W3C home > Mailing lists > Public > www-rdf-logic@w3.org > April 2001

Re: RDF semantics: applications, formalism and education

From: pat hayes <phayes@ai.uwf.edu>
Date: Tue, 3 Apr 2001 14:37:17 -0700
Message-Id: <v0421010bb6efee0a4cb3@[130.107.66.237]>
To: Graham Klyne <GK@NineByNine.org>
Cc: www-rdf-logic@w3.org
>It is with great interest that I have followed the thread about the 
>adequacy of RDF.  I think Peter F. Patel-Schneider captured a theme 
>with these comments:
>
>>1/ There are many places in the RDF and RDFS documents where vitally
>>important statements about the semantics of RDF or RDFS are presented with
>>no formal backup, and in a totally inadequate fashion.
>
>[...]
>
>>2/ The triple model does not provide sufficient representational power to
>>transfer interesting semantic content.
>
>While fully accepting both of these statements, I'd like to try and 
>make a case for not giving up on RDF:
>
>
>Regarding point (2):
>
>I shall assume that by "interesting semantic content", a level of 
>understanding about some domain of discourse adequate for the 
>purposes of a range of reasoning activities is meant.  I guess this 
>would be roughly what is required for software agents to negotiate 
>autonomously and flexibly on behalf of their human "masters".
>
>I think there is a range of applications for which the expressive 
>power of basic RDF is adequate.  (e.g. RSS, calendaring, directory 
>metadata, message metadata, web page metadata, etc., etc.)  I think 
>these will typically be applications in which human participants are 
>still directly involved.  For these applications, the expected value 
>of using RDF is (a) to leverage common tools, and (b) to ease the 
>movement of (RDF-coded) data between different applications, or to 
>be able to combine data from multiple applications.  Prosaic stuff, 
>but still valuable.

Agreed, and any standard needs to accomodate this good stuff. But 
this is not an argument for RDF in particular, as almost any notation 
could do this. (Also, by the way, I suspect that things like metadata 
will expand to fill whatever expressive power is made available for 
them, and they are limited at present more by the tools that have 
been available for them than by their intrinsic simplicity.)

>I also happen to think there are some simple rule-based systems 
>(e.g. simple access control, directed searching, system component 
>matching) that can be built using a kind of limited inference 
>framework similar to that of RDF(S), which falls considerably short 
>of FOL.

Of course we need more limited expressivity than full FOL. The 
overall shape of the expressivity/efficiency tradeoff has been 
thoroughly explored and is now reasonably well understood, and again 
there are many alternatives to RDF on this curve.

>Where I think things get interesting is if we can use this dull old 
>common metadata format as a foundation on which to build the more 
>interesting semantic content and reasoning elements.  Because, if we 
>can do this, the "semantically aware" components potentially have 
>access to a wealth of raw data for their cogitations.

I agree.

>Then the question I ask is:  Is RDF adequate as a basis for building 
>definitions of richer semantic capabilities?  (I don't ask if it's 
>the best base to build from, or complete, just if it's an adequate 
>foundation.  Someone reminded me recently that in the Internet 
>standards area "the Best is the enemy of the Good".)  I think the 
>answer to this question, from what the logicians among us have been 
>saying, is "Yes".

Well, my answer is emphatically NO. This is my primary concern, in fact.

>I think there is value in the simplistic framework of RDG precisely 
>because it is simple.  For an Internet standard I think it would be 
>a mistake to build up the base RDF so that the simple functions are 
>no longer simple.

Einstein once said: keep everything simple, but not too simple. Of 
course simplicity is to be sought after, but only if the basic 
functionality that one needs is made available. Also there are 
different kinds of simplicity. If you want to minimise the number of 
symbols, you can write all of higher-order logic using just lambda 
and Scheffer stroke, but nobody will be able to read it. There are 
ways of stating full FOL which are deceptively simple in the syntax 
but devlishly subtle in the semantics. Often, computational 
efficiency requires syntactic restrictions which are awkward and 
complicated to state and which people find irritating, but the 
machines burn rubber.

>Something I have learned is that Internet standards are often most 
>successful when built in relatively small incremental stages.  I 
>think that, roughly speaking, RDF is the right size of step and 
>would resist trying to put full semantic expression (whatever that 
>may be) into the RDF core.

Just to clarify, I (the original grouser on this thread) did not 
intend to suggest that 'full semantic expression' should be in the 
core. In fact I don't really have a problem with the RDF core *if it 
is seen as a core*. It is just a datastructuring tool, and in that 
role it is as good as any; my problem is that it isn't described that 
way, but is put forward as a universal semantic model. And in that 
role, it just isn't up to the task.

>The fact that we might build different semantic frameworks on that 
>core is a real strength at this time.

I think you are using 'semantic' in a different sense. It makes sense 
to have different extensions to a core. It doesnt make sense to have 
different ways of interpreting a core (or any other formal language), 
since that renders it incapable of communicating content; if the same 
formalism has alternative semantics, then what I mean when I send 
something to you might not be what you read it as saying. And if this 
IS the language we use to communicate in, we have no way of even 
discussing what the differences might be, since everything I say to 
try to clarify your misunderstanding can itself be misunderstood. 
This is not a source of strength; it is the curse of Babel.

>I don't think this is anything not already said, but it seems to 
>have been lost in the recent debate about the semantic adequacy of 
>RDF.
>
>
>Turning to point (1):
>
>I think that, in some respects, the RDF specification is its own 
>worst enemy, and that the lack of comprehensive formal underpinnings 
>may be part of the reason.  As one who intends to be an active 
>participant in the RDF review WG, I hope to do my part in rectifying 
>this.  I believe other participants will do likewise.
>
>But I think we have a significant job of education ahead of us:  (a) 
>education of the specification writers in the subtleties of formal 
>underpinnings (e.g. is it a "model"?  I know I have much to learn), 
>and (b) education of the specification users;  this means being able 
>to draft the specification in such a way that the expectations of 
>system developers who don't have a background in formal systems are 
>set appropriately.
>
>I believe that truly outstanding experts in the field of logic and 
>semantics are engaged here, and I truly hope that you will help us 
>to get the formalities right this time, and to effectively 
>communicate the necessary elements to the target audience of Web 
>systems developers.

Yes, we all need to go through a learning process. For what its 
worth, I havn't learned so much in one year for a long time.

Pat Hayes

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Received on Tuesday, 3 April 2001 17:35:22 GMT

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