- From: Sandro Hawke <sandro@w3.org>
- Date: Thu, 04 Apr 2002 12:52:15 -0500
- To: www-rdf-logic@w3.org
This is an old issue. Pat Hayes explained very nicely a year ago [1] that RDF M&S has always had existentially-quantified variables (in the subject role), and to take them out would be a change. Many others (including myself at the time) supported this position, including the RDF Core WG. Some agreed that's what M&S said, others simply thought existentials would be good to have. But I want to revisit the question more fundamentally. What good are existentially quantified variables in a language like RDF? [2] Why would we possibly want them? I no longer know a good reason. Here are four reasons I have heard for them, and my counter-arguments. Reason 1: Because people often express themselves without giving URIs for things. Using pronouns like "something" feels like a very natural and comfortable way to communicate. Response: While we may not explicitely have names for things, the ability of natural language to "go meta" means you can always talk about "the thing you meant when you said '...'". So we can still identify the denotations of terms used in conversations, even if no explicit identification was given at the time. Alternatively, who cares. This is a formal system, and any appeals to human qualities are likely to be misleading anyway. Reason 2: We were getting a lot of noisy URIs in our knowledge bases. Response: This isn't really a model-theory issue. A best-practices approach might be to suggest that urn:uuid's be used for any identifiers which are both not meant for humans and not tied to any on-the-web knowledge base. User interfaces shouldn't be showing URIs anyway, but they could hide these even more. Another approach is to have meta-data, with triples saying that the URI "urn:uuid:57c259ca-47ed-11d6-8002-0050ba4812a6" is not meant for humans, or something. (Note that such an assertion must be about the above string, not the object denoted by that string. That object may be denoted by many URIs, some of which are prettier than others.) Reason 3: Because we can use existential variables for querying RDF. (Pat suggests this in [1]. It was my reason a year ago.) Response: I've seen a lot of RDF query languages now, and this approach is not being used. In my own query language designs, I've found them unsatisfactory. Reason 4: One should be able to commit to a statement like "Someone on E-Bay is selling a signed copy of 'Weaving the Web'." That statement is different from "Someone on E-Bay is selling the signed copy of 'Weaving the Web' which has the URI http://example.com/copy7". The second one entails the first, of course. Response: Why do you want to make the first statement? What bad thing will happen if you call it <urn:uuid:57c259ca-47ed-11d6-8002-0050ba4812a6> ? Are you worried that someone else will say something about <urn:uuid:57c259ca-47ed-11d6-8002-0050ba4812a6>? Like "it's missing page 7?" But they could do that anyway (with existentials) by using various more-expressive ontologies, perhaps talking about all the books on E-Bay, or talking about the terms in your original statement. When you used a URI for E-Bay, were you committing to any facts? Which ones? I think that's the real issue here. I think a URI enables a client to learn more about a thing, but the use of a URI in one document cannot be taken to mean that someone asserting the document believes any facts outside the document. ... but then how do you know what was meant? If I write you a check for $100, how do you know if I meant $US or $Canadian? The fuzzy vision of the Semantic Web involves the meaning of URIs being established by some kind of consensus reality, I think. But how does one commit to a statement in such fuzzy terms? Anyway, maybe existentials will be useful as being free from consensus reality issues, but I don't see how. I want to see the argument you present to a judge when you're suing someone for breech-of-RDF-assertion. Maybe it comes down to the natural-language comments in the ontology. Do you need to explicitely commit to an ontology, or just use terms from it. What if two ontologies exist with the same URIs? Which one are you committing to? I think the only solid ground is to explicitely import facts (including natural-language documentation) from some URI, giving a secure hash of the contents if you're worried it might change. (but that's not the way RDF is being used today...) The reason to leave them out is that they complicate things. Its worth noting, too, that if you really want to make an existential statement, you can do it in some higher-layer logic language. Of course, it wont be understood by as wide an audience. But that brings us back to original question: why would you want someone to understand it? -- sandro [1] http://lists.w3.org/Archives/Public/w3c-rdfcore-wg/2002Apr/0014.html [2] I don't want to get too into what a language-like-RDF might mean, so let's just say RDF/XML without anonymous resources, or N-Triples without namedNodes, and hope that gets the point across.
Received on Thursday, 4 April 2002 12:53:39 UTC