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ELECTRONIC NEWSLETTER ON REASONING ABOUT ACTIONS AND CHANGE
Issue 98014 Editor: Erik Sandewall 2.2.1998
Back issues available at http://www.ida.liu.se/ext/etai/actions/njl/
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********* TODAY *********
We carry today a further exchange between Michael Gelfond and Hector
Geffner, continuing the discussion from the previous Newsletter.
We also advertise the posting of major parts of the discussions at the
recent Commonsense 1998 workshop, which contains a lot of interesting
and timely material for reasoning about actions and change. It is also
a major step in the development of the present medium for onlline
research communication. The specifics follow below.
A clarification: when Hector Gelfond wrote "This is in reply to
Michael's message" in last Friday's Newsletter, he was referring to
Michael Gelfond and noone else.
********* NEW INITIATIVES *********
--- COMMONSENSE WORKSHOP CONTINUES DISCUSSIONS ONLINE ---
--- AND IN OUR NEWSLETTER ---
The recent workshop on Formalization of Commonsense Reasoning in London
on January 7-9 featured 23 presented papers, as well as two panel
discussions. The discussions at the workshop were very lively and
informative, and it was decided to make an experiment with pursuing
the discussions online. The result is now available in the ETAI web
structure, and to go there directly please use
http://www.ida.liu.se/ext/etai/nj/fcs-98/listing.html
The structure rooted at that URL contains a discussion summary that the
participants have had a chance to correct and augment (although additional
amendments may be forthcoming), but it is also intended as a starting
point for continued discussion. Thus, the online workshop discussions
will proceed in the same manner as our discussions about ETAI received
papers and our existing on-line panels. Also, it is of course not
restricted to workshop participants: every reader of the Newsletter
is invited to take this opportunity to ask questions to the authors.
Just like in a question period in a seminar, this is for the benefit
of all: if one person thought something required further explanation
and asked a question, then chances are that several others will also
find the question and the answer useful. In fact, this style of
presentation may be an excellent way of explaining a piece work, just
like a list of "Frequently Asked Questions" and their answers may be
more readable than a plain text containing the same information.
In other words, you are making the author a favor by asking a good
question to her or him.
However, in one respect the discussion about workshop papers differs
from our earlier online discussions: we don't send out the initial
part by e-mail, simply because the resume of questions and answers is
too voluminous. Follow-up questions and comments concerning a point
that was first addressed in the discussion at the workshop will have to
be understood by relating to the existing online structure (URL above).
Although many of the workshop papers addressed reasoning about actions
and change, some of them concerned other aspects of commonsense
reasoning. (Mutatis mutandis, there are some aspects of reasoning
about actions and change that don't really qualify as formalization of
commonsense). However, although the online discussion is run through
the Actions and Change Newsletter and Colloquium, we'll be generous and
include all the articles. This means in particular that researchers
outside the constituency of the present Newsletter may also be interested
in seeing the discussion and in participating. If you know someone who
might be interested on that account, please forward this Newsletter to
her or him.
********* DEBATES *********
--- ONTOLOGIES FOR ACTIONS AND CHANGE ---
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| FROM: Michael Gelfond
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Dear Hector,
You write
> As I see it, in the "basic" approaches to the frame problem (Reiter's
> completion, Sandewall form of chronological minimization, language L, etc.
> in one way or another, action rules are compiled into transition functions
> of the form f(a,s) - where $s$ is a state and $a$ is an action - that
> describe the set of possible state trajectories. Observations in such
> models just prune some of the trajectories and hence have a "monotonic"
> effect (i.e., predictions are not retracted, at most they are made
> inconsistent).
If you replace L by A then I agree.
A model of L however consists of a transition function, initial
situation and the ACTUAL PATH - a sequence of actions which actually
happened so far.
Observations not only prune "the set of possible trajectories" but also
change the actual path in each model. This gives the nonmonotonicity
of entailment in L.
Is this nonmonotonicity "in the set of observations"?
You continue:
> In other models, Michael's and Luis' included, actions are represented
> in the state (in one way or the other) and hence abduction to both fluents
> and actions are supported. Such models *are* non-monotonic in the set of
> observations. Actually the only apparent difference in such models between
> actions and fluents is that the former are assumed to be "false by default"
> while the latter...
You are right when you say that in L occurrences of actions are assumed
``false by default'' and fluents are assumed "to persist by default".
But there are other ``standard'' differences. Actions label arcs of the
automaton, while sets of fluents label its states. There is also a syntactic
difference: occurs(A,S) requires action as the first parameter, at(F,S)
requires F to be a fluents, etc.
Michael
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| FROM: Hector Geffner
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Dear Michael,
I meant the language A, not L.
I've read your paper with Chitta and Provetti, but don't have it with me.
As far as I recall, and following what you say, yes, definitely that
account is non-monotonic wrt to the observations.
Also a correction: I referred to a AAAI94 paper of mine on this topic;
I should have said AAAI96.
Hector
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