******************************************************************** ELECTRONIC NEWSLETTER ON REASONING ABOUT ACTIONS AND CHANGE Issue 97020 Editor: Erik Sandewall 11.11.1997 Back issues available at http://www.ida.liu.se/ext/etai/actions/njl/ ******************************************************************** ********* TODAY ********* Today, Tom Costello responds to Alessandro Provetti's comment about the answer by Rob Miller and Antonis Kakas to the first one of Tom's two questions about their article. (In order to follow that more easily, check our structured webpage for the interactions about their article). Also, Rob Miller answers back to Hector Geffner in the ontologies discussion. ********* 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 Antonis and Rob, (and Alessandro) While the languages L_0 and L_1 of Baral et al. give truth conditions for propositions stating happens, precedes and holds, they do not give truth conditions for causes propositions. Like Baral and Gelfond, and Kartha and Lifschitz their models are functions from sequences of actions to (sets of) states. Because of this they conflate domain descriptions that are not conflated by models that are functions from states to sets of states (Res etc.). Consider the following domain description, stated in A. A causes F initially F or in L_0 A causes F F at S0 These have the same functions from sequences of actions to sets of states as, A causes F A causes G if not F initially F or in L_0 A causes F A causes G if not F F at S0 However, if we consider functions from states to sets of states, then these have different models. Thus domain descriptions that were distinguished by A, are conflated by later languages. These later q approaches conflate models that intuitively differ. I agree with Alessandro that E type languages can give semantics to h-propositions. My complaint is that current approaches fail to give semantics to all their propositions. As A and E type languages do not have a proof theory, save by being translated into other approaches, it seems strange that they do not even have a model theory for all their propositions. In Antonis and Rob's case they lack truth conditions for some of their propositions, and worse, it seems that it is not even possible to define truth conditions. The same problem arises for casual statements in Baral et al., Baral and Gelfond, and Kartha and Lifschitz. Other models of A type languages do not have this problem of collapsing domain descriptions A considered distinct, for causal statements, for instance, E. Giunchiglia, N. Kartha and V. Lifschitz, "Representing action: indeterminacy and ramifications". Therefore, I argue that action language models should define truth conditions for all their propositions, and further, should ensure that intuitively different models are distinct. Tom ********* DEBATES ********* --- NRAC PANEL DISCUSSION ON ONTOLOGIES FOR ACTIONS AND CHANGE --- -------------------------------------------------------- | FROM: Rob Miller -------------------------------------------------------- Hector Geffner (ENAI 10.11) wrote: > 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 > .... > > 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). That's right. 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 (see e.g. http://www.dcs.qmw.ac.uk/~rsm/project.html#Other for a bibliography). 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)? If we did more of this type of work, we'd stand more chance of being able to (in Hector's words) "package the theory for the outside world". Rob ******************************************************************** 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/ ********************************************************************