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

Re: What do the ontologists want

From: pat hayes <phayes@ai.uwf.edu>
Date: Tue, 15 May 2001 19:59:42 -0500
Message-Id: <v0421011bb7277b56aa49@[205.160.76.173]>
To: "Jonathan Borden" <jborden@mediaone.net>
Cc: www-rdf-logic@w3.org
>pat hayes wrote:
>
> >
> > >In any case, the ability to create a reference to something has proven
> > >itself useful in my practical experience.
> >
> > Sure, but be careful what you mean by 'reference'. A pointer to a
> > position in a data structure is one kind of reference; a name
> > denoting an entity is a different notion of reference. Both are
> > useful, but trouble ensues if you use one for the other carelessly.
> > Quotation creates a name (of a character string), not a pointer.
>
>The issue regarding the distinction between names and addresses has been
>often addressed. It is not clear to me that one can support making an
>absolute distinction. Frequently 'pointers' are used as names and names as
>pointers, especially in the markup world.

Yes, I agree the issue gets complicated when one considers markup 
languages and tries to understand them in logical-semantic terms. 
Even hoary old LISP poses some tough challenges for a coherent 
semantic theory, I think for much the same kinds of reason.

> >
> > >The reason I have suggested that quotation be implemented in a fragment
> > >identifier syntax is that URI references are how we refer to things on
>the
> > >web, and a statement is one of the things we might want to refer to.

(I should have said this last time) I agree entirely that we might 
want to refer to statements from time to time.  But like salt, this 
should be used in moderation.

> >
> > I confess to still not knowing what a URI is supposed to be beyond a
> > URL, and not believing what I have read about them. URLs are file
> > addresses, a species of pointer, and those I do understand. URLs do
> > not denote what they point to. The relationship between a URL and the
> > file it locates is not the same as that between a logical name and
> > what it denotes (for example, the locatee is not determined relative
> > to an interpretation, but is fixed by the operational circumstances.)
>
>I agree that there currently exists alot of fuzziness in this regard. I am
>in the process of trying to clean this up by providing a (hopefully) logical
>framework. http://www.rddl.org/SchemaAlgebra is a work in progress
>addressing some of these issues.

Great.  Thanks for the pointers and exposition. I will try to digest them.

> >
> > >Quotation is related to query and in the same way as XPath serves as a
> > >simple query language for XML,
> >
> > I beg to differ. XPath provides a way to refer to a position within
> > an expression, which is exactly what quotation does not provide.
> > Quotation simply refers to the quoted expression as an atomic whole,
> > and provides no way to access its structure, syntax, parts or
> > anything else.
>
>Not true. Quotation allows perfectly good access to structure and syntax.
>For example:
>
>'(says 'Jon '(isColor 'sky 'blue))
>
>what is the problem with access to structure?

Ah, but now you have changed the quotation rules (if I follow you). 
This seems to be quotation as in LISP, where it really means 
something different: the quote mark is a tag to prevent evaluation. 
One can indeed think of this as a kind of structural quotation, where 
instead of a character or lexical string being denoted, the quoted 
expression denotes a datastructure (or perhaps an abstract 
mathematical structure indicated by the datastructure conventions) 
containing lexical items. Yes, of course: just as expression 
quotation in English text preserves the character-string linear 
structure, structural quotation preserves the 
expression-subexpression structure. That is what McCarthy called 
'abstract syntax' (in an old paper which everyone should read at 
least twice.)  OK, if this is what you mean by quotation, then it is 
more similar to quasi-quotation in logic.

> > If you want to do that, you need something like
> > quasi-quotation (an extended form of quotation invented by WVO Quine
> > which allows quantification over substrings inside quoted
> > expressions),
>
>exactly.
>
> > and if you want to be able to refer to the *content* of
> > quoted expresions, then you also need something that maps between
> > descriptions of expressions (such as quoted strings) and the actual
> > expressions that those descriptions describe; what is usually called
> > a truth-predicate.
>
>Perhaps we are using different meanings for the term quotation. So far I am
>talking about using quotation in the simplified stripped down version of RDF
>that my initial post to this thread defined. This stripped down version of
>RDF currently exists purely at the syntactic level.

So it has no semantics at all? I doubt if that is really what you 
mean, since then it would have no point to it, as far as I can see. 
Certainly the RDF literature seems to *want* RDF to actually mean 
something.

>Generally the 'something' that maps a character string to an actual
>expression is called a parser/evaluator.
>
>Bottom line: in order to be useful, quotation
> > needs to be supercharged and be supplemented with truth-predicates
> > (or principles of reflexion, or various other exotic pieces of
> > linguistic machinery).
>
>My assumption is that DAML+OIL intends generally to provide an acceptable
>semantic meaning to RDF (abstract syntax) statements.

I would phrase it rather differently: DAML+OIL has a syntax of its 
own with an associated semantics. Some of that syntax is  RDF, some 
is RDFS, and it is all parsable as RDF triples. But the semantics of 
most of DAML+OIL requires one to understand it as DAML+OIL, not 
merely as RDF.

> >
> > >an RDF path language might serve as a simple
> > >query language for RDF. For example:
> > >
> > >?x says "The sky is blue"
> >
> > OK, if you bear in mind that this really does mean only that ?x says
> > a certain string of characters. It (quite literally) says absolutely
> > nothing about skies being blue, not even that ?x said that. If you
> > want to say that ?x asserts that the sky is blue, you need something
> > which can map back from the quoted string "The sky is blue" to
> > whatever proposition that quoted string is asserting in whatever
> > language ?x is talking in.
>
>why not:
>
>(says ?x '(isColor 'sky 'blue))

Well, again, you have switched to structural quotation; but still my 
point applies just as well. This says that ?x says a certain 
structure (because that is what a quoted expression denotes.) But 
what ?x actually meant was probably something about the color of the 
sky. If you want to draw any conclusions about the content of what ?x 
said - for example, to judge if it is true or false - you need to 
somehow map from this structure to its content. The map may of course 
be implicit in the semantic rules of your language, but this way 
madness lies. Or at any rate, I think it does.

>or in the triple fragment id syntax:
>
>#triple(:says ?x <#triple(:isColor :sky :blue)>)
>
>
>note: as a _query_ this quotation universally quantifies x (assuming the
>'universe' is the current database)

Sure, I have no problem with the ?x, which is safely outside any 
taint of reification or quotation.

>
>in an analogous fashion to an XPath which returns an XML node set on
>execution, this would return a set of statements on execution.
>
> >
> > I'm not saying that this can't all be done. I am saying that it is
> > exotic, not simple, and requires elaborate syntactic and semantic
> > machinery to be done without producing paradoxical results.
>
>If this stuff were easy we'd all be home now.

Amen to that. Thanks for feedback.

Pat Hayes

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Received on Tuesday, 15 May 2001 20:59:48 UTC

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