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Re: The semantics of blank nodes

From: Frank Manola <fmanola@mitre.org>
Date: Thu, 14 Nov 2002 13:03:48 -0500
Message-ID: <3DD3E584.4566DDC7@mitre.org>
To: www-rdf-interest@w3.org

MDaconta@aol.com wrote:

> In a message dated 11/13/2002 2:47:52 PM US Mountain Standard Time, fmanola@mitre.org writes:
>    >The point I disagree with above is when you say, "the blank node
>    >represents an address" because no where do we connect it with
>    >the concept (or Class) "Address".  As you say below,
>    >this could be resolved if we specify a type and then that clears up the
>    >semantic confusion.  I would guess that a blank node then really just
>    >means "thing" (unless we type it).
>    This depends on what "means" means.  If I create a resource, or a blank 
>    node, I get to decide what it means, and anyone who understands my 
>    vocabulary or the structure I'm using (through some means or other) 
>    might share that meaning (e.g., they might understand that the thing at 
>    the end of my "address" property was an address).  As far as 
>    machine-interpretable meaning is concerned, though, *any* resource, 
>    blank node or not, just means "thing" unless you provide some additional 
>    information about it (like a type, which of course the machine has to 
>    understand too).  That is, it isn't just blank nodes that have to be 
>    typed to have this meaning:  resources with URIs do too.  URIs don't 
>    carry meaning with them (or at least, they aren't supposed to be 
>    interpreted as carrying meaning in RDF).
> Understand.  Identity and typing are orthogonal.  Except where a well-known
> identity conveys well-known semantics but that cannot be assumed.  
> This brings up a tangential issue on the semantics of a URI identity
> beyond uniqueness.  Has there been any thought in RDF Schema to creating
> a utility property to express that a URI is retrievable (same issue as 
> namespace retrieving an RDDL document).  Or is it better to use a urn scheme?
> I am not that familiar with the urn scheme but in your example in the primer:
> 1. <?xml version="1.0"?>
> 2. <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
> 3.             xmlns:dc="http://purl.org/dc/elements/1.1/"
> 4.             xmlns:ex="http://www.example.org/terms/">
> 5.   <rdf:Description rdf:about="http://www.example.org/index.html">
> 6.        <ex:creation-date>August 16, 1999</ex:creation-date>
> 7.        <ex:language>English</ex:language>
> 8.        <dc:creator>
> 9.           <rdf:Description rdf:about="http://www.example.org/staffid/85740">
> 10.           </rdf:Description>
> 11.        </dc:creator>
> 12.   </rdf:Description>
> 13. </rdf:RDF>
> How is an RDF application processing this to know that the URI on
> line 5 is an accessible web page but the URI on line 9 is just an identity?
> Especially when both specify http: as the protocol.  I know this is off topic
> but some pointers on this would be helpful.  One way I can think of is
> to add the <dc:type> property to the web page and then you can assume 
> anything without a dc:type is just an identity (not a foolproof technique unless
> the dublin core folks add a dc:type value for "identifier" or "name").

A utility property for "retrievable" would be pretty coarse-grained
wouldn't it?  I'd be looking for more descriptive vocabulary about what
kind of thing it was, and so on, with how to access it (if it were
accessible) being additional information.  Anyway, it seems to me the
most straightforward way to find out whether something with an http URI
is accessible is to try to access it.  I know this has been debated a
lot, but certainly in some quarters there's a lot of support behind the
idea of using http URIs even for things that are just supposed to be
identities, with the idea that you might put some descriptive
information about them there (as in the RDDL document you mentioned).  

>   <snip />
>   You generally make statements about classes of things in schemas.  So 
>   the place to say that the property <address> has a range of class 
>   <Address> would be in a schema.  In the original example, though, I 
>   wanted to represent the specific address of a specific staff member. 
>   That means that the thing on the end of Joe's <address> (or <livesAt>) 
>   property needs to be something that represents an individual address, 
>   not the class of addresses.  So I need to use either a blank node, or a 
>   resource with a URI.  Either way, adding a type property would convey 
>   more information about that instance (to someone or something who 
>   understood what class <Address> meant).
> Agreed.  In terms of best practice, it may be good to mention that in 
> the primer.

I *do* mention it in the Primer (though perhaps not as explicitly as you
might like).  Check out the new one at
http://www.w3.org/TR/2002/WD-rdf-primer-20021111/  The material on blank
nodes (as well as of other things) has been expanded.


Frank Manola                   The MITRE Corporation
202 Burlington Road, MS A345   Bedford, MA 01730-1420
mailto:fmanola@mitre.org       voice: 781-271-8147   FAX: 781-271-8752
Received on Thursday, 14 November 2002 13:03:57 UTC

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