******************************************************************** ELECTRONIC NEWSLETTER ON REASONING ABOUT ACTIONS AND CHANGE Issue 97014 Editor: Erik Sandewall 30.10.1997 Back issues available at http://www.ida.liu.se/ext/etai/actions/njl/ ******************************************************************** ********* TODAY ********* It is very fascinating to see this Newsletter develop! Gradually, a new form of scientific communication is taking shape. Today's issue contains the answers of authors Kakas and Miller to three questions about their paper, namely two questions which were stated in issues a few days ago, and one question which is also in today's Newsletter. This is really in the spirit of a global, on-line scientific colloquium, where physical distance doesn't count. The other interesting thing is that we see the emergence of "midi" size contributions - significantly shorter than the conventional article, but significantly bigger and more thought out than the plain, newsgroup style message. Both in the answer by Kakas and Miller to the question by Thielscher, and in today's question by Costello, the authors decided to package the core of their arguments as separate research notes. These notes are attached to the immediate contributions, but will be published in a more journal-like style so that they can be cited in their own right. This development is a natural consequence of the rapid pace of our medium: when the turnaround is counted in days or even hours, instead of months, then there is no occasion to write full articles in each cycle. Conversely, although traditional journals often maintain a section for "research notes", their use is much more restricted in the journals because of the publication delays. Besides the continued discussion about the Kakas/Miller article, we also feature a contribution by Pat Hayes on the topic of ontologies. ********* ETAI PUBLICATIONS ********* --- DISCUSSION ABOUT RECEIVED ARTICLES --- ======================================================== | AUTHOR: Antonis Kakas and Rob Miller | TITLE: Reasoning about Actions, Narratives and Ramification ======================================================== -------------------------------------------------------- | FROM: The authors | TO: Michael Thielscher | FOR: Question re definition of initiation and termination points -------------------------------------------------------- Hello Michael, Thanks for your comments about Definition 14 of initiation and termination points. You are of course right to say that the definition requires the least fixed point construction, so perhaps we should have made this explicit within the definition itself. We omitted this from the paper in an attempt not to overload the definition with too much formalism, but perhaps its omission is causing more rather than less confusion. (Hudson Turner emailed us a similar comment to your's a little while ago.) So yes, the initiation and termination points are defined by a least fixed point construction (along the lines we say after the definition). The version of the definition that makes this explicit is unfortunately a little too full of mathematical notation to write here in plain text or html format, so we've made it available as a postscript file, at http://www.dcs.qmw.ac.uk/~rsm/fixpoint.ps You'll see that the operator corresponding to the least fixed point does indeed have an interpretation as argument. But there's no problem with this, because the interpretation is already fixed at the beginning of the definition. It's necessary include this argument in order to deal with preconditions of c-propositions. For example, consider the following domain (with time as the naturals): Take initiates Picture when {Loaded} Take happens-at 2 -Picture holds-at 1 We want 2 models, one in which Loaded is true at 1, and one in which Loaded is false at 1. In the former model, 2 should be an initiation point for Picture, but in the latter it shouldn't. Rob and Tony -------------------------------------------------------- | FROM: The authors | TO: Tom Costello | FOR: Question re truth conditions for t-propositions -------------------------------------------------------- Hello Tom, Thanks for your comments and observations. Regarding your specific comments about the Language E, then you're right - from a formal point of view there is no concept of truth or falsity as regards h- and c-propositions. So, from the definitions, it doesn't even make sense to talk about "the set of true h-propositions". For your example, the semantics simply "disregards" the h-proposition "A happens-at 0", because the occurrence of A at 0 that this represents at the syntactic level has no effects. There's no problem with this from a formal point of view, but it does mean that E, and languages like it, are very restrictive. That's why they're perhaps best regarded as stepping-stones towards formalisations or axiomatisations written in fuller, general-purpose logics. (However, and as we hope we and others have illustrated, they do have a use in discussing and illustrating approaches to particular issues - in our case, to ramifications - in a relatively intuitive and uncluttered way, and also in proving properties of classes of logic programs.) This is where work such as that of Kartha (translating A into various versions of the Situation Calculus) is valuable. In the case of the Language A, Kartha's translations bring out the fact that there is an implicit completion of causal information (A's e-propositions) in A's semantics. Much the same thing is true of h- and c-propositions in E. (This is why adding truth functions for h- and c-propositions in E models would be trivial but rather superfluous). We discussed this in more detail in our first paper on E (in the Journal of logic Programming). As we've said in both papers, it's our intention to explore these issues further by developing translations analogous to Kartha's for E. You might also be interested to look at the papers by Kristof Van Belleghem, Marc Deneker, and Daniele Theseider Dupre, who have developed a language ER similar in many respects to E, but more expressive and with a correspondingly more complex semantics (which includes truth conditions for the equivalent of h- and c-propositions). (We've described this briefly in Section 5 of our paper.) As regards your general point about "A type languages", it would be interesting to get some comments from "A type people" about this. Perhaps "not sufficiently expressive" is a better phrase than "not sufficiently formal". (On this general theme, Mikhail Soutchanski made another good point in the recent ENAI when he pointed out that it's much easier to combine theories of action written in classical logic with other commonsense theories, e.g. of space or shape, than if specialised logics are used.) Rob and Tony -------------------------------------------------------- | FROM: Tom Costello | RE: Question `Why didn't you write everything in classical logic?' -------------------------------------------------------- A question on the choice of approach: Why didn't you write everything in classical logic?". Personally, I find it much more natural to consider classical logical languages than A-type languages. The enclosed postscript file, http://www.ida.liu.se/ext/etai/rac/notes/1997/12/njracn.ps, is a translation of the proposed E language to a classical language, which I feel makes much clearer the advantages and disadvantages of the proposal. -------------------------------------------------------- | FROM: The authors | TO: Tom Costello | FOR: Question re `Why didn't you write everything in classical logic?' -------------------------------------------------------- Hello Tom, We've no objection to using classical logic. Indeed, in both our E papers we've mentioned our intention to translate E into classical logic and other general-purpose formalisms, in order to gain the obvious benefits. (An obvious candidate as a target for this translation is something like the classical logic Event Calculus in [Miller & Shanahan 1996].) As you indicate in your question, different researchers will find different approaches more natural. We chose to initially express our ideas on ramification in this form because we found it relatively intuitive and uncluttered, and convenient for proving properties of logic programs that we want to use for various applications. As we've stated in our answer to your previous question and in our first paper on E, these specialised languages are perhaps best regarded as stepping-stones towards formalisations or axiomatisations written in fuller, general-purpose logics. It's great that you have in fact used E in exactly this way. Please publish! One point about your relations "init" and "term" in your classical logic translation. You say that you should take the "smallest relations ... that satisfy the above [axioms partially defining the relations]". But it turns out that this "smallest relation" idea is still not quite sufficient for eliminating the kind of anomalous models that Michael Thielscher was drawing attention to. So you really do need a least fixed point notion or equivalent somewhere in your axiomatisation, where the associated operator generates the least fixed point starting from a pair of empty sets (see our answer to Michael's question). Of course, another reason for using the specialised language approach was to illustrate that the Language A type methodology could be applied using ontologies other than that of the Situation Calculus. We're not sure if authors of Language A type papers would reply to your question in the same way, so it would be interesting to get some other responses from this community. Rob and Tony ********* DEBATES ********* --- NRAC PANEL ON ONTOLOGIES FOR ACTIONS AND CHANGE --- -------------------------------------------------------- | FROM: Pat Hayes | RE: On the misuse of the word 'ontology' -------------------------------------------------------- Before responding to the responses to my comments about the situation calculus, a note on terminology. The 'situation calculus', 'event calculus', etc., are all just styles of writing axioms in a first-order logic (with suitable modifications to allow circumscription, etc..) The word 'calculus' doesnt point to anything more substantial than a choice of vocabulary and an axiomatic style. (Contrast the useage in 'lambda calculus', for example.) This isn't anything to regret in itself, but it does mean that to talk about something being an 'extension' to a calculus becomes rather fuzzy. There is no end to the relations and axioms one might add to a first-order theory; and if we also allow the axioms of the theory to be altered and the intuitions which motivated them to be replaced by different intuitions, then we can make any set of axioms into any other set of axioms, so all talk of this or that 'calculus' becomes meaningless. Ray Reiter seems to have done this for the situation calculus. Whatever it is, this 'extended ontology' that Ray describes [see ENAI 27.10] bears almost no similarity to the ontology and axiomatic style expounded by McCarthy about 30 years ago (and still used by Ray, along with everyone else, as late as 1991 in his paper in the McCarthy festschrift). It has a different vocabulary, different axioms and is based on different intuitions (which are directly opposed to those motivating the original situation calculus) and has different formal properties. Contrast, for example, Reiter and McCarthy on what a 'situation' is meant to be: McCarthy (1969): "A situation is the complete state of the universe at an instant of time." Reiter (1997): "Even at this late stage in AI, many people still don't understand what a situation is, so here's the secret: A situation is a finite sequence of actions. Period. It's not a state, it's not a snapshot, it's a *history*." Evidently Ray is talking about something different from McCarthy. Nothing wrong with this, of course: I've done it myself from time to time. (Consider my old naive physics 'histories' ontology. World-states are a special case of histories, and there's a systematic translation of situation-vocabulary into history-vocabulary; does that mean that the 'liquids' axiomatisation is written in an "extended" situation calculus?) Now, it may be said that the field has advanced, and its up to old fogies like me to adapt ourselves to the new terminological consensus. Just as 'frame problem' now means almost everything from Hume's problem of induction to the cost of memory, the meaning of 'situation calculus' has moved with the times. (As Mikhail Soutchanski says, "the SC of 1997" is different from the SC of, say, 1991.) I've made a similar point to Erik, who carelessly used 'ontology' to mean what it meant for about a thousand years, thus risking confusion with the new West-coast sense of 'ontology' (ie. a sorted first-order theory presented in an object-oriented notation, with comments in Managerese.) But, as Erik said, we still need a name for the old sense; and we still need a name for the situation calculus as it was everywhere from 1965 until around 1994 and still is in most of the world outside Toronto. How about 'gofsitcalc'? Whatever we call it, in any case, that's what I was talking about. More substantive comments to follow. ******************************************************************** 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. 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