******************************************************************** ELECTRONIC NEWSLETTER ON REASONING ABOUT ACTIONS AND CHANGE Issue 97022 Editor: Erik Sandewall 13.11.1997 Back issues available at http://www.ida.liu.se/ext/etai/actions/njl/ ******************************************************************** ********* TODAY ********* Costello and Miller have not agreed yet, but the disagreement becomes clearer for every day. In the ontologies debate, Miller asked a few days ago why we aren't doing more research on ***** (details below) instead of focussing so much on ##### for example. Today, Sandewall tells the true story of what happened to his graduate students who tried to work on ***** some five years ago. ********* ETAI PUBLICATIONS ********* --- DISCUSSION ABOUT RECEIVED ARTICLES --- The following debate contributions (questions, answers, or comments) have been received for articles that have been received by the ETAI and which are presently subject of discussion. To see the full context, for example, to see the question that a given answer refers to, or to see the article itself or its summary, please use the web-page version of this Newsletter. ======================================================== | AUTHOR: Antonis Kakas and Rob Miller | TITLE: Reasoning about Actions, Narratives and Ramification ======================================================== -------------------------------------------------------- | FROM: Tom Costello -------------------------------------------------------- Dear Rob and Tony, Your definition of truth for c-propositions seems very unintuitive to me. I would think that if A terminates F if G, then A terminates F if G,H Your definition does not give this result. 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. If the propositions are expressed in a standard logic, then I understand them using the definition of truth in a model. However, your propositions are not in a standard logic, and therefore, to understand what A terminates F if G means, I have to know when it is true. Your paper introduces a new type of proposition, F whenever G_1, ... ,G_n There are some obvious choices for truth conditions for this type of proposition. In particular it can be understood that every this is obeyed at every time-point, or that this is a property of every "possible" state, not every "actual" state. Without knowing which notion this proposition is trying to express, I cannot understand what the proposition says. I do not think truth conditions are a side point to the main theme of your paper. As you say, action languages are supposed to be "understandable and intuitive". Languages cannot be understood without semantics. I cannot find the referenced paper of Van Belleghem, Deneker and Dupre on their home pages. Can you supply a URL. Yours, Tom ********* DEBATES ********* --- NRAC PANEL DISCUSSION ON ONTOLOGIES FOR ACTIONS AND CHANGE --- -------------------------------------------------------- | FROM: Erik Sandewall -------------------------------------------------------- Rob, You wrote: > ... But I think that an important wider problem that we have to tackle > within "reasoning about action and change" is how to synthesise or > combine very different approaches to modelling dynamic systems within a > single "commonsense" framework. For example, I'd like to see more > research along the lines of Erik Sandewall's 1989 work on combining > reasoning about actions with modelling using the differential calculus. > It's true that there has been a small amount of subsequent work on this > theme since then ... But not much compared with, say, work on extending > state-transition based approaches to deal with ramifications in evermore > sophisticated ways. Why is this so? Why don't we put much effort into > addressing challenges such as Kuipers' - on combining the Situation > Calculus with Q.R. (see Kuipers' book, p. 201)? ... Unfortunately, the answer to this question is a brutal one: publication problems. At least, that's the conclusion I have drawn from the experience of our group. The following is what happened after our start on hybrid systems in 1988-89. A key new result in the 1989 papers (KR and IJCAI) was that minimization or restriction of change generalized nicely to minimization or restriction of discontinuities. (The particular use of chronological minimization as a restrictor on the set of models was of secondary importance, I think; one can do it in other ways). The weak spot that we identified at the same time, and which was clearly spelled out in the papers, was that some additional model selection criterion was necessary, since we still got some unintended models. There were two options: modifying the logic itself, or introducing concepts from other disciplines. The first approach was pursued by two of our graduate students at the time, Tommy Persson and Lennart Staflin. Their first paper in this direction was accepted at ECAI 1990, but then they ran into the wall. One more of their papers is still available as a departmental report; it was called "Cause as an Operation on a Logic with Real-valued Fluents and Continuous Time". The article is available at http://www.ida.liu.se/publications/cgi-bin/tr-fetch.pl?r-90-45+abstr and the abstract goes as follows: > We propose a new method for characterizing the discontinuities in > processes that are mostly continuous. We introduce a causal operator > that is used to specify when the value of a fluent has a cause. A > discontinuity in a fluent is allowed if the fluent's value immediately > after the discontinuity has a cause. The causal operator is incorporated > in a temporal logic with continuous time and real-valued fluents. The > resulting logic is a nonmonotonic logic suitable for representing > physical models of real world situations. We define a selection function > which given a set of models returns a subset of the models. This > selection function defines a nonmonotonic entailment operator. The > intuitive idea behind the selection function is that is should select > all models where all discontinuities are ``specified'' as allowed. In other words, they proposed what is known today as a causal approach. The paper was rejected for IJCAI 1991. Around the same time, my journal style article which combined and extended the results in the 1989 KR and IJCAI papers was rejected for the A I Journal, with vitriolic reviews. The other approach, which we also investigated, was to bring in aspects of real physics. We started cooperation with people who had that competence, and in particular with our colleagues in control theory. This led to work on the use of bond graphs, which is a classical energy-based method for modelling physical systems, and uniformly applicable to systems from different domains (mechanical, electrical, hydraulic, etc.). Members of our group (Stroemberg, Soederman) developed a generalization of bond graphs to take account of abrupt changes (that is, combining continuous and discontinuous) by introducing a "switch" concept in a clean way. Yet another approach was the use of hybrid transition systems, which are a generalization of the transition systems that come from the theory of real-time systems. Additional members of our group (Nadjm-Tehrani and Stroemberg) used hybrid transition systems for modelling actions, analyzing safety conditions ("is it possible that if I drive this way, I may crash into the car in front of me?"), etc. For both bond graphs and transition systems, the idea was to import methods from other areas into AI and KR. In both cases, our people were able to publish successfully in the neighboring discipline, but not in AI, or at least most of the AI submissions were rejected. Reviewers tended to say either that this was not relevant for AI, or that although possibly relevant, more would have to be done in order to reach the presumed high quality standards that we require in our field. It goes without saying that after a few experiences of this kind, these (then) Ph.D. students turned away from AI and continued their work in the areas where they were better received. They were also put off by what they considered as idiotic comments by reviewers, for example, to the effect that the proposed modelling system was not capable of accounting for the sudden occurrence of asteroids on the scene. When these things happen, it is our discipline that stands to lose. There were great opportunities at that time for bringing in fresh concepts into KR, and for integrating them with what we are otherwise doing. On the other hand, time does not stand still while we fumble, and if our area does not deal in a timely fashion with new problems, then there are others who will. It is also important to note that this resistance to new ideas is not reciprocal. This year's HART conference (Hybrid And Real Time systems) had no trouble accepting my paper on relating high-level and low-level descriptions of actions, which was an extension of my invited paper at last year's ECAI. This panel discussion has already touched on the remarkable persistence of situation calculus in our field. The field's unwillingness to accept and use outside knowledge for dealing with continuous change is equally remarkable. Erik ******************************************************************** This Newsletter is issued whenever there is new news, and is sent by automatic E-mail and without charge to a list of subscribers. To obtain or change a subscription, please send mail to the editor, erisa@ida.liu.se. Contributions are welcomed to the same address. Instructions for contributors and other additional information is found at: http://www.ida.liu.se/ext/etai/actions/njl/ ********************************************************************