- From: Jeff Heflin <heflin@cse.lehigh.edu>
- Date: Thu, 25 Apr 2002 16:01:26 -0400
- To: Drew McDermott <drew.mcdermott@yale.edu>
- CC: www-rdf-logic@w3.org
Drew, Thanks for pointing out the terminology issue. When talking in "Web circles," I have a habit of using the term "definition" in a much looser way than understood by logicians and other KR people. I do this because people from other communities seem to grasp the term better than if I say "axiom" or "description." I usually hope that those who think of "definition" in the technical sense, realize that is not what I mean, but I should always be careful to point this out, just to avoid confusion. Also, I think your two points about importing ontologies being different from pointing to a web page are well-stated and I whole-heartedly agree with you. Jeff Drew McDermott wrote: > > [Jeff Heflin] > My personal opinion is that if you're using an ontology language, every > term you use must be defined in some ontology (even if only to say that > it is a class or property). > > I agree with the sentiment, but please let's not use the word > "definition" the way you and Dan B. are using it. Ontologies express > relationships among terms, but they almost never define them. People > colloquially speak as though a statement like "living things are > partitioned into vegetables and animals" is a definition of, > say, "vegetable" and "animal," because it is, in some sense, a > "declaration" of these symbols, and in the computer world it is > usually the case that declaring something is necessary and sufficient > to define it. But (as I know you know) in a KR language that is not > the case. In fact, as Pat Hayes has argued, it is hard to say what the > *logical* difference is between the "paritition" statement above and > the seemingly humbler statement that "Sally is a vegetable." > > But you're completely correct that importing an ontology is different > from pointing to a web page or even a set of assertions. At the risk > of repeating what you said, here are the two key reasons why: > > 1. The purpose of an ontology is to allow agents to draw conclusions, > and in particular to detect inconsistencies (e.g., type errors) in > datasets. A dataset without its associated ontology has no > rationale that I can see. If I give you a set of facts about > numbers, but I view Peano's axioms as an option that you can take > or leave, then the set of facts could be taken to be saying > anything at all about any topic at all; what in the world is the > point? > > 2. Ontologies are *small* and *internally consistent.* I think the > argument about importing assumes that once I start following > pointers to the web I could wind up anywhere. That may be true in > general, but it's not true for ontologies. Whoever designed an > imported ontology didn't just throw together some stuff they found > on Google. They had to think through many tough questions, and at > every stage slight changes from the design decisions taken would > have introduced subtle gaps or inconsistencies. Chances are the > designers had to backtrack several times as these pitfalls were > encountered. > > Point (2) has as a consequence that if my dataset imports Ont-1, I can > be confident that all the ontologies Ont-1 imports are, in the > designers' minds, *coherent pieces of Ont-1.* If they weren't, the > designers would have imported something else or built what they > needed. Furthermore, the chain of imports is likely to be shallow; > and if Ont-1 imports Ont-2 and Ont-3, following the import links from > Ont-2 and Ont-3 is likely to get you to a common ancestor more > frequently than chance would predict. Coherent theories just don't > look like balls of string. > > -- Drew McDermott
Received on Thursday, 25 April 2002 16:01:30 UTC