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Antonis Kakas and Rob Miller
Reasoning about Actions, Narratives and Ramification

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 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.