Re: rdf inclusion

Tim Berners-Lee <timbl@w3.org> wrote:
>
>On Wednesday, May 8, 2002, at 02:34 PM, Jeff Heflin wrote:
>
>>Pat Hayes wrote:
>>>
>>>>Jeff Heflin wrote:
>>>>>  ... Since I was the initial proponent of daml:imports on the Joint
>>>>>  Committee, let me address this issue. You are absolutely correct that
>>>>>  the imports statement must be used. Simply refering to a namespace does
>>>>>  not include any ontology definitions. You must make the imports
>>>>>  statement explicit. Period. ...
>>>>
>
>Neither half of this is correct by itself, IMHO.  Each is too sweeping.
>
>When you use a term (eg Property) in a namespace, its meaning is defined
>by the definer of that namespace.  You are (unlike in english)
>bound to use a term according to its creator's definition.

I don't understand what this is supposed to mean. If we are talking 
about assertional languages (which includes RDF, RDFS and DAML+OIL) 
then there is no such thing as a 'definition' of a term. There are 
only assertions involving it. If A publishes a web page using the 
term T and says a lot of stuff there about it, and then B uses that 
same term T, what exactly is B committed to? Does B, simply by 
*using* the term, assent to everything that A says about it?

Also, what is that 'bound to'? I see no way that any kind of web 
publication by A can *enforce* any behavior or assumptions on B. The 
best that one could do would be to have some public 'rules of 
inference behavior' which might for example say that if B publishes 
some content using a term 'belonging' to A, then C is entitled to use 
anything that A says using the term when drawing conclusions from 
what B says in what B publishes, and B is just as responsible for any 
such consequences as B would be had the consequences been drawn 
solely from what B has published. This is a kind of cut principle for 
inferential responsibility.

It might be worth trying to draw up some of these rules. We are going 
to need them, once the SWeb gets going in the real world.

>The specifications of HTTP are defined such that if you dereference
>a term, then the information you get, modulo forgery, is  a published
>statement about the term by its owner.

?? About the term?? Surely not. When I access http://www.google.com/, 
what I get isn't about "http://www.google.com/" . It might be about 
Google, but that's a different claim altogether.

>The information which anyone may get in this way may be useful,
>When you use such a term, you can be held to the implications of
>your statements according to the specifications.

You can? By whom, and under what circumstances? What does 'being held 
to' amount to? I think that until questions like this are answered 
clearly, this whole discussion is meaningless.

>When the term's owner
>publishes a schema, then this makes public a currently rather
>poor subset of the information about the term.
>
>You certainly don't have to explicitly import the schema or anything
>else to be bound by the meaning of the term.
>
>However, that doesn't mean that every processor should consider your
>document as containing the statements of the schema.  An inference 
>engine should generally feel at liberty to suck in such information 
>IF it exists, and IF it is of
>a form which will be useful.  But it is not part of the document.

Right, that makes sense. Its something more like a licence rather 
than an instruction.

>
>So it is maybe useful to have a statement which asserts that the document's
>meaning explicitly includes a given resource's meaning.   It isn't essential
>at the RDF level.  Shouldn't be part of the core.
>If it is an RDF statement (rather than magic syntax) then it becomes
>interesting because it gives web access capability to an inference engine.
>
....<snip>

>>This
>>isn't too bad when you can't express a contradiction, but once you
>>include additional semantic primitives (as DAML+OIL does) and scale for
>>use in distributed environments you are bound to get contradictions if
>>you simply merge the information sources. Some have suggested that in
>>such situations that you just select one of the contradictory axioms and
>>throw it out. However, if different systems choose different things to
>>throw out, then they no longer agree on the semantics of terms used. The
>>only way around this problem would be to have a consistent, world-wide,
>>determinisitc method for resolving these contradictions. Even if it was
>>possible to design such a thing, it would be highly impractical because
>>people often cannot agree on things (some of the worst cases of this
>>lead to wars, etc.) We can never expect the Semantic Web to be a single
>>consistent knowledge base!
>>
>You are absolutely right in that.  RDF was never expect to imply 
>that, and in fact a whole lot of energy has been expended from time 
>to time explaining that to people.

No doubt the inventors of RDF did not intend to impose global 
consistency; such an ambition is obviously slightly insane. However, 
as a matter of fact, this insane assumption is built into the very 
architecture of RDF as it exists. RDF provides no way to make any 
other assumptions. It provides no way to agree or disagree, no way to 
define, no way to negotiate, no way even to question, any content. It 
only provides a simple way to make elementary assertions using a 
global vocabulary. It is obvious that this will not work unless there 
is some global coherence to the assertions made using the global 
vocabulary, unless there is some way to negotiate content and handle 
contradictions.

>RDF provided only reification as a very crude tool for doing fancier 
>things.  I found that allowing an RDF set of statements (graph, 
>"model" for some, or better "formula") to be itself gets you out of 
>that hole, and allows you to handle a heterogeneous world

I don't see how. It makes RDF more logically expressive, but the 
globality assumption is built into the restriction on RDF that it use 
only urirefs as identifiers. Those are globally unique names, 
supposed to have a scope that covers the whole planet and transcends 
(in an ideal world) all future changes. That already embodies the 
insanity of the global-consistency picture.

Pat

Received on Thursday, 23 May 2002 12:10:33 UTC