******************************************************************** ELECTRONIC NEWSLETTER ON REASONING ABOUT ACTIONS AND CHANGE Issue 98077 Editor: Erik Sandewall 9.10.1998 Back issues available at http://www.ida.liu.se/ext/etai/rac/ ******************************************************************** ********* TODAY ********* Murray Shanahan has received several sets of questions to his ETAI submitted article, which equals his invited paper at ECAI 1996. In today's issue he answers two sets of questions from Paulo Eduoardo Santos. ********* ETAI PUBLICATIONS ********* --- DISCUSSION ABOUT RECEIVED ARTICLES --- The following debate contributions (questions, answers, or comments) have been received for articles that have been submitted to 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: Murray Shanahan | TITLE: A Logical Account of the Common Sense Informatic | Situation for a Mobile Robot | PAPER: http://www.dcs.qmw.ac.uk/~mps/robotics_long.ps.Z | REVIEW: http://www.ida.liu.se/ext/etai/ra/rac/010/ ======================================================== -------------------------------------------------------- | FROM: Murray Shanahan -------------------------------------------------------- > I agree that Separation avoids the Hanks-McDermott problem , but doing > so you get a restriction in the theory, since you are not able to write > formulae using Initiates (or Terminates) and Happens together (on the > contrary it won't be clear where to place such formulae in the > circumscriptions). Thus, we are not able to > express (and infer) some facts. It's true that there are some (very few) kinds of formulae that we cannot write using this method. For example, if we wanted to express the fact that EITHER flicking the switch terminates the light being on OR it was initially dark, then there's no obvious way to do it. (This is Tom Costello's example.) However, I'm a little sceptical that we do ever want to mix these different kinds of information in this way. In this example, I suspect that another layer of reasoning is called for, which explains observations (such as the fact that it is dark), and this may update the Initiates/Terminates part of the theory, or may update the initial situation description. On the other hand we can, and often do, write other sorts of formulae that mix Initiates, Terminates and Happens predicates. In particular, triggered events can be described using formulae of the form: Happens(a,t) if Initiates(a,f,t) These formulae can be included in the circumscription of Happens, and this doesn't cause a problem, since the assumption is that the meaning of Initiates is already fixed by separate the circumscription of the Initiates and Terminates part of the theory, and that this formula is, so to speak, just a "consumer" of the Initiates predicate. > Then, why do not use negation-as-failure instead, since, for the > particular case of circumscription we are > interested in, the former is equivalent to the later > > In which sense circumscription is more powerful than negation-as-failure > in this framework? We could use negation-as-failure (or rather, say, predicate completion). Using circumscription does allow for the addition of, for example, disjunctive facts, however. Predicate completion is only defined for a certain class of theories. Event though this class encompasses most of what we're interested in, there doesn't seem any point in ruling out exceptions. > I could not understand the last formula of page 14. There a > biimplication is used defining the region g > with the formula Pi . My interpretation of this is that Pi is not an > approximation of g but it describes > exactly g . Is that what it means? This is exactly what it means. Formula (5.7) on the same page is an example. > My question is: what do you do if there is no M2 that explains Psi ? Then there really is no explanation. The robot is stuck. Murray Shanahan -------------------------------------------------------- | FROM: Murray Shanahan -------------------------------------------------------- > Why didn't you circumscribe Initially and Happens in parallel in the > first conjunctive term of the expression? > > Why do we have to circumscribe Abspace letting Initially vary, and > not circumscribe only Initially (or both Initially and Abspace )? Well, it may be possible to do things that way. I was trying to follow as closely as possible the approach I'd taken in my AIJ paper "Default Reasoning about Spatial Occupancy", where I introduced a predicate similar to AbSpace. But anyway, I think you may have uncovered a bug in my formalisation. As you'll see in other papers and in Chapter 15 of my book, usually when I present the event calculus I use two Initially predicates: Initiallyp to say that a fluent holds and Initiallyn to say that it doesn't hold. A counterpart to axiom (EC1) for not HoldsAt and Initiallyn is also included. (Using two predicates like this allows us to distinguish fluents that (1) are inertial and true from time 0, (2) are inertial and false from time 0, and (3) are non-inertial from time 0.) I tried to get away with just one predicate here, because I thought the examples didn't need the Initiallyn predicate. I need to look into it properly, but I suspect that I need to reintroduce Initiallyn to get the minimisation of AbSpace right. It's only a minor modification, but at the moment I don't think it works. Murray Shanahan ******************************************************************** 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/ ********************************************************************