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Message-id: <199410191921.AA20385@dante.cs.uiuc.edu> X-Sender: phayes@dante.cs.uiuc.edu Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Date: Wed, 19 Oct 1994 14:24:09 -0600 To: Danny Bobrow <bobrow@parc.xerox.com>, dwig@markv.com, Fritz Lehmann <fritz@rodin.wustl.edu> From: phayes@cs.uiuc.edu (Pat Hayes) Subject: Re: CCAT: TIME: Fantasyland?/Various issues Cc: cg@cs.umn.edu, srkb@cs.umbc.edu, M.J.Johnson@qmw.ac.uk, anquetil@iro.umontreal.ca, beancar@cucis.cis.columbia.edu, bill@violin.att.com, billrich@vnet.ibm.com, brayman@zuben.ca.boeing.com, buteau_brandon@prc.com, cassidy@micra.com, cbwillis@netcom.com, cyre@vtvm1.cc.vt.edu, dick@glue.umd.edu, doudna@aol.com, doug@csi.uottawa.ca, fletcher.mcleancsd@xerox.com, fritz@rodin.wustl.edu, ged@cs.rmit.edu.au, gerbe-o@immedia.ca, grau@falcon.depaul.edu, kivs@bgcict.bitnet, kremer@cpsc.ucalgary.ca, kschoi@cs.kaist.ac.kr, lukose@peirce.une.edu.au, moulin@ift.ulaval.ca, oh@vax2.cstp.umkc.edu, peterman@informatik.uni-hamburg.de, roger@ci.deere.com, s.griffin@mcs.surrey.ac.uk, shmyaeng@mailbox.syr.edu, sowa@turing.pacss.binghamton.edu, thompson@zuben.boeing.com, wei@oucsace.cs.ohiou.edu, willems@cs.vu.nl Sender: owner-srkb@cs.umbc.edu Precedence: bulk

At 9:12 AM 10/18/94 -0700, Danny Bobrow wrote: >It seems that the concepts such as every last Friday of every third >month are not concepts of time, but concepts that derive from our >ability to count, select and name. I can ask for every tallest student >from every third classroom, etc. Once we have chunks, and names for >chunks of time (MTWHFSS, or Jan Feb, or 1884) and an ordering among >names, then we can use our ability to compose selection functions on >ordered sets The fact that some calendar programs select some of these >is "just" user convenience (no small matter if you want to sell). So we >need a theory of selection from ordered sets, and notions of exceptions. > I think this is both right and maybe wrong. (This is the logician speaking, remember ;-) Right because these concepts are surely definable in terms of simple temporal notions like 'interval' and 'duration' and then a suitably powerful way of describing sets of things and integers and so on, and the process is going to be very like defining the tallest student from every classroom, etc. All a clock is, is a starting time and an 'beat' interval which it keeps on counting, for example. In fact there are some generally useful ideas that can be defined very abstractly, such as the sequence of n'th things from a sequence of sequences, etc.; there might be a useful general-purpose theory of counting and selecting. However....there are some tantalising possibilities. For example, the thirteen simple-interval relations which James Allen described form a complete algebra, and this algebraic perspective turns out to be a useful and productive way to think about them. Suppose we allow intermittent intervals: is there a collection of relations on them which has the same kind of role that the Allen relations plays for simple intervals, ie is there a useful algebra of relations-between-intermittent-intervals? People have looked at this but I dont know of a definite answer. The questions go beyond whether we *can* describe this stuff (answer, yes) to whether it repays further effort to see if it can be described in other ways. And the answer to THAT question, in my view, is whether the results from it (ie the 'theory' of intermittent intervals) are likely to be of any use to anyone. Thats what Im trying to discover by pestering the world through the internet. And theres another slight qualification. Im *pretty sure* that most temporal ideas can be described in terms of simple ones like point, interval and duration plus general-purpose ideas like the n'th in a sequence. But Im never going to be *entirely certain*, and so its always worth actually trying to do it, just to keep testing the basic time theories. You never know when a flaw might show up. Pat ---------------------------------------------------------------------------- Beckman Institute (217)244 1616 office 405 North Mathews Avenue (217)328 3947 or (415)855 9043 home Urbana, IL. 61801 (217)244 8371 fax Phayes@cs.uiuc.edu ----------------------------------------------------------------------------