-Antoine Champin.

References

[RDFM&S]

[RFC2396]

[[[TBD]]]