Electronic Newsletter Actions and Change

Electronic Newsletter on
Reasoning about Actions and Change


Issue 97019 Editor: Erik Sandewall 10.11.1997

The ETAI is organized and published under the auspices of the
European Coordinating Committee for Artificial Intelligence (ECCAI).

Today

The ontologies panel continues unabated, today with a contribution by Hector Geffner. Also, the session on the paper by Kakas and Miller today contains the first occurrence of a continuation - beyond questions to the authors and answers to the questions, which we have had so far, there is a follow-up question posed by Alessandro Provetti.

If you are using our web pages for referring to earlier discussion, you may appreciate that there is now a separate web page for each panel discussion. It is accessed via the HTML version of Newsletters (just click the session's headline in the Newsletter issue) or via the ordinary entries to the Actions and Change web structure.


ETAI Publications

Discussion about received articles

Additional debate contributions have been received for the following article(s). Please click the title of the article to link to the interaction page, containing both new and old contributions to the discussion.

Antonis Kakas and Rob Miller
Reasoning about Actions, Narratives and Ramification


Debates

NRAC Panel Discussion on Ontologies for Actions and Change

Hector Geffner:

Rob Miller says:
  My problem with this is that, in general, dynamical systems in the everyday world can't be realistically modelled as state transition systems, because they involve things like continuous change, actions or events with duration, partially overlapping events, interuptable events, etc. ...
My point is that action languages - in any dress you like - are just a convenient means for specifying (and in certain cases reasoning with) dynamic systems. That is the main lesson I think of the Yale Shooting Problem(s) and a lot of the work on temporal non-mon. Namely, the meaning of a rule like:

    if loaded and shoot then not alive
is that the only state trajectories s0, s1, s2, ..., that are possible are the ones in which `alive' is false at si+1 when loaded and shoot are true at si.

You can formulate the idea in many ways (suitable circumscriptive policy, Erik's version of chronological minimization, predicate completion, ...), but it is the same idea: rules specify possible state transitions, observations prune possible state trajectories.

Now, Rob is right; dynamic systems come in different varieties; e.g.,

  1. discrete time, discrete value space
  2. discrete time, continuous value space
  3. continuous time, continuous value space
  4. ....

Rules like the one above (with first order extensions, etc) are good for specifying systems of Type 1 only. Yet it's not difficult to see how systems of Type 2 could be specified as well.

Actually there are other type of mathematical models for the type of problems that Rob has in mind as the "Semi-Markov Decision Processes" (probabilistic continuous processes - like queuing systems - that are controlled at discrete time intervals).

My point is that we are not inventing new mathematical models of dynamic systems. What we are inventing are suitable structured languages for specifying and in certain cases controlling those systems. That's what STRIPS is about.

In my view, the KR/control enterprise is about developing richer versions of STRIPS suitable for specifying and controlling not only systems of Type 1, but also Markov Decision Processes, Partially Observable MDPs, Semi-MDP's, etc etc.

How we will measure success?

When we can model and control some dynamic systems that cannot even be modeled using non KR methods.

- Hector Geffner