[SCL] SCL spec

[Murray Altheim]

pat hayes wrote:
>> [small correction to my last message to avoid any confusion.]
>>
>> I wrote:
>> [...]
>>
>>> 4. Finally, as I've done in my XCL proposal, all components of
>>>    the language that may be referred to conceptually either in
>>>    discussions or more particularly by tools should be given a
>>>    unique identifier. Topic Maps introduced a concept called
>>>    Published Subject Indicators (PSIs) that are basically just
>>>    stable URIs used as identifiers. The idea here (which would
>>>    be useful in Topic Map, RDF, or any other web-related system)
>>>    is that if two things identify themselves as having the same
>>>    PSI, they are semantically equivalent.
>
> Oh dear, that is a VERY strong statement (and it seems to me rather 
> naive, to be honest.) Semantically equivalent in what language, with 
> what model theory? Do you mean they denote the same entity in all 
> possible models? (Please don't say yes to that question, at least not 
> quickly: see the uri at w3c.org email archive for some heated discussions.) 
> This whole matter of there being a global semantics for URIs seems to me 
> to be woefully under-analyzed at present, and almost everything written 
> about it is rather naive. The REST model (and the diluted version 
> assumed in RFC 2396) isn't up the task without being extended, for sure.

Perhaps I'm being unclear. It's not that machines ever declare two
things as having identity, but rather people. There are no semantics
whatsoever in URIs, in my opinion. They're just strings used as
identifiers (some of which may resolve to something, but that for
the purposes of this discussion is not important). This isn't some
instance of nominalism, this is unambiguous identifiers within
specific domain namespaces being used to identify concepts.

>>>  For example, if OWL
>>>    has a concept "http://www.w3.org/2002/07/owl#" and my Ceryle
>>>    project has a concept "http://purl.org/ceryle/psi/authoring/#Thing",
>>
>> [...]
>>
>> Cut and pasting I neglected to add "Thing" to the OWL identifier.
>> This should be "http://www.w3.org/2002/07/owl#Thing" being compared
>> with "http://purl.org/ceryle/psi/authoring/#Thing". Sorry if this
>> caused any confusion.
>>
>> Without this feature it's difficult (at a tool level) to claim
>> identity between systems for even things such as "true" or "false".
> 
> Why would you expect it to even be meaningful to claim identity between 
> *systems* ? Of course it is difficult to claim identity across systems 
> which have different underlying semantics: it ought to be difficult, 
> because it is meaningless.  That is one of the main purposes of the 
> application of SCL to the Web, to try to provide a globally coherent 
> semantic framework which can underlie the various surface notations.

Again, I was unclear. The purpose of PSIs is to identify concepts
unambiguously. If you and I have two systems, and I'm trying to
map the interpretation of concepts within your system to the
interpretation of concepts within mine (e.g., I can determine that
my "forall" is the same as your "forall" by reading the documentation
of both systems, therefore their two PSIs identifying the concepts
can be mapped as equivalent).

>> [perhaps someone can answer me privately on this question: I can't
>> find "true" or "false" among OWL or RDFS. Is it there and I'm
>> missing it, or not? Thanks.]
> 
> Not explicitly. What do you expect those names to mean? There is a 
> 'universally true' class, ie the universe class, which is rdfs:Resource 
> in RDFS and owl:Thing in OWL; similarly the 'universally false' (empty) 
> class in OWL is owl:Nothing. Different versions of OWL take different 
> views on the relationship between rdfs:Resource and owl:Thing, by the 
> way. In OWL-Full they are the same; in OWL-DL, owl:Thing is a subset of 
> rdfs:Resource, because the OWL-DL universe doesn't contain classes or 
> properties, for example.
> 
> These languages do not have names for what in logic would be called the 
> truth-values themselves.

Fair enough. I was thinking in terms of truth values, not classes. In Cyc
(of which I'm most familiar) it defines #$True as

    An instance of #$TruthValue. #$True is logical truth in Cyc; this is
    the abstract logical notion--not to be confused with Lisp's T, nor
    with the English word `true'.

So if in my Ceryle system I implement an ontology using a root concept
of "Thing" and I agree with Cyc's #$Thing, shouldn't it be possible to
consider them equivalent, and create a map equivalency between them
(i.e., I as a human state the relation so that my computer will consider
them equivalent)?

Would you say that Cyc's set of constants for #$implies, #$not, #$and,
#$or, #$forAll, #$thereExists, would these have identity (semantically)
with the SCL list of tokens, "implies", "not", "and", "or", "forall",
"exists"? If so (and this I would imagine can be determined by reading
the documentation of each), thenn (is there an equivalent to iff?) we
can map the PSIs between them:

          http://purl.org/xcl/1.0/#implies
    maps to
          http://www.cyc.com/cycdoc/vocab/fundamental-vocab.html#True

          http://purl.org/xcl/1.0/#exists
    maps to
          http://www.cyc.com/cycdoc/vocab/fundamental-vocab.html#thereExists

   et cetera.

assuming the publishers of Cyc decided to use their HTML URLs
as PSIs. This would simply be a matter of them declaring that to
be the case, and to conform to the PSI recommendations/best
practices, to publish some metadata about the set. You can see
this at:

   http://purl.org/xcl/1.0/#psimeta

Now, I'm perhaps (okay, likely) naive, but I'm not sure what else
one can do to state the equivalence between two concepts.

[Actually, since the Cyc publishers might be listening, I'd certainly
advocate "publishing" the PSI set for Cyc. Last time I talked with
Doug he said that the individual URLs for each Cyc constant could be
considered stable. One could publish them under their current URLs,
or add each constant name to some base URL. The former would be
preferred, IMO, as the OASIS technical committee handling PSIs has
recommended that PSIs resolve to human-readable documentation (if
I remember correctly). You could create a PURL domain for Cyc do
this redirecting in that manner so that the Cyc PSIs would be
independent of the web site domain, perhaps a strategy for gradual
transition over to opencyc. So these two would be equivalent, the
first redirecting to the second:]

    http://purl.org/cyc/fundamental-vocab.html#True
    http://www.cyc.com/cycdoc/vocab/fundamental-vocab.html#True

[At some point, you'd just change the redirect from cyc.com to
opencyc.org... any questions on this just ask.]

> Pat
> 
> PS. You might find the translation of OWL into Lbase (a version of SCL) 
> helpful in keeping track of things 
> http://lists.w3.org/Archives/Public/www-archive/2003Mar/att-0018/LBASE-new.html 
> 
> (This is slightly out of date now on the XML literal details, so ignore 
> that stuff at the beginning; and its incomplete as OWL has grown since 
> it was written, but it will give you the idea.)

Thanks -- will do. I must admit my first reading of LBase didn't
stick in my head, but I think I understand a few of these things
a bit better now.

Ahh, just occurred to me. I see in Cyc both #$tuple and #$Relation.
During the telecon we talked about n-tuples, whereas I'd been thinking
syntax-wise more along the lines of #$Relation.

Now, a question came up (I think from John) about whether Topic
Maps are/will be important technology. I believe so, but that's
just my opinion. What I think is more important is that for the
CL specs we include PSIs, so that *whatever* technology is used
in implementing CL-based systems, there's a way to unambiguously
label each language component (what did John call these? grammar
things? can't remember now. I'd been calling them syntax
"productions"). So if RDF or TM fails or succeeds, if URLs are
still being used as identifiers in ten years, CL concepts will
still be identifiable.

Enough for now. Chris, when you edit your next version of the spec,
could you be so kind as to perhaps add in square bracketed tokens
for each part of the abstract syntax you think should be identified
as a "grammar thing"? (gad, my memory today is shot). If you look
at the XML Recommendation

   http://www.w3.org/TR/REC-xml

you can see what I mean by production numbers ([1], [2], etc.).

Murray

......................................................................
Murray Altheim                    http://kmi.open.ac.uk/people/murray/
Knowledge Media Institute
The Open University, Milton Keynes, Bucks, MK7 6AA, UK               .

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