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RE: Review of RDF Based semantics [2nd]

From: Michael Schneider <schneid@fzi.de>
Date: Thu, 9 Apr 2009 00:31:25 +0200
Message-ID: <0EF30CAA69519C4CB91D01481AEA06A00125F681@judith.fzi.de>
To: "Zhe Wu" <alan.wu@oracle.com>
Cc: <public-owl-wg@w3.org>
>-----Original Message-----
>From: Zhe Wu [mailto:alan.wu@oracle.com]
>Sent: Wednesday, April 08, 2009 9:10 PM
>To: Michael Schneider
>Cc: public-owl-wg@w3.org
>Subject: Re: Review of RDF Based semantics [2nd]
>
>Michael,
>
>That is quick!
>>> A few minor comments as follows.
>>> - Section 2.1 talks about generalized RDF triples. I am wondering
>>> whether it is useful
>>>   to restrict the use of literal values as predicates.
>>>
>>
>> The text currently states that the RDF-Based Semantics *MAY* be
>applied
>> to graphs with generalized RDF triples, and generalized RDF triples
>> may contain literals in predicate position. This does not mean that
>> implementations must support such extended RDF graphs, but they will
>> not lead to problems *if* implementations support them.
>>
>> Technically, there is no problem with having literals in predicate
>> position, since they denote individuals under the RDF-Based Semantics,
>> and, as it is true for all individuals, these data individuals can
>> be properties as well.
>>
>> Whether having literals in predicate position is useful or not
>> will certainly depend on the respective application, but such
>> questions are outside the scope of this specification document.
>>
>> However, the strong relaxation may become relevant to other standards.
>> For example, the same notion of "generalized RDF" is assumed by the
>> following important work-in-progress specifications that have a
>> dependency on the OWL 2 RDF-Based Semantics:
>>
>>  * OWL 2 RL (see 2nd paragraph of §4.3 in the Profiles Editor's Draft
>at
>> [1a])
>>  * RIF+OWL (see Section 3.1.1 of the current Editor's Draft at [1b])
>>
>> So I do not intend to do a change.
>>
>>
>I think I am OK with it. 

Good. :)

>It's just that it is not intuitive to
>understand an assertion like "x 3.1415 y"
>even though there is a well defined "meaning" for it.

Note that even with "traditional" RDF one can still write

  :pi owl:sameAs 3.1415
  x :pi y

So if someone really wants to express such things, then you can't 
stop him, anyway. By disallowing generalized RDF you just place 
(semantically ineffective) syntactic obstacles in his way. 

Generalized RDF does not expand the semantic expressivity of the 
OWL 2 RDF-Based Semantics. Hence, the *MAY* in Section 2.1 is 
actually harmless, it just eases the life a bit for those who care.

>>> - Section 5, in the paragraph starts with Unscoped variables. "x"
>>> denotes an arbitrary individual in the universe.
>>>  I am wondering if "element" is a better word than "individual" in
>this
>>> context (mainly
>>>  to avoid confusion with owl individual)
>>>
>>
>> The term "individual" is just what is meant here. The text sais:
>>
>> [[
>> Unscoped variables: If no scope is explicitly given for a variable
>"x",
>> [...]
>> then x is unconstrained, which means that x in IR,
>> i.e. "x" denotes an arbitrary individual in the universe
>> ]]
>>
>> The idea behind this convention is that if nothing specific is said
>about
>> a variable x, then x stands for an individual. The reason is that all
>> elements of the domain (IR) of an OWL 2 Full interpretation are
>individuals.
>>
>> So I do not intend to change this.
>>
>>
>Hmmm. When an owl user talks about individual, he/she probably means
>those x in his/her ontology
>such that x type owl:Thing holds. Here IR is about the interpreted
>domain and we call elements of IR individuals.
>Isn't it a bit confusing?

Under the OWL 2 RDF-Based Semantics, just as for the original version,
the term "individual" refers to the elements of the domain of an 
OWL 2 Full interpretation. In this way the term is used several times
in the Introduction section, as well as in Section 4. There is no other 
meaning of the term "individual" as far as the OWL 2 RDF-Based Semantics 
is concerned.
 
Concerning "owl:Thing" and "IR": owl:Thing actually represents IR aka 
the whole domain:

  ICEXT(I(owl:Thing)) = IR .

>>> - In Section 6, second last paragraph,
>"IEXT(I(owl:topObjectProperty)) =
>>> IR x IR"...
>>>  for which there are no corresponding domain and range axiomatic
>>> triples.
>>>  Why?
>>>
>>
>> This is a reasonable question! The text was even confusing, as I see
>now.
>> I have changed the text, please check!
>>
>> DIFF:
>> <http://www.w3.org/2007/OWL/wiki/index.php?title=RDF-
>Based_Semantics&diff=21
>> 611&oldid=21279>
>>
>>
>
>I don't quite understand it. The same kind of argument applies to the
>pair of owl:complementOf and owl:disjointWith, right?

No. Table 3 does /not/ specify the /exact/ property extension
for these two properties, but only sais that their property extensions
are /subsets/ of IC x IC. This is much less specific than in the case
of owl:topObjectProperty, which sais "= IR x IR". The subset relationship
can actually be expressed by two axiomatic triples with rdfs:domain and 
rdfs:range, the "=" relationship cannot.

>Cheers,
>
>Zhe

Cheers,
Michael

--
Dipl.-Inform. Michael Schneider
Research Scientist, Dept. Information Process Engineering (IPE)
Tel  : +49-721-9654-726
Fax  : +49-721-9654-727
Email: michael.schneider@fzi.de
WWW  : http://www.fzi.de/michael.schneider
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Received on Wednesday, 8 April 2009 22:32:09 GMT

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