Just say not

Pat Hayes,
For me your "Catching the Dreams" essay [1] tells us the sorted story of why
the Semantic Web seems to have zigged into complexity when some of us though
it would just zag. Thanks for writing it ... I'm posting this to
RDF-Interest, logic, comments, and semanticweb in the hopes that more people
will get a chance to read your essay in its entirety.
[1] http://www.aifb.uni-karlsruhe.de/~sst/is/WebOntologyLanguage/hayes.htm
But I want to ask some particular question inspired by your passages ...
[[
 Considered as content languages, description logics
 are like logics with safety guards all over them. They
 come covered with warnings and restrictions: you
 cannot say things of this form, you cannot write rules
 like that, you cannot use arbitrary disjunctions, you
 cannot use negation freely, you cannot speak of
 classes of literals, and so on. A beginning user might
 ask, why all the restrictions? It's not as if any of these
 things are mysterious or meaningless or paradoxical,
 so why can't I be allowed to write them down on my
 web page as markup? The answer is quite revealing:
 if we let you do that, you could write things that our
 reasoning engines might be unable to handle. As long
 as you obey our rules, we can guarantee that the
 inference engines will be able to generate the answers
 within some predetermined bounds. That is what DLs
 are for, to ensure that large-scale industrial ontologies
 can be input to inference machinery and it still be
 possible to provide a guarantee that answers will be
 found, that inferential search spaces will not explode,
 and in general that things will go well. Providing the
 guarantee is part of the game: DL's typically can be
 rigorously proven to be at least decideable, and
 preferably to be in some tractable complexity class.
]]
... and then ..
[[
 I think that what the semantic web needs is two
 rather different things, put together in a new way.
 It needs a content language whose sole function
 is to express, transmit and store propositions in a
 form that permits easy use by engines of one kind
 and another. There is no need to place restrictions
 or guards on this language, and it should be
 compact, easy to use, expressive and syntactically
 simple. The W3C basic standard is RDF, which
 is a good start, but nowhere near expressive
 enough. The best starting-point for such a content
 language is something like a simple version of KIF,
 ..
]]
So what (if anything) would we sacrifice if the semantic web adopted a
language that included the basic sentential operators (and, or, not, =>,
<=>) as primitives? Specifically what inference algorithm would become
intractable ? Could that intractability be eliminated with a simple
assumption: select only those facts and axioms that apply to a narrow
context prior to starting any inference process?
Could we use the test case example as per:
[2] http://lists.w3.org/Archives/Public/www-webont-wg/2002Mar/0127.html
Somebody says:
 :page1 dc:title "ABC"
Then I want to contradict their assertion:
 :page1 (is not dc:title) "ABC"
It seems to me that DanC's way of saying that in [2] using DAML is
needlessly complicated.
Why can't I just say:
 :not_title :negates dc:title
and then
 :page1 :not_title "ABC"
where I have imported a rule for negation... perhaps coded something like in
my mentograph [3]:
(<=> (not (p A B) ) (and (not_p A B) (:negates not_p p)))
[3] http://robustai.net/mentography/notArrow.gif
Now obviously both of those assertions cannot consistently exist in the same
context (sorry for using the 'C' word). So, hopefully just as obviously, we
need to introduce the 'C' word in the next version of a semantic web
language. Hmmmm ... how come I don't see the big c mentioned in [4] ?
[4] http://www.w3.org/TR/webont-req/
What would be the real problems (if any) of this simplicity ?
Seth Russell

Received on Friday, 8 March 2002 16:49:52 UTC

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