Another Word For It Patrick Durusau on Topic Maps and Semantic Diversity

February 7, 2013

The Semantic Web Is Failing — But Why? (Part 2)

Filed under: RDF,Semantic Web — Patrick Durusau @ 4:30 pm

Should You Be Using RDF?

Pay Hayes (editor of RDF Semantics) and Richard Cyganiak (a linked data expert), had this interchange on the RDF Working Group discussion list:

Cyganiak: The text stresses that the presence of an ill-typed literals does not constitute an inconsistency.

Cyganiak: But why does the distinction matter?

Hayes: I am not sure what you mean by “the distinction” here. Why would you expect that an ill-typed literal would produce an inconsistency? Why would the presence of an ill-typed literal make a triple false?

Cyganiak: Is there any reason anybody needs to know about this distinction who isn’t interested in the arcana of the model theory?

Hayes: I’m not sure what you consider to be “arcana”. Someone who cannot follow the model theory probably shouldn’t be using RDF. (emphasis added) Re: Ill-typed vs. inconsistent? (Mon, 12 Nov 2012 01:58:51 -0600)

When challenged on the need to follow model theory, Hayes retreats, but only slightly:

Well, it was rather late and I had just finished driving 2400 miles so maybe I was a bit abrupt. But I do think that anyone who does not understand what “inconsistent” means should not be using RDF, or at any rate should only be using it under the supervision of someone who *does* know the basics of semantic notions. Its not like nails versus metallurgy so much as nails versus hammers. If you are trying to push the nails in by hand, you probably need to hire a framer. (emphasis added) Re: Ill-typed vs. inconsistent? (Mon, 12 Nov 2012 09:58:52 -0600)

A portion of the Introduction to RDF Semantics reads:

RDF is an assertional language intended to be used to express propositions using precise formal vocabularies, particularly those specified using RDFS [RDF-VOCABULARY], for access and use over the World Wide Web, and is intended to provide a basic foundation for more advanced assertional languages with a similar purpose. The overall design goals emphasise generality and precision in expressing propositions about any topic, rather than conformity to any particular processing model: see the RDF Concepts document [RDF-CONCEPTS] for more discussion.

Exactly what is considered to be the ‘meaning’ of an assertion in RDF or RDFS in some broad sense may depend on many factors, including social conventions, comments in natural language or links to other content-bearing documents. Much of this meaning will be inaccessible to machine processing and is mentioned here only to emphasize that the formal semantics described in this document is not intended to provide a full analysis of ‘meaning’ in this broad sense; that would be a large research topic. The semantics given here restricts itself to a formal notion of meaning which could be characterized as the part that is common to all other accounts of meaning, and can be captured in mechanical inference rules.

This document uses a basic technique called model theory for specifying the semantics of a formal language. Readers unfamiliar with model theory may find the glossary in appendix B helpful; throughout the text, uses of terms in a technical sense are linked to their glossary definitions. Model theory assumes that the language refers to a ‘world‘, and describes the minimal conditions that a world must satisfy in order to assign an appropriate meaning for every expression in the language. A particular world is called an interpretation, so that model theory might be better called ‘interpretation theory’. The idea is to provide an abstract, mathematical account of the properties that any such interpretation must have, making as few assumptions as possible about its actual nature or intrinsic structure, thereby retaining as much generality as possible. The chief utility of a formal semantic theory is not to provide any deep analysis of the nature of the things being described by the language or to suggest any particular processing model, but rather to provide a technical way to determine when inference processes are valid, i.e. when they preserve truth. This provides the maximal freedom for implementations while preserving a globally coherent notion of meaning.

Model theory tries to be metaphysically and ontologically neutral. It is typically couched in the language of set theory simply because that is the normal language of mathematics – for example, this semantics assumes that names denote things in a set IR called the ‘universe‘ – but the use of set-theoretic language here is not supposed to imply that the things in the universe are set-theoretic in nature. Model theory is usually most relevant to implementation via the notion of entailment, described later, which makes it possible to define valid inference rules.

Readers should read RDF Semantics to answer for themselves whether they understand “inconsistent” as defined therein. Noting that Richard Cyganiak, a linked data expert, did not.

The next series starts with Saving the “Semantic” Web (Part 1)

3 Comments

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