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Re: subclasses (RDF vocabulary definitions)

From: Richard H. McCullough <rhm@cdepot.net>
Date: Fri, 22 Nov 2002 05:45:18 -0800
Message-ID: <002601c2922d$65770e00$bd7ba8c0@rhm8200>
To: "Jon Hanna" <jon@spin.ie>, <www-rdf-interest@w3.org>
1. Jon, I agree with everything you say here.
2. I think you have also answered Brian McBride's complaints that my discussion has been too terse, and has not identified any "logical flaw" in RDFS.  RDFS has no "logical flaw"; it allows you to add additional statements to correct the false alternatives which are permitted by previous statements.  Brian, if my position is still not clear, please ask me some more questions.
3. Obviously, many of you are having trouble understanding my unfamiliar KR language.  It's all crystal clear to me only because I have been using KR for 6 years.  (Being the inventor doesn't hurt either.)  The best advice I can offer you is "think triples".  KR already has the ability to accept triples like "man  rdfs:subClassOf  animal" instead of the equivalent "man  iss*  animal", if you prefer the former.
4. If you compare KR and RDFS, I think you will find that a KR description is 10 times shorter and 10 times easier to read.
P.S. One of the things that makes RDFS more verbose is the multiple layers of definition, which then require qualifiers to distinguish the names, e.g.: rdfs:Class instead of Class.  The OWL level should be the starting point, and should be the default with no qualifiers required for its names.  That's my practical advice for language design.
Dick McCullough 
knowledge := man do identify od existent done
knowledge haspart list of proposition

  ----- Original Message ----- 
  From: Jon Hanna 
  To: www-rdf-interest@w3.org 
  Sent: Friday, November 22, 2002 4:14 AM
  Subject: RE: subclasses (RDF vocabulary definitions)

  > The English is
  >    When RDFS says that man is a subclass of animal
  >    what it means is that either (1) the set of all men
  > is identical to the set of all animals or (2) the set
  > of all men is a proper subset of the set of all animals

  This is quite true. Indeed it is true that either (1) or (2) holds, so
  <:_man> <rdfs:subClassOf> <:_animal> isn't wrong as such.

  Adding other statements we could say "the set of all men is not identical to
  the set of all animals" (hence implying (2)). Or "the set of animals is an
  rdfs:subClassOf of the set of all men" (hence implying 1). Or we can say 1
  directly or 2 directly.

  We can also say A. The set of all dogs is an rdfs:subClassOf the set of all
  animals and B. The set of all dogs is disjoint with the set of all men
  (which hence implies 2 - since to be disjoint from another subset it must be
  a proper subset, although we may or may not care about that at the time).

  Hence the language is not lacking in its ability to express all of these
  possibilities, but some possibilities may require more verbose collections
  of statements than others.

  While the interaction of these statements may seem pointlessly indirect,
  there are advantages. The intention is that these statements may be used by
  applications to do something useful. Such applications generally won't want
  all of the possibilities above. An application that wishes to know "is
  Richard H. McCullough an animal" can find that from knowing that "Richard H.
  McCullough is a man" and man is an rdfs:subClassOf animal.

  In a distributed environment it can be useful to be able to transmit as
  close to the minimum information needed for the task as possible (I don't
  know if these issues have affected KR or not).

  Even in a non-distributed environment it is generally easier for programs to
  maintain the relationships between "identical sets", "subset" and "proper
  subset" by breaking them down into more atomic concepts. That is the program
  can either:

  1. Define "proper subset" as "is subset and is not identical".
  2. Define "subset" as "is proper subset or is identical".
  3. Define identical as "is subset and is not proper subset".

  This isn't the only way to do this, but it is a way to do so that causes
  comparisons between the three concepts to happen at clearly defined point,
  and hence less error-prone than alternatives that would compare the concepts
  directly at various different times.

  Hence we need to decide which of these we use. Since 1, 2, and 3 above are
  logically equivalent we should do so on the basis of practical concerns.

  Inference of a type from a known type is likely to be the primary practical
  use of rdfs:subClassOf (for example, knowing that what one program calls
  "the type of document I produce" can be used as what another program calls
  "Any stream of bytes").
  This inference can be made if either one is a subset of the other (proper or
  not). The programs involved won't care if it is a proper subset or not.

  As such it makes sense to allow this inference of type to occur with the
  briefest statement, with definitions of whether the two classes involved are
  a proper subset being more verbose. Similarly knowing that two classes are
  identical is likely to be more commonly needed information than knowing that
  one is a proper subset of the other. It makes sense therefore to make
  "subset" easier to express than "proper subset".

  In summary:

  1. I believe that all the relevant possibilities can be expressed in the
  language. Hence theorists should be happy.

  2. The most useful statements to applications are the most concise. Hence
  hackers should also be happy.

  While a case for a xxx:properSubsetOf could be made for notational
  convenience it could be defined in OWL anyway.
Received on Friday, 22 November 2002 08:45:21 UTC

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