- From: David Booth <david@dbooth.org>
- Date: Fri, 18 Mar 2011 14:44:51 -0400
- To: Kjetil Kjernsmo <kjekje@ifi.uio.no>
- Cc: public-rdf-dawg-comments@w3.org, Tim Berners-Lee <timbl@w3.org>, SW-forum Web <semantic-web@w3.org>
On Fri, 2011-03-18 at 13:43 +0100, Kjetil Kjernsmo wrote: > [ . . . ] > What does the URI http://www.kjetil.kjernsmo.net/foaf > identify? Apart from a foaf:PersonalProfileDocument, is it an RDF Graph or an > RDF Document? The key problem is that you are making the implicit assumption that the URI identifies exactly the same thing in all RDF graphs. I have been calling this myth #1: http://dbooth.org/2010/ambiguity/paper.html#myth1 This is only true for *one* interpretation http://www.w3.org/TR/rdf-mt/#interp of *one* graph. (And by "interpretation" I'm talking about interpretations as described in the RDF Semantics.) Unless you have explicit disjointness assertions, in one graph the URI could ambiguously identify a foaf:PersonalProfileDocument, an RDF Graph and an RDF Document. But in another graph that adds a few disjointness assertions, it may identify only a foaf:PersonalProfileDocument. And in a third graph that adds *different* disjointness assertions, it may identify only an RDF Document. This is illustrated in this section: http://dbooth.org/2010/ambiguity/paper.html#inconsistent-merge This does *not* exactly mean that the meaning of a URI is "context dependent". A URI is global in scope and (should) have a globally scoped identity. But the fact is that above and beyond any globally scoped identity, different graphs may *further* constrain the URI's identity in different ways. That's what happens when you add more assertions to a graph, as illustrated in Figure 10a: http://dbooth.org/2010/ambiguity/paper.html#figure-10a This is not a design flaw of semantic web architecture, it is an inescapable fact of life, and we need to learn to live with. And, aside from outright errors, this is a key reason why joining datasets with owl:sameAs often leads to inconsistencies. See http://dbooth.org/2010/ambiguity/paper.html#sameAs [ . . . ] > Surely, a resource can't be both an RDF Document and an RDF Graph? As long as the classes of RDF Documents and RDF Graphs are not declared disjoint in your graph, then a URI certainly *can* (ambiguously) denote both an RDF Document and an RDF Graph. Ambiguity of reference is an inescapable fact of life. See "In Defense of Ambiguity" by Pat Hayes and Harry Halpin: http://www.ibiblio.org/hhalpin/homepage/publications/indefenseofambiguity.html It does *not* necessarily mean that anyone was sloppy or erroneous. Rather, it means that there are limits to our ability to avoid ambiguity, and this has far reaching consequences in semantic web architecture -- consequences that underly issues that have plagued the community for years, ranging from the seemingly endless debates of the TAG's httpRange-14 issue to more recent criticisms of owl:sameAs. -- David Booth, Ph.D. http://dbooth.org/ Opinions expressed herein are those of the author and do not necessarily reflect those of his employer.
Received on Friday, 18 March 2011 18:45:24 UTC