Re: Further remarks

From: pat hayes (
Date: 10/29/02

  • Next message: pat hayes: "Re: Revised daml-time.pddl"
    >    [Jerry]
    >    As the expert in branching futures, how would you feel about ....?
    >Well, speaking as the expert in branching futures, the current
    >ontology is a bit incoherent.  It is mostly concerned with time
    >_measurement_.  From that point of view it would be meaningless to
    >talk of branching time, because even if you think there are multiple
    >versions of next week, they presumably all start Wednesday at the same
    >The only place where the ontology connects to actual events is in
    >section 2.5, where the 'holds' predicate is introduced.  But the
    >treatment there is sketchy.  For instance, we have the following:
    >	during(e,T) & inside(t,T) --> at-time(e,t)
    >Intuitively, this would be true only if 'e' is some kind of
    >proposition, such as "The cat is on the mat."  If 'e' is "Sir Fred
    >bowing three times to Queen Sally," in what sense does that hold at
    >every instant of an interval over which it occurs?  Well, one can
    >craft one's ontology any way one wants, but in branching time there is
    >a real problem.  Suppose that in one branch, after the first bow Sir
    >Frank rushes in and knocks Sir Fred to the floor.  In another, Frank
    >is a little late and Fred completes his three bows.  Now pick a time
    >instant in the middle of the first bow.  By the axiom above Fred is
    >bowing three times at that instant, even though in some futures he
    >never gets to bow 2.
    >I think it would be cleaner to separate time measurement from events
    >and other eventualities.  The axioms can all stay the same, but we
    >drop the idea that a time instant is a situation in the McCarthy-Hayes
    >sense.  Instead, we would have a function
    >     (sit-at ti pw)
    >which denotes the situation that obtains at time 'ti' in the possible
    >world 'pw'.  (If you would rather avoid time instants altogether, you
    >can also map an interval to a situation sequence or history or
    >something; I will leave that to others.)  Then you set up
    >relationships among various possible worlds in the usual ways.  Then
    >you make 'holds' a predicate on situations (or histories, mutatis
    >mutandis).   Branching time is the case where if pw1 is an alternative
    >to pw2, then they share a prefix and nothing thereafter.
    I agree, except for the last sentence. I think that the idea of 
    branching time is close to incoherent if we really want time to be 
    something like a continuum, or even dense. (When does it branch, for 
    example, and when does it just go on rolling along?) It is better to 
    distinguish possible-timelines and times in a timeline, as orthogonal 
    distinctions. One can think of these as two modalities, respectively 
    possibility and tense. The discrete sit-calc picture is then a 
    branching 'structure' defined over a set of timelines with a common 
    time metric, like little arrows going from one line across to 
    another. The structure is branching, more or less by definition; but 
    the timelines aren't and they needn't even have a common past. Then 
    it makes sense to ask things like, how long would it have been since 
    you last went to Australia if you hadn't gone to Sydney last year?
    Possibility really hasn't got anything intrinsically to do with time. 
    We can make firm predictions about the future, and we can be 
    uncertain about the past. Thinking about planning tends to get these 
    muddled together since one assumes sufficient knowledge about the 
    present and not about the future (which is why one is planning in the 
    first place, presumably), but that's only one way of thinking and may 
    not be the primary one.
    IHMC					(850)434 8903   home
    40 South Alcaniz St.			(850)202 4416   office
    Pensacola              			(850)202 4440   fax
    FL 32501           				(850)291 0667    cell	          http:/	   for spam

    This archive was generated by hypermail 2.1.4 : 10/29/02 EST