Today's issue contains Michael Thielscher's answers to the referee's comments for his accepted article, and his comments on the ETAI reviewing and publication procedure. Also, Jixin Ma on the ontology of time.

Additional contributions have been received for the discussions about the following article(s). Please click the title of the article in order to see each contribution in its context.

Michael Thielscher
A Theory of Dynamic Diagnosis

Michael Thielscher:

First of all I can only side with Rob in saying that the experience of publishing an article in the novel way was both exciting and instructive. It took some time for the discussion on my paper to get going, but in the end it proved very useful and led to important improvements. It was, however, never as lively as, say, the discussion on Tony Kakas and Rob Miller's submission. I recently told Rob that I envied the two of them for their paper receiving so much attention. Although his impulsive remark was that there are two sides to everything, I guess that in the end the authors of articles much debated upon can be most happy about the public attention. Thus a lesson that might be learned from the experience with ETAI so far is that the more controversial a paper is, the more is gained by submitting it to the new publication scheme. And of course this holds vice versa: ETAI seems to profit most from controversial papers.

The editor of the Newsletter raised the question whether networked articles should be as self-contained as articles in the classical medium. Indeed the new medium offers new possibilities. If there is a good general introduction to the topic of one's paper, then adding a link might often be a better idea than just copying the contents in one's own words. In this way an article could be made accessible for a readership with truly different background. For classical journal papers, authors always have to struggle with the problem of how much background they should provide. Some papers even include choicepoints of the form "The reader who is familiar with topics x, y, z,... may skip sections a, b, c, ..." The new medium offers the exciting possibility of instead writing "The reader who is not familiar with x, y, z, ... should first follow the links l, m, n, ..." This is of course much less time-consuming and can thus be used with virtually no effort to make a paper suitable for almost everyone. Although I doubt that today too many useful electronic links exist which may serve this purpose, maybe sometime in the future there will be.

One suitable supplement to any ETAI paper is readily available already today, namely, the electronic public discussion. I support Rob's suggestion that everyone who downloads an accepted ETAI paper should be strongly encouraged to also print out and append the discussion page. My feeling is that this truly new feature is among the greatest advantages of the novel publication style.

Jixin Ma:

In ENRAC 3.5 (980521), Sergio wrote:

I am actually skeptic about the need of a temporal domain which includes time-intervals. There are many convincing arguments that a temporal domain consisting of time-points is good enough in many different situations (Newtonian mechanics and Thermodynamics, for instance, as well as Sandewall's underlying semantics for K-IA), and I see no reason why I should pursue a different path.

You said here "a temporal domain consisting of time-points is good enough in many different situations". "many"? Is this a convincing argument for general treatments? Anyway, the fact that "you see no reason why you should pursue a different path" does not mean others don't see/have the reason (see below).

... about the convenience of using intervals are based on the belief of the need of them.

I supposed you did so, that is why I originally invited you to make a backward step and give some convincing argument(s) on the plausibility of this approach. According to the standard scientific methodology, in fact, we shall build on top of already existent solutions, and be consistent. Just to make an example, suppose one refuses a classical notion (continuity?), and encounters the problems that this notion was used to solve (the dividing instant?); it is surely not consistent to justify the need for a novel approach via the claim that the problems he encountered can not be solved by the notion he just refused.

Have you applied the above arguments to that one proposed by youself? Sorry, I am here again using your question to ask you.

Anyway, while I (and many others) have seen the convenience of using intervals, I can also see the need of them. In fact, there have been quite a lot of examples (many) in the literature that demonstrated the need of time-intervals (or time-periods). Haven't you ever encountered any one of them? Or you simply cannot see anyone of them is convincing?

All right, let's just have a look at the example of throwing a ball up into the air. As I shown in ENRAC 1.4 (98033) (one may disagree with this), the motion of the ball can be modelled by a quantity space of three elements: going-up, stationary, and going-down. Firstly, or at least, we can see here the convenience of using intervals. In fact, we can conveniently associate the property that "the ball changes its position" with some time-intervals. Secondly, let's see if we indeed need time-intervals. Without the notion of time-intervals (neither primitive nor derived from time-points), can you just associate such a property with time-points? Yeah, we may associate it with a pair of points. However, this doesn't mean that the property holds at these points. What it really means is that the property holds for the time periods denoted by the pair of points, Are these time periods in fact time intervals? So, we do need the notion of time intervals, don't we?

It is important to note, up to now in the above, I just talked about the need of the notion of intervals. As for how to characterise intervals (e.g., are intervals taken as primitvie or derived structures from time-points?) is another important issue, and this issue, again, has been addressed in the literature for a long time. I think I don't need to repeat this.

The Point Is: while we were/are discussing/arguing about some broader issues on temporal ontology, you just jumped in and asked "why an alternative notion of continuous structure is needed at all?" First of all, the "continuity" (or more truly, density) is not the main issue we are talking about. The fundamental question is if we need to address and how to addess time intervals. Based on such a discussion, in the case that intervals are taken as temporal primitive, then, we are talking about how to characterise some corresponding issues including dense/discrete structures. But your questions and arguments/replies do not seem follow this. As stated in the former replies from both Pat and myself, first of all, the dense structure does not have to be characterised in terms of the only form of the so-called "axiom of completeness". Also, in the case where time-intervals are involved (even they are still point-based, let alone in the case they are taken as primitive), such an axiom doesn't simply apply. In fact, I have shown this to you two times with different notations. I will show this again and point out more problems in detail below in my response to your reply to Pat.

Concerning the dividing instant problem, which seems to summarize what is left from your objections, please read below.

As I said in my former reply to you, your approach does not solve the DIP at all. In fact, it seems that you don't realise the DIP in the way as we are talking about (see below).

In reply to Pat Hayes (ENRAC 24.4.1998):

As posted in my original message, I have not yet seen any explanation why an alternative notion of continuous structure is needed at all?

Still not yet?

You and Jixin Ma proposed the "dividing instant problem", apropos of the problem of switching on the light, and argued the axiom of completeness inadequate for solving that problem. The formulation I gave in ENRAC 24.4.1998, with today's minor adjustment, gives the evidence on how the axiom of completeness is, instead, safe with respect to the dividing instant problem. You and Jixin based your argument on the fact that I do not allow the domain  S  to hold points "and" intervals, so that if  S  admits just intervals then the dividing point  p  can not exist. I refuted that argument by simply observing an interval from the real line may have equal end-points.

Do you remember that it is yourself who specially claimed that your domain  S  contains points or (exclusive-or) intervals? As I suggested, and also as you have now realised, to fulfill the axiom of completeness, you must allow your intervals to be possibly some singletons (i.e., a set of single point). In other words, if your  S  contains intervals, it should also contain singletons (points). The real problem is that, even you allow your intervals to be singletons, the Dividing Instant Problem is still there, and in fact more obviously. Do you agree with this?

The closed intervals   [p, q]   and   [q, r]  , with  p < q < r , do not fulfill the relation  <p, q> < <q, r> , hence they do not make a valid counterexample.

Pat's example becomes invalid only after you made the "minor adjustment" that replaces the relation   <   in your hypothesis  <s1, t1> < <s2, t2>  by   <  , that is  <s1, t1> < <s2, t2> . (Is this an alternative?)

So, you do need alternation, don't? (And this is just for the case when you construct intervals out of points. In the case where intervals are taken as primitive, the need of such alternative is indeed more conceptually necessary). However, your adjustment is not enough, or you haven't reached the proper form for general treatments. In fact, you need address the issue regarding different cases. To see this, you may just consider the difference between the case where at least one of  <s1, t1>  and  <s2, t2>  is "closed" at  t1  (  = s2 ), and the case where both  <s1, t1>  and  <s2, t2>  are "open" at  t1  (  = s2 ). In the former case, you need use   <   in the hypothesis; otherwise, Pat's example will be a valid counterexample. In the latter case, you need use   <   in the hypothesis; otherwise, your axiom cannot not prevent a "gap" between   <s1, t1)   and   (s2, t2>  , that is, there is no guarantee that the singlton   [t1, t1]   is contained in  S  (Do you think this is consistent with the "classical" concept of contiunity?).

Jixin