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    ELECTRONIC NEWSLETTER ON  REASONING ABOUT ACTIONS AND CHANGE        
Issue 98069             Editor: Erik Sandewall             16.9.1998
Back issues available at http://www.ida.liu.se/ext/etai/actions/njl/
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                    *********  TODAY  *********

The discussion between Eugenia Ternovskaia and the authors Marc
Denecker, Daniele Theseider Dupre, and Kristof Van Belleghem has
referred several times to the work of Michael Thielscher on the same
general topic, i.e. on ramification. Today we have his comments to the
discussion.



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The following debate contributions (questions, answers, or comments)
have been received for articles that have been submitted to the ETAI and
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        ========================================================
        |  AUTHOR: Marc Denecker, Daniele Theseider Dupre, and Kristof 
        |          Van Belleghem
        |  TITLE:  An Inductive Definition Approach to Ramifications
        |  PAPER:  http://www.ida.liu.se/ext/epa/cis/1998/007/tcover.html   
        |          [provisional]
        |  REVIEW: http://www.ida.liu.se/ext/etai/received/actions/009/aip.html
        ========================================================

--------------------------------------------------------
|  FROM: Michael Thielscher
--------------------------------------------------------

Dear Eugenia, Marc, Daniele, Christoph,

I'd like to second Eugenia's view on the "Counter" counter-example,
which implies that the intended behavior can be obtained after all
with my approach. The natural "inter-state constraint" in this example
would be

  -Holds(out,s) & Holds(out,do(a,s)) & Holds(count(n),s)
    -> Holds(count(n+1),do(a,s))

( in conjunction with Holds(count(m),s) & Holds(count(n),s) -> m=n )

Then if you consider the causal relationship

   out  causes  count(n+1)  if  count(n)

( in conjunction with count(n) causes -count(m) if m=/=n )

the only consistent successor state would show the expected increment
of the counter by one. (A side remark: Inter-state constraints like the
above can be expressed in a novel Situation Calculus-style formulation
of the Fluent Calculus; if you're interested, have a look at
http://pikas.inf.tu-dresden.de/~mit/publications/conferences/JELIA98.ps)

You have raised an interesting question in this context, namely,
whether and how causal relationships can be extracted from such
inter-state constraints. The current procedure is not directly
applicable because it is based on formulas without situation or time
argument. My feeling is that a generalization must be possible--there
definitely is a formal correspondence between the above state
constraint and the causal relationship--but working out the details
will require some brain work.

Cheers.
Michael


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