W3C home > Mailing lists > Public > www-rdf-logic@w3.org > June 2002

Re: the mythical RDF inference engine was: Re: What is truth anyways?

From: patrick hayes <phayes@ai.uwf.edu>
Date: Wed, 12 Jun 2002 17:46:32 -0500
Message-Id: <p05111a01b92d7a0efa51@[]>
To: "Jonathan Borden" <jonathan@openhealth.org>
Cc: www-rdf-logic@w3.org

>patrick hayes wrote:
>>  ... If there's a CWM agent out there  disguised as an RDF
>>  agent, and my RDF agent reads its output, then all of *its*
>>  inferences are likely to be corrupted by invalid RDF conclusions that
>>  the CWM agent has drawn.  I just want each spec to wear its meaning
>>  on its sleeve, as it were, *as part of that spec*. And CWM and N3 are
>>  NOT parts of the RDF spec; and in fact, it is easy to prove that the
>>  claimed meanings for much of N3 are not writable in anything that
>>  could possibly conform to the RDF spec.
>Right. The problem is that RDF is not (currently) making itself a good
>platform for such _other agents_ particularly WebOnt/OWL to as you say "wear
>its meaning on its sleeve"
>Since we agree that useful inferencing engines are going to extend RDF in
>some way, rather than direct the work of RDF _inward_ toward describing how
>RDF inferencing ought work (do I care ? :-) perhaps a bit more effort ought
>be directed at figuring out how other systems might use RDF triples _as
>>  >Indeed since base RDF itself is just a bunch of assertions, I'm not sure
>>  >what inferencing you can do _with RDF alone_ rather it seems just the
>>  >mechanism for carrying 'facts'. I worry that all this emphasis on
>>  >inferencine _within_ RDF is over constraining the ability to do
>>  >_on_ RDF.
>>  The problem is that last phrase is meaningless. Writing all the
>>  triples backwards is inferencing on RDF. Erasing the RDF and
>>  replacing it with a quote from the Koran is inferencing on RDF. If
>>  you disagree, tell us what YOU mean by inferencing on RDF.
>Shrug... this last statement is no more or less meaningless than anything
>else I choose to say in English.

No, that kind of move isn't acceptable in this kind of discussion. We 
aren't just having a conversation over a beer here, we are trying to 
pin down a technical spec of a language to be used for content 
exchange between software agents. Words like 'inferencing' take on 
some weight, and need to be made precise. There is no precise 
ordinary English sense for words like this'.  To make them 
sufficiently precise we need to appeal to the relevant technical 
field, and when you do, you will arrive at the question I asked. 
Talking about the validity of inferencing is meaningful only with 
respect to a semantics of some kind.

>But OK, I'll bite. OWL might choose to represent individuals and facts about
>individuals _in RDF_. OWL might have a model theory that provides for
>certain inferences to be drawn given these individuals and a bunch of

OK, stop right there. You said OWL is representing facts in RDF. To 
me that sounds like saying that Im talking English in French: it 
simply doesn't make sense. Which language are you referring to? RDF 
or OWL? Because they are *different*.

>That might be useful. I might describe these inferences as being
>drawn _on_ the particular facts. Nonetheless we consider such facts as data
>from which we draw conclusions. What is the big deal?

The big deal is, you have to say which formal language you are using. 
Look, map this all over to programming languages. Suppose I was to 
say: what's all the fuss about PERL and LISP and JAVA and stuff? 
Arent they all programming languages? And don't all programming 
languages describe algorithms? So why can't I just, like, write my 
JAVA in PERL and then a PERL engine could read it. That would be 
useful. What is the big deal?

If you use an informal sense of 'algorithm' this makes sense, just as 
your idea makes sense if you use an informal  notion of 'fact' .

>>  >So why not just say that RDF is a bunch of assertions
>>  Becasue that is an extremely dangerous thing to say: it sounds
>>  meaningful but in fact is meaningless. A bunch of assertions *in what
>>  language*?? In RDF? If so, then it is INCORRECT to use non-rdf-valid
>>  processes on them.
>Huh? INCORRECT according to whom?

According to the RDF spec., which defines a notion of validity for 
RDF inference. Its not "my" model theory, its the RDF core WG's model 
theory. If you want RDF to not have a model theory, take up the 
matter with Brian McBride.

>Certainly this is (actually) begging the
>question if you define CORRECT as according to your model theory, eh :-)

If things go according to schedule, this is how the W3C will soon be 
defining it.

>I am just trying to be practical. A bunch of RDF triples is like a
>database -- well really a single table with three columns. Why not define
>"rdf-valid" loosely -- let whomever is using the triples decide what is
>valid. Isn't that how most all software works: there are data structures and
>algorithms which operate on the data. A triple is a data structure, right?

Wrong. It is not a datastructure, it is a description in a 
(admittedly rather simple) relational logic. I know that OWLs task 
would be a hell of a lot easier if only RDF *were* just a 
datastructure language, but that isn't what the RDF authors intended, 
or what the W3C spec says.

>>  >and forget about this
>>  >pure RDF inferencing engine that doesn't seem to exist and let the folks
>>  >care about inferencing decide what inferences might be drawn from a
>>  >bunch of facts and be done with it.
>>  The whole point of having a precise account of meaning in the
>>  language spec is to sanction some inferences and not others. If we
>>  can allow anyone to draw any conclusions from an RDF graph, then RDF
>>  doesn't differ from XML.
>Well N-triples looks alot different to me than XML. And a triple store fits
>into a single relational table with three columns, so that is the
>different -- perhaps not earth shattering, but what magic are you looking

Im not looking for any magic at all. My purpose is to try to make it 
clear to everyone that appeal to magic is right out.

>>  In fact, it doesn't differ from ASCII or
>>  graffiti: its just character strings that you can do whatever the
>>  hell you like with and call it inference. So why are we even
>>  bothering with it?
>ASCII, or to be modern, Unicode, will get you pretty far. But yeah, that's
>about right, we have Unicode, XML and RDF. And on top of that we develop
>OWL, DAML-S etc. Why not just consider RDF as a standard way to transmit
>graphs, or collections of triples. Isn't that useful enough?

Well, that would have been one way to go. It certainly would have 
made my life easier. But the decision to not go that way was made 
long before I got involved, and I don't think that this suggestion 
will fly.


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Received on Wednesday, 12 June 2002 18:46:36 UTC

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