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Re: What do the ontologists want?

From: Jim Hendler <jhendler@darpa.mil>
Date: Thu, 17 May 2001 04:04:57 -0400
Message-Id: <p04330100b729300ffa93@[158.63.53.121]>
To: David Allsopp <dallsopp@signal.dera.gov.uk>
Cc: www-rdf-logic@w3.org
>Jim Hendler wrote:
>
>
>If so, how are these statements (the actual message content) 'contained'
>by the message object, since it can't point at them without reification?
>
>+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>For example, we have the triples
>
>message_01 --rdf:type--> agent:Message
>message_01 --agent:timeStamp--> "4:00PM Tuesday"
>
>and the content of the message, such as:
>
>Airport_01 --status--> Operational
>
>How do we assemble these such that we know that the timestamp
>appies to the content? Or do we associate the two by convention?
>+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>
>
>(One could nest a serialized RDF/DAML document within the message object
>(i.e have a 'content' property that points to a rather long string
>literal) but that doesn't seem to be what you're suggesting).
>
>Alternatively, the description of the Message object merely accompanies
>other statements in a document, and it is up to the recipient to notice
>it and take appropriate implementation-dependent action.
>
>Or perhaps I have misunderstood the intended usage; I want to be able,
>in principle, to determine the timestamp, origin etc of any individual
>statement about any resource in my database/RDF Model. I am unclear how
>or if putting a timestamp on a message object would translate to a
>timestamps on its constituent statements (which are gathered together
>along with those from many other messages in some form of database).
>
>Regards,
>
>David Allsopp
>DERA Malvern

OK - several possibilities - the one I'd suggest is to create a 
content property, but not fill it with a string, fill it with an 
anonymous node which itself is the appropriate RDF for the content:

message01 :a Agent:AgentMessage ;
   Agent:timeStamp "4:00 Tues" ;
   Agent:MsgContent
     [ Airport:AirportName AirportList:Airport_01;
       Airport:AirportStatus "operational"] .

Now, before people yell too much (and with apologies to Pat, I'm 
going to ignore some deeper issues and be a real "scruffy") - there 
is a formal semantics as to what this means with respect to DAML+OIL 
(remember, we defined the AgentMessage class and its properties in 
DAML+OIL in an earlier message) -- but the important thing here is 
the operational semantics we give it -- in other words, if I define 
the ontology with these sorts of properties, there are some in the 
RDF community who might claim that I asserted that the Airport 
actually is operational.  But I would argue that I have not done so 
-- you cannot find this message in any useful way outside of the 
MsgContent it appears in, and I would make sure my applications knew 
not to make that inference.
  There is the issue of whether other applications would know not to 
make that inference upon reading my RDF/DAML, but again, notice that 
the anonymous node really isn't the same as making the assertion 
since that graph never really "appears by itself" -- not something 
easily formalized without a lot of work (and that is what the whole 
reification debate really seems to turn on), but in practice it 
doesn't seem to me to be a tough issue -- I find it hard to believe 
someone would develop an application that would assume I had asserted 
this anonymous class to explicitely be true (see Pat's messages), and 
if they haven't, then all they can really assert is that some 
particular document (the one this appeared on) seemed to imply that 
the Airport was operational - again, this could cause problems, but I 
don't see that there are any show stoppers to fielding things with 
this syntactic sort of approach (or at least any that aren't almost 
as bad using reification).
  What I've done is basically to mimic what the reification is 
supposed to do -- break my quoted message into a set of arcs/links 
that have appropriate content - but I've named the links myself, and 
used the operational semantics to make sure appropriate things happen 
- not elegant, but very powerful.
   There's a lot of other issues in the use of this stuff - but for 
all practical purposes a lot of us have been using techniques like 
this for a long time, and they work just fine.

  That said, let me again restate that I do NOT offer this as a great 
mechanism that solves all problems -- rather, I was trying to show 
that for practical purposes one can do a lot of tagging and 
assertions about assertions by careful use of ontologies coupled with 
reasonable operational semantics.  The eventual semantic web may want 
better, but for those of us who want to start building now, 
mechanisms like this one should work just fine for a while
  -Jim H.

p.s. For some random mutterings on tags see : 
http://www.cs.umd.edu/users/hendler/Tags.html

For some pointers to stuff about agents, ontologies, and the Sem Web see
http://www.cs.umd.edu/users/hendler/ontologies.html

-- 
Prof. James Hendler		Program Manager
DARPA/ISO			703-696-2238 (phone)
3701 N. Fairfax Dr.		703-696-2201 (Fax)
Arlington, VA 22203		jhendler@darpa.mil
Received on Thursday, 17 May 2001 04:03:45 UTC

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