SUO: Re: W3C approves RDF and OWL as recommendations
The recent interchange between Azamat Abdoullaev and
Danny Ayers raises some important questions about
logic and ontology in general and about the SUO and
the semantic web in particular. I don't agree 100%
with either one, but I agree more with Asha (about 75%)
than with Danny (about 25%). In any case, the questions
address important issues about the goals and directions
for both the SUO and the semantic web.
I'll start with Danny Ayers' conclusion:
DA> The bottom line is that the W3C's initiatives and
> the loftier aims you describe aren't mutually exclusive.
> I'd go further and suggest that the former may indeed
> help bootstrap development of the latter.
I agree with the first statement, but disagree with the
second. The biggest weakness of the semantic web is
illustrated by the popular "layer-cake" diagram (see
attachment). As that diagram shows, the W3C started
with a purely syntactic foundation consisting of Unicode,
the URI naming scheme, and the XML language. There is
nothing wrong with including those notations as important
parts of the W3C strategy. But what is wrong is the absence
of an architecture that addresses the semantic issues.
I agree with Azamat Abdoullaev's statement:
AA> the administrators running the World Wide Web Consortium
> rushed the matter by recommending OWL as an ontology
> language standard fit for structuring the Web data,
> documents, and applications. Since, beside the well-known
> merits, the language has bad conceptual faults which make
> it fall short of wide commercial use.
As an example of the weakness of OWL, I would cite the following
claim, which is dangerously and inexcusably misleading:
DA> As I see it the OWL language attempts to find the sweet-spot
> between expressive capability and decidability in the context
> of the web. From my own little experiments, I'd suggest it
> does quite a good job.
The most serious misconception is the idea that decidability is
a property of a language. That is totally misguided. The correct
statement is that decidability, like solvability or unsolvability,
is a property of a problem. There is an enormous number of
problems associated with the WWW, with varying degress of
solvability and with varying requirements for expressive power.
One example of the need for more expressive power than OWL is the
query language, of which SQL is the most successful commercial
example. The WHERE clause in SQL queries and constraints has
the full expressive power of first-order logic, which goes far
beyond the capabilities of OWL. Yet all the problems to which SQL
queries and constraints are applied can be solved in polynomial
time. By any measure of success, SQL certainly hit a "sweet spot",
and the failure of OWL to match that sweetness is a serious mistake.
Another serious limitation of OWL is the kludgy syntax which was
inherited from RDF, which is a very watered down version of Guha's
original proposal, which he only intended as a preliminary step
toward a more suitable language. Unfortunately, the absence of
published guidelines for the RDF semantics let developers treat
the RDF building blocks as a Lego kit that lead to an incoherent
florescence of exotic forms that had no common semantics. Guha
and Hayes eventually published a sound semantics, but only after
many applications of RDF had been built without any semantics.
AA> The language is lacking many significant features of
> relations, both formal and real. Among real, first of all,
> the relation of cause and effect is fatally missing.
> Such defects come from the approach used, purely
> set-theoretical and formal logical, while any content-based
> (world) ontology distinguishes internal and external relations,
> avoiding their reduction to relational properties.'Being a
> parent' is merely a relational property, while 'parentage'
> (parenthood, but not the act or process of parenting) is
> a relation of parent to child. Or, more generally, the
> relational property of being a cause is just a monadic
> reduction of causality, the relation of cause to effect.
DA> Cause and effect can be modelled using OWL - this is
> something found in OWL-S:
>
> http://www.daml.org/services/owl-s/1.0/owl-s.html
Indeed, a relation named "cause" can be represented in OWL.
But there is much more to the concept of causality than
just a simple relation. For a brief outline of some
of the issues, I suggest a paper I started to write,
and which I hope to complete sometime:
http://www.jfsowa.com/ontology/causal.htm
Processes and Causality
DA> I can't actually see any part of that which couldn't be
> represented fairly directly using RDF/OWL. As to the processing
> of the material thus represented, maybe what you describe falls
> outside of the capability of OWL's Description Logic - but
> that doesn't mean such logic couldn't be layered on top.
I have no quarrels with using a description logic as a part
of a more complete system of logic and ontology. But I
strongly disagree with the idea of starting with the bottom
layers before any work has been done on the overall
architecture. For a view of the kind of design that emerges
from that approach, I recommend the following web site:
http://www.winchestermysteryhouse.com/story.html
I agree with Abdoullaev that Aristotle's framework is an
outstanding example of the kind of work that is needed
for an upper ontology, and I believe that the Aristotelian
foundation he recommends should be included in the MSO
(Multi-Source Ontology) that is being proposed for the SUO:
http://www.eis.com.cy/
However, I also agree with Ayers' cautionary note:
DA> Whether there is a single universal ontology is a moot point.
> There's certainly a lot more that can be done in the field,
> but I would suggest that we can learn a lot from applying
> what we have already, approaching the problem from the
> direction of what we do know, rather than that of what we don't.
Where we disagree is whether OWL is a satisfactory example
of what we know. The Aristotelian basis of the EIS ontology
has been known for over two millennia. FOL has been known
for over a century, and the SQL application of FOL has been
in commercial use for the past 30 years. Another example is
the Horn-clause subset of logic, which has been implemented
in many commercial applications for almost as long as SQL.
Although I agree that the possibility of a single universal
ontology is "a moot point", there is a lot more that can be
done to accommodate multiple ontologies. I have recommended
the following paper as an outline of a richer, broader approach,
which could be supported by something like the MSO and WebKB:
http://www.jfsowa.com/pubs/signproc.htm
Signs, Processes, and Language Games
John Sowa
