******************************************************************** ELECTRONIC NEWSLETTER ON REASONING ABOUT ACTIONS AND CHANGE Issue 97028 Editor: Erik Sandewall 27.11.1997 Back issues available at http://www.ida.liu.se/ext/etai/actions/njl/ ******************************************************************** ********* TODAY ********* Today's issue contains a contribution by Vladimir Lifschitz to the discussion about action languages in the ontology panel. ********* DEBATES ********* --- NRAC PANEL DISCUSSION ON ONTOLOGIES FOR ACTIONS AND CHANGE --- -------------------------------------------------------- | FROM: Vladimir Lifschitz -------------------------------------------------------- Erik, In ENAI 21.11 you discuss advantages of the F&F approach, in comparison with action languages, and you write: > No need to define new languages all the time, and of comparing > newly published languages with previously published ones. We stay > with the same language, which is sufficiently expressive right from > the start to last for a while ... I'd like to understand this better. The need to define new action languages arises when we want to describe aspects of reasoning about action that have not been understood in the past. Here are some examples. 1. Ramification constraints vs. qualification constraints. A fact about fluents sometimes allows us to conclude that an action has an indirect effect, and sometimes that it has an implicit precondition. (It functions sometimes as a "ramification constraint" and sometimes as a "qualification constraint.") Example: the objects that I have in my pocket are in the same place where I am. After I come home with a comb in my pocket, the comb will be in my home also; this is an indirect effect. Since knives are not allowed in airplanes, I can't board an airplane with a knife in my pocket; this is an implicit precondition. 2. Asymmetry of ternary constraints. Consider a spring-loaded suitcase with two locks. Its state can be described by three fluents: lock 1 is open; lock2 is open; the suitcase is closed. The constraint is that these fluents cannot hold simultaneously. Consider a state in which one of the locks is open and the suitcase is closed. When I open the other lock, this causes the suitcase to open. (This action does not cause the first lock to close, which, logically speaking, is another possibility.) 3. Interaction between concurrently executed acions. Consider lifting the opposite ends of a table upon which various objects have been placed. If one end of the table has been raised, the objects on the table will fall off. But if both ends are lifted simultaneously, the objects on the table will remain fixed. These phenomena could not be described in the original action language A and in some of its successors. New, more expressive languages had to be designed. I am wondering what the status of examples 1-3 in the F&F framework is. Would you be able to formalize them in your original language, which you described as sufficiently expressive right from the start? My understanding of the possibilities of F&F is not sufficient to answer this without your help. But about the situation calculus I know that new syntactic features had to be added to it to address these problems, such as the predicate Caused (similar to Holds, but not quite the same), and an addition operation on actions (to represent concurrent execution). Vladimir ======================================================================== Patrick, In ENAI 21.11 you write: > To clarify my "current" working view of the role action languages > should play, I simply quote the original intuition Vladimir himself > at one time had about their role in his paper, "Two Components of an > Action Language": > > "Originally, action languages were meant to play an auxiliary role. > The primary goal was to represent properties of actions in less > specialized formalisms, such as first-order logic and its nonmonotonic > extensions, and the idea was to present methods for doing that as > translations from action languages." > > My group currently uses this approach and it is also one of the > cornerstones of the Features and Fluents methodology. Indeed, your TAL is essentially an action language. There are minor differences in style between TAL and the languages that I've been working on. What you write as [t1,t2] move(p,l) ~> [t1] !(place_of(p) == l) -> [t1,t2] place_of(p) := l I would maybe represent this as move(p,l) CAUSES place_of(p)=l, IMPOSSIBLE move(p,l) IF place_of(p)=l. This is slightly more concise because t1, t2 are suppressed. I am wondering whether you would lose any important expressivity if you changed your macros in a similar way. Vladimir ======================================================================== Tom, In ENAI 21.11 you write: > The most difficult issue that I am aware of, is that it is unclear whether > > A causes F if G > > means that > > in every actual state S where G is true, then F is true in R(A,s), > > or the similar, but different > > in every possible state S where G is true, then F is true in R(A,s). I would say it's the latter. Vladimir ======================================================================== Pat, In ENAI 25.11 you write: > Vladimir Lifschitz writes: > > > > It seems to me that the semantics of an action language cannot be > > described by specifying truth conditions for its propositions. The > > problem is the same as with nonmonotonic languages in general. Take, > > for instance, the closed-world database {P(1),P(2)}. The negation of > > P(3) is a consequence of this database, but this fact cannot be > > justified on the basis of truth conditions for P(1) and P(2). > > > > But it can be justified on the basis of the truth conditions for P(3), > which is just as much a proposition of the language as the first two. Good point. Our closed-world database determines the model in which P(3) is to be evaluated according to the truth conditions of classical logic. Similarly, a domain description in the language A determines the transition diagram in which a value proposition is to be evaluated when we want to determine whether it is a consequence of the description. Further you write: > Nonmonotonic logics are not classically truthfunctional, but they do have > truth conditions. Here I cannot fully agree with you. A default theory in the sense of Reiter is defined by its axioms and its defaults; we have truth conditions for axioms, but not for defaults. Vladimir ******************************************************************** 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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