******************************************************************** ELECTRONIC NEWSLETTER ON REASONING ABOUT ACTIONS AND CHANGE Issue 98013 Editor: Erik Sandewall 30.1.1998 Back issues available at http://www.ida.liu.se/ext/etai/actions/njl/ ******************************************************************** ********* TODAY ********* Should actions be first-class citizens in logics of actions and change, so that it is possible to have action variables, to quantify over them, to minimize models with respect to the set of actions in them, etc? We have already touched on this topic before, namely in the discussion about the Kakas/Miller article (question number 2, initiated by Tom Costello and with a number of follow-up comments). Now the same question comes up again, but in another perspective, in a reply by Hector Geffner to a question by Michael Gelfond. ********* DEBATES ********* --- ONTOLOGIES FOR ACTIONS AND CHANGE --- -------------------------------------------------------- | FROM: Hector Geffner -------------------------------------------------------- This is in reply to Michael's message. As I see it, in the "basic" approaches to the frame problem (Reiter's completion, Sandewall form of chrnological minimization, language L, etc), in one way or another, action rules are compiled into transition functions of the form f(a,s) - where $s$ is a state and $a$ is an action - that describe the set of possible state trajectories. Observations in such models just prune some of the trajectories and hence have a "monotonic" effect (i.e., predictions are not retracted, at most they are made inconsistent). In other models, Michael's and Luis' included, actions are represented in the state (in one way or the other) and hence abdution to both fluents and actions are supported. Such models *are* non-monotonic in the set of observations. Actually the only apparent difference in such models between actions and fluents is that the former are assumed to be "false by default" while the latter are assumed "to persist by default". Interestingly, there seems to be another difference between actions and fluents in those models, and that is, that actions, unlike fluents, are not allowed to appear in the head of rules. Actually I'm not sure whether this is the case in all such models, but it is true at least in mine AAAI94. The reason I excluded actions from rule heads was precisely "to prevent actions from having an effect on beliefs about the past" (just think of a rule like $hungry then eat$). Again "the causality principle". I wonder now what's the reason the same constraint is in force in most other models of such type (if that's so). Any ideas? Hector Geffner ******************************************************************** 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/ ********************************************************************