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    ELECTRONIC NEWSLETTER ON  REASONING ABOUT ACTIONS AND CHANGE        
Issue 97024             Editor: Erik Sandewall             19.11.1997
Back issues available at http://www.ida.liu.se/ext/etai/actions/njl/
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                    *********  TODAY  *********

The discussion about action languages vs direct use of logic has
prompted a contribution by Vladimir Lifschitz in today's issue.



                   *********  DEBATES  *********

---  NRAC PANEL DISCUSSION ON ONTOLOGIES FOR ACTIONS AND CHANGE  ---

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|  FROM: Vladimir Lifschitz
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I would like to respond to some of the comments on action languages
published in ENAI 13.11 and 17.11.

Tom Costello writes to Tony Kakas and Rob Miller regarding their new
action language:

> The reason I ask for truth
> conditions for your propositions is that I cannot understand what the
> intuitive consequences of a set of propositions should be, unless I
> understand what the propositions say.

> As you say, action languages are supposed to be
> "understandable and intuitive". Languages cannot be understood without
> semantics.

It seems to me that the semantics of an action language cannot be
described by specifying truth conditions for its propositions.  The
problem is the same as with nonmonotonic languages in general.  Take,
for instance, the closed-world database {P(1),P(2)}.  The negation of
P(3) is a consequence of this database, but this fact cannot be
justified on the basis of truth conditions for P(1) and P(2).

Patrick Doherty writes:

> The danger we find with the trend in using A language approaches is
> that it often appears to be the case that one is taking a relatively
> simple surface language and translating into what turns out to be 
> something along the lines of classical logic, but in a rather 
> indirect and complex manner.

> On the other hand,
> if provided with well-understood and modular translations into 
> classical logic, it is much easier to evaluate progress and 
> simplify comparisons.

It is impossible, unfortunately, to translate an action language into
classical logic in a modular way, because classical logic is monotonic,
and action languages are not.  The best we can achieve is a modular
translation from an action language into a nonmonotonic formalism,
such as circumscription, whose semantics can be characterized by a
nonmodular translation into classical logic.  This is indeed indirect
and complex.  But we have to pay this price for the convenience of
reasoning about actions in classical logic.

Vladimir Lifschitz

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