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.

Pat Hayes

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.