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Re: Semantics101, 2 and 3

From: Jos de Bruijn <jos.debruijn@deri.org>
Date: Tue, 28 Jun 2005 10:15:37 +0200
Message-ID: <42C10729.6090505@deri.org>
To: Adrian Walker <adrianw@snet.net>
CC: public-rule-workshop-discuss@w3.org, "Boley, Harold" <Harold.Boley@nrc-cnrc.gc.ca>, tim.glover@bt.com

Hash: SHA1

Adrian Walker wrote:

> Hi All --
> Firstly, many thanks to Sandro et al for the Workshop report and
> minutes, and to Ben, Harold, Michael and Said for the Strawman
> powerpoint.
> As Ben pointed out in the closing minutes of the workshop, there is
> a lot of good conceptual work from the logic programming community
> (well beyond the initial work on Prolog) that should inform any
> Web Rules effort.
> In particular, we have an opportunity to "ride on the shoulders of
> semantic giants" (or at least of tall people).
> Reasoning with rules over the semantic web is going to be a
> conceptually complex process. To bring order and accountability to
> such a process, we would do well to focus on semantics,
> authorability and understandability early on.
> There are at least three kinds of semantics to consider:
> 101 -- tagging data and predicates with their real world types, as
> in xml or rdf
> 102 -- model theory based semantics that say what *should* be
> deducible from a collection of rules and facts
> 103 -- executable English sentences that document what we mean by
> our predicates
> I think we all pretty much understand the 101 part.
> For 102, there is the question of *which* semantics. The leading
> candidates would appear to be (a) stratified range-restricted
> datalog with negation [1], or (b) the well-founded semantics [2].
> Note that (a) is two-valued -- true or false -- while (b) is
> three-valued -- true, false or unknown. There are many successor
> papers to each of [1] and [2].

(a) and (b) are equivalent for the stratified case, so the
well-founded semantics could easily extend stratified negation. In WRL
we have chosen to use Perfect Model semantics for the simpler language
variant (WRL-Flight) and well-founded for the more complex variant
Perfect Model semantics is slightly more general than stratified,
because it allows locally stratified programs, which is more
appropriate in F-Logic based languages (and WRL is based on F-Logic).

> The need for semantics103 can be seen by reading the jena-dev and
> other SW-related lists. There, one daily sees smart people getting
> thoroughly confused about the meanings of RDF and OWL constructs
> and reasoning processes when translated into English.

When your language consists of rules, the reasoning process is much
easier to understand than for such complex formulas as OWL axioms.
In general, simpler language constructs are easier to understand than
more complex ones. Plus, the meaning of some constructs in RDF(S) and
OWL is not intuitive to everyone.

I'm not really sure what you meant by "executable English sentences".
Did you mean a natural-language like surface syntax for the language?

Best, Jos

> In our work, we have fielded an online system [3] that meets the
> 103 need without getting deep into NL research. However, the
> system depends on having a firm basis at the Semantics102 level.
> Thus, this is more than 'syntatic sugar' [4]. For example, a
> two-valued closed world semantics102 can support, at the
> semantics103 English level, an open world flavored predicate such
> as "so far as is known at the moment, flight 678 is on time".
> So, here is a proposal for prioritization, as Harold has suggested.
> A. Add to the top of the "semantic layer cake'"a "rules
> authorability layer". Perhaps just rename the FOL++ block in the
> strawman ppt .

> B. Rank the available Semantics102 candidates from the literature
> [1,2,...]
> I hope all of this makes sense. If anyone would like to set up a
> time to call us at (USA) 860 830 2085, I'll be glad to discuss.
> Thanks in advance for comments on the list too.
> -- Adrian
> [1] K. R. Apt, H. A. Blair and A. Walker, Towards a theory of
> declarative knowledge, in J. Minker, ed., Foundations of Deductive
> Databases, pp 89-148, (Morgan Kaufmann, Washington, 1988).
> [2] Van Gelder, A., K.A. Ross, and J.S. Schlipf. 1991. The
> Well-Founded Semantics for General Logic Programs. Journal of the
> ACM 38(3):620--650.
> [3] Internet Business Logic, online at www.reengineeringllc.com
> <http://www.reengineeringllc.com/>
> [4] http://www.w3.org/2004/12/rules-ws/paper/19
> <http://www.reengineeringllc.com/>.reengineeringllc.com
> <http://www.reengineeringllc.com/>
> Adrian Walker Reengineering LLC PO Box 1412 Bristol CT 06011-1412
> Phone: USA 860 583 9677 Cell: USA 860 830 2085 Fax: USA 860
> 314 1029

- --
Jos de Bruijn, http://www.uibk.ac.at/~c703239/
+43 512 507 6475         jos.debruijn@deri.org

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Received on Tuesday, 28 June 2005 08:15:53 UTC

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