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RE: Subjects as Literals, [was Re: The Ordered List Ontology]

From: Michael Schneider <schneid@fzi.de>
Date: Sun, 4 Jul 2010 14:41:49 +0200
Message-ID: <0EF30CAA69519C4CB91D01481AEA06A001F07D0B@judith.fzi.de>
To: <nathan@webr3.org>
Cc: "Linked Data community" <public-lod@w3.org>, "Semantic Web" <semantic-web@w3.org>, "Pat Hayes" <phayes@ihmc.us>
On Behalf Of Nathan wrote on Friday, July 02:

>Pat Hayes wrote:
>> On Jul 1, 2010, at 11:18 AM, Yves Raimond wrote:
>>> "A literal may be the object of an RDF statement, but not the subject
>>> or the predicate."
>>
>> Just to clarify, this is a purely syntactic restriction. Allowing
>> literals in subject position would require **no change at all** to the
>> RDF semantics. (The non-normative inference rules for RDF and RDFS and
>> D-entailment given in the semantics document would need revision, but
>> they would then be simplified.)
>
>I have to wonder then, what can one all place in the s,p,o slots without
>changing the RDF semantics? literal and bnode predicates for instance?

Short answer: you can put URIs, literals and bNodes in all three positions
of a triple, without needing to touch the current RDF semantics spec. 

Longer answer:
 
The syntactic restrictions of RDF (no literal subjects, no literal or bnode
predicates) are not "checked" by the semantic conditions of the RDF
semantics. Every node in a triple is (in a first "step") simply related to
some resource in the interpreted universe. For URI nodes this is probably
clear, but even literals are meant to represent resources: all datavalues
are treated as resources in RDF! bNodes are a bit different, since they do
not stand directly for a specific resource, but indicate that some resource
/exists/. 

But, in the end, all you receive from the interpretation of all the nodes
occurring in an RDF triple is essentially a binary relationships between two
resources. And the rest of the semantics specification of RDF(S), and even
of OWL Full, works exclusively on these kinds of sets of binary
relationships, below the "syntax surface", so to speak. So the whole stack
of RDF-based semantics is ready to be used with "generalized" RDF out of the
box -- just waiting for the RDF syntax to give up its restrictiveness. :-)

As a special note: Even bNode predicates, and even literal predicates are
allowed in the RDF semantics and make sense (in a technical sense, at
least), because: 

  (a) Properties are also treated as regular resources by the RDF semantics.
They have some binary relation "attached" to them (in order to allow to use
them to relate two resources), but you can still treat them like any other
resource, e.g. relate themselves to another resource by some other
property).

  (b) Literals stand for data value resources, and nothing stops a data
value resource from being used as a predicate resource. (I leave it to the
philosophers here in the list to debate whether this is good use or bad use
in practice.) 

>variables or formulae as in n3?

For formulae: Certainly "no", since formulae are normally not interpreted to
denote resources, but are assertions being interpreted by a truth value. 

For variables: Firstly, /existentially/ quantified variables are already
treated, since they are represented by bNodes. Remains /universally/
quantified variables, as they exist in N3. Supporting them would definitely
need /some/ extension of the RDF semantics, since it has at least to be said
how such variables are interpreted (this is only said for URIs, plain and
typed literals, and for bNodes so far in the spec, see Sections 1.4 and 1.5
of the RDF Semantics [1]). I just cannot tell how much extension would be
required without more deeply thinking about it. So this question keeps open
from my side at the moment. Maybe Pat has an answer?
 
>read as: if a new serialization/syntax was defined for RDF what are the
>limitations for the values of node/object and relationship specified by
>the RDF Semantics?
>
>Best,
>
>Nathan
>
>ps: apologies if this is a dumb question, I fear i'd still be hear next
>year trying to answer it myself though ;)

Not dump at all. Technically pretty demanding, in fact.

Cheers,
Michael
 
[1] <http://www.w3.org/TR/rdf-mt/>

--
Dipl.-Inform. Michael Schneider
Research Scientist, Information Process Engineering (IPE)
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Email: michael.schneider@fzi.de
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Received on Sunday, 4 July 2010 12:42:27 UTC

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