|Issue 98060||Editor: Erik Sandewall||31.7.1998|
The News Journal for Reasoning about Actions and Change started exactly one year ago - the original monthly webpage appeared on July 31, 1997. As most of you know, we switched to the system with direct-mail Newsletters in September, and that's clearly the major reason why the interactions have been very lively since. The present one is the 97:th Newsletter, so another reason for celebration is coming up soon.
Just in time for this anniversary, we have now completed the official version of ETAI 1997. (Please remember that each annual volume contains those accepted articles that were submitted during that year, except of course those requiring major revision, so the delay of a number of months is quite normal). The completed volume is available, each paper in its own file(s), on the Linkoping E-Press server, and the ETAI webpages for the Journal have been modified accordingly. In particular, responding to a suggestion by Rob Miller in an earlier Newsletter, we now have an overview page for the annual volume that gives parallel access to the article itself and to the discussion about the article.
At this point, I feel that the basic idea with this electronic journal has become very clear in the webpage structure. While in the midst of Newsletter activities - reading and contributing to them - one may sometimes have the impression that this is the ETAI, or more precisely one of its sections. At the end of the year, however, we have the articles that have been discussed, revised, and refereed through the Newsletter-based interaction process, and for posterity ETAI will of course be the collection of those articles and their commentary. Please take a look at the ETAI journal page (accessible from the main ETAI page) to see things in this perspective.
All of this has of course been produced by you the subscribers/ peers/ participants/ colloquium members. One other key notion in the ENRAC, in particular, is that debate contributions ought to be produced with a look-and-feel that matches their quality and the work that got into them, or in brief, that everyone who contributed to the discussions shall be able to feel pride for it and be able to refer to it. This is one of the reasons why the News Journal (containing the accumulated contents of Newsletters, one News Journal issue per month) is produced not only in HTML form but also in postscript via latex. I feel that much of what is written in our debates has a lasting value, and that it is worth referring to. Please feel free, therefore, to cite ENRAC debates just like you would cite any other journal. It does have official status, and its contents can be characterized in the same way as for other journals, in terms of ISSN number, volume, page, and so on. In particular, in order to refer to the discussion about an article, the appropriate citation is to the News Journal issue where the comment was made. The News Journal is organized with separate segments for separate articles being discussed.
Today's Newsletter contains a contribution to a `panel' discussions: Jixin Ma's answer to the recent contribution by Erik Sandewall in the debate about the ontologies of time.
In Newsletter ENRAC 27.7 (98059), Erik wrote:
| Jixin has observed that sometimes it is necessary to assume an
intervening point between two intervals, for example, for being able
to say that a ball that has been thrown vertically up into the air
will be motionless (zero velocity) at a certain point in time. For
some other actions or events it is desired not to have an
intervening (time-)point, as is the case for two successive
intervals, one where a switch was "on" and the next where the switch
was "off". Sergio Brandano has already observed (ENRAC 23.4) that
the timepoint domain will then be agent-specific. It will also be
local to each scenario, so that if additional actions are added to
the scenario description, one must change the timepoint domain
accordingly. In fact, it even becomes necessary to revise the
timepoint domain each time a query is asked for a scenario, which
seems a bit odd.
| I suppose one could get used to this mode of thinking, and to accept
that time is in the mind of the beholder. However, here are two
examples where the punctuated time approach leads to absurd
A. Jim fires a model rocket and observes its flight. At the moment
when it reaches the top of its trajectory, he turns a switch. If the
flight of the rocket is modelled like Jixin proposes to model the
throwing of a ball, then it requires that there exist a point for
the clocktime when it is at its apex, but at the same time the fact
that Jim turned the switch implies that there must not be any such
point. Therefore, if the scenario description includes the statement
that Jim turned the switch at the same (clock-)time as the rocket
reached the apex, then it is semantically inconsistent.
Suppose two lights, Green light and Red light, are both switched at the same time point P. By commonsense, we have that
(C) "GreenOff Meets GreenOn" and "RedOff Meets RedOn".
In addition, assume that by some reason we impose that the Green light is, for instance, On, at the switching point P, how are we going to express the situation for the Red light? In other words, is the Red light "Off" or "On" at the switching point P? Galton was afraid that this would lead to the Deviding Instant Problem again.
Let me sort out this problem first and then show that Erik's example can be dealt with similarly.
Since we have no information about the the state of the Red light at the switch point P, we may just use two successive intervals, I1 and J1, to express the scenario as:
Since (GR) allows us to express
However, if we in addition specially impose that, for instance, the Red light is Off at the swiching point P, we can still express the whole scenario as:
Again, since (GR*) allows us to express
Now, let's come back to Erik's example A:
First of all, I don't think the time theory criticized is semantically inconsistent. Yeah, for the modelling of the throwing of a ball, it requires that there exists a point referring to the apex. However, the fact that Jim turned the switch does not necessary imply that there must not be any such point, especially if one insists that "at the moment (point?) when it (the ball) reaches the top of its trajectory, he (Jim) turns a switch". I guess what Erik actually means here is that we don't have any information about the state of the object (a light?) being switched at the switching point. What we do know is just that the "On" state is immediately after the "Off" state. Therefore, similar to the treatment to the above "Two Lights Problem", we can express Erik's scenario as:
| B. Tom and Bob compete for eating icecream cones. They start with
four cones each, and have to eat them as fast as possible, starting
at the same time. The referee rings a bell when one of them has
finished eating all his cones. If one of the contestants tries to
cheat by dropping icecream on the ground, he also rings the bell in
order to call off the contest. The bell sounds at time |
Now, let me try to model this interesting example:
For Tom, we have:
Similarly, for Bob, we have:
|Duration(I1+I2+I3+I4) < Duration(J1+J2+J3+J4)|
Similarly, if Bob finishes his last cone before Tom, i.e.,
|Duration(J1+J2+J3+J4) < Duration(I1+I2+I3+I4)|
As for the cheating:
|Holds(TomDroppingIcecream, K) ^ During(K, I1+I2+I3+I4)|
|Holds(BobDroppingIcecream, L) ^ During(L, J1+J2+J3+J4)|
I feel that's it, though I wonder if I missed some of the specifications of the example.