******************************************************************** ELECTRONIC NEWSLETTER ON REASONING ABOUT ACTIONS AND CHANGE Issue 97013 Editor: Erik Sandewall 28.10.1997 Back issues available at http://www.ida.liu.se/ext/etai/actions/njl/ ******************************************************************** ********* 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. -------------------------------------------------------- | Antonis Kakas and Rob Miller | Reasoning about Actions, Narratives and Ramification -------------------------------------------------------- FROM: Tom Costello In your paper you have three types of proposition, h, t and c-propositions. In your definition of an interpretation, you give enough information to establish truth conditions for t-propositions. The following is the obvious truth condition for t-propositions. A t-proposition, F holds-at T, in true in an interpretation E, if E(F,T) = true. However, you do not seem to have enough information to give truth conditions for h or c-propositions. Consider the domain language with one time-point 0 and one fluent F and one action A. Then the domain description, A happens at 0 F holds at 0 has one model, (F,0) -> true The domain description F holds at 0 has the same model. However, these two descriptions differ on the h propositions. Thus from an interpretation you cannot determine the set of true h-propositions. For a logic to model distinct sets of propositions by the same structure is problematic for many reasons. As a general point, A type languages are not sufficiently formal in defining when a proposition in true in a model. This has led to errors like the above in A-type languages. Some papers have used a function from sequences of actions to sets of fluents, rather than a labeled transition function/relation from sets of fluents to sets of fluents, to give semantics to action languages. The former collapses domain descriptions that differ on causal propositions, while the latter does not. Guinchiglia, Kartha and Lifschitz are an example of the use of the latter. I know of no paper that explicitly gives truth conditions for all propositions in an A-type language ********* DEBATES ********* --- NRAC PANEL ON ONTOLOGIES FOR ACTIONS AND CHANGE --- +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Murray Shanahan: Let's play music and not argue about violins. Stop! Enough! This discussion has quickly degenerated into childish bickering. There is little value in a debate of the form: A: You can't do X in B's formalism. B: Yes you can. But you can't do Y in A's formalism. A: Yes you can. But you can't do Z in B's formalism. and so on. . . No doubt, suitably modified, you can do whatever you need to in any of the formalisms. (Why does Ray write "sensing actions in the event calculus: not likely"? Rob Miller has work in progress on this theme. History should tell us that such claims are dangerous. A few years ago we were saying "continuous change in the situation calculus: not likely".) Why this possessivenss about formalisms? I'm proud to say I've written papers using both situation calculus and event calculus, and my book covers both extensively. It would be so much more valuable if we sought relations between different formalisms and tried to understand the space of possible action formalisms. The most pertinent comment I've read in this debate so far was Pat Hayes's when he wrote: > One of the biggest failures > of the KR community generally is that it is virtually impossible to > actually publish a knowledge representation itself! One can talk about > formalisms and semantics and equivalences etc. etc., . . . > but this is all part of the *metatheory* of knowledge > representation. But when it comes to actually getting any representing > done, we hardly hear about that at all. It's as if we were violinists in an orchestra who, instead of making music, spent all their time arguing over who has the nicest violin. Let's make some music. Let's use our formalisms to build theories, and then let's see how those theories fare when used in anger. Then perhaps we'll actually make some progress in common sense reasoning. +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Erik Sandewall: Let's structure the arguments. Murray, I agree with you that possessiveness about formalisms is a bad thing, but let's not give up this discussion so hastily. After all, it is important to understand what is the range of expressiveness of our current major formalisms. What we need, I think, is * Concrete, well founded arguments (not just "X has recently showed that formalism F can do phenomenon A") * A structure to the topic, maybe along the lines of the table in Ray Reiter's position statement for the ontologies debate * A broader scope, considering other properties of a formalism than just its expressiveness. Are there entailment methods which work correctly for the whole range of expressiveness that is being claimed? Wrt the first item, a concrete and well founded argument may need a little more space than just a few lines in a discussion, while on the other hand it does not require a full paper. The notes structure of the present Newsletter and News Journal may come in nicely here. In the Newsletter web pages where the present two panels started (21.10 and 27.10), clicking the title of a position statement leads one to a postscript file for that statement; that presentation of the statement will also go into the monthly News Journal edition. These notes have a journal-like "look and feel" and will be citable; they are one step more formal than what you find in a newsgroup. All newsletter participants are invited to submit their comments in that form (latex preferred). Wrt structure of the topic, why don't we build on Ray's table - contributions addressing specific combinations of "representation aspect" and "ontology" (possibly correlated with a formalism) are invited. I'll try to set up a web-page structure where every such combination obtains its own thread of messages. One reason why this discussion will be useful is to clear up some misunderstandings. For example, Michail, when you write > From the other hand, the event calculus and other "narrative time-line > languages" do not have any term that would keep record of what part of > the narrative had been done before the moment when a failure happened... you express a misunderstanding bordering on an mistake. Since each interpretation in a narrative time-line approach contains one history of the world along the time-line, it can also contain the actions that are (were) performed in that history, or up to a point in time in that history. Then the history of past events is not expressed as a term, of course, but why would that matter? In the work on range of applicability for entailment methods, as reported in the "Features and Fluents" book, I started out with a narrative timeline approach simply because it seemed more natural for dealing with events with extended duration and overlapping intervals, and with continuous change. However, it became clear during the work that a simple generalization of the time-domain definition made it possible to include situation calculus as a special case, and that virtually all the formal results about the properties of various nonmonotonic methods carried over without any difficulty. In that sense there is no contradiction between sitcalc and narrative timeline approaches, although I still like to think of the former as a special case of the latter. On the other hand, I have also noticed that it is apparently much easier to get articles published if they use situation calculus. This may possibly be due to notational chauvinism (a natural consequence of possessiveness) on the side of some reviewers: If one really believes that (e.g.) the situation calculus is the best answer to all problems, then why accept a paper from someone that hasn't seen the truth? If our research area is going to conserve an older approach to such an extent that essential new results can't make it through the publication channels, then the whole area will suffer. There, in fact, is an additional reason why we may have to sweat out this discussion about the capabilities of different ontologies and formalisms: not in order to bicker, but to increase the acceptance of each other's approaches. ******************************************************************** 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. 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