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RE: Subjects as Literals

From: Michael Schneider <schneid@fzi.de>
Date: Tue, 6 Jul 2010 14:03:19 +0200
Message-ID: <0EF30CAA69519C4CB91D01481AEA06A001F07F4E@judith.fzi.de>
To: "Toby Inkster" <tai@g5n.co.uk>
Cc: "Linked Data community" <public-lod@w3.org>, "Semantic Web" <semantic-web@w3.org>, "Pat Hayes" <phayes@ihmc.us>
Toby Inkster:

>On Mon, 5 Jul 2010 17:43:17 -0500
>Pat Hayes <phayes@ihmc.us> wrote:
>> Well, nobody is suggesting allowing literals as predicates (although
>> in fact the RDF semantics would easily extend to this usage, if
>> required, and the analogous structures are allowed, and do have
>> genuine use cases, in ISO Common Logic.)
>Actually, I have suggested allowing them just to make things simpler -
>URIs, blank nodes and literals would all be allowed in any position.
>However, a statement with a literal in the predicate position would be
>officially defined to have no meaning.

So, if 

    :s "lit" :o .

must not have a semantic meaning, what about

    "lit" rdf:type rdf:Property .

? As, according to what you say above, you are willing to allow for literals
in subject position, this triple is fine for you syntactically. But what
about its meaning? Would this also be officially defined to have no meaning?
And if so, only this special kind of typing triple with rdf:Property as its
object? Or all typing triples with literal subjects (this would exclude one
of Pat's usecases)? Or even all triples with literal subjects?

But, assume that this triple does not have a semantics, what about

    :p rdfs:range rdf:Property .
    :s :p "lit" .

This triple actually /has/ a semantic meaning in the /current/ specification
of RDFS. And this meaning indeed includes the meaning of the previous
example, namely that the resource denoted by the literal "lit" is an
instance of the class rdf:Property, that is, the literal denotes a property.

Or what about

    :p owl:sameAs "lit" .
    :s :p :o .

This is also allowed in current RDFS, and in OWL *) this triple even has the
same meaning as

    :s "lit" :o . 

So we are back at the start.

Any idea, how to cope with this all? Semantically? Syntactically?


(* To be precise, the two examples are equivalent under the OWL 2 RDF-Based
Semantics [1], which is the semantics of OWL 2 Full, and which explicitly
allows literals in all positions of a triple.

[1] <http://www.w3.org/TR/owl2-rdf-based-semantics/#topic-ont-generalrdf>

Dipl.-Inform. Michael Schneider
Research Scientist, Information Process Engineering (IPE)
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FZI Forschungszentrum Informatik an der Universität Karlsruhe
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Received on Tuesday, 6 July 2010 12:03:54 UTC

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