Ontologies for time
In ENRAC 20.8 (98064), Erik wrote:
| One of the cases you mention is where two intervals Meet in direct
succession; one is where the first interval Meets a point which in
turn Meets a second interval; in the two remaining cases one or the
other interval includes that point. - I am afraid there's a
misunderstanding here, since I was referring to the domain used in
each of the interpretations. For each particular interpretation, it
must certainly be determined whether or not there is a point between
the two intervals. Therefore, different scenarios will sometimes
differ with respect to their domains for the type of "point" (and
maybe also for the type "interval"?) if one insists on dealing with
dividing instant situations by using domains where for certain
clocktimes there is no corresponding (time)point. Sometimes,
different models for the same scenario will also differ in that
respect.
Now to the examples. I will take for granted that we talk about
timepoints and intervals that are related along the lines of Pat's
core theory, only with the adjustment that intervals are not
entirely determined by their endpoints: there can be up to four
intervals for each pair of endpoints, because you allow these
intervals to be either open or closed at each end. (The interval
will then be defined as closed if there exists a point beginning
resp. ending it, otherwise it's open).
| Yeah, there's a misunderstanding about the time domain, and actually
it was my fault since I didn't make it clear in my former message. In
fact, the time domain (actually the time theory as a whole) which I
was referring is that which takes both intervals and points as
primitive on the same footing (neither intervals are constructed out
of points, nor points are defined as the "meeting places" or other
limiting structures of intervals), rather than the time lines of
Pat's core theory. An interval can meet other intervals and/or
points, but a point can only meet intervals (including moments) (see
Ma and Knight, Comuter Journnal 94). Therefore, such a theory allows
all the following cases:
- interval I1 Meets interval J1 (without any information as whether
there is a point, say P1, which Finishes I1, or Starts J1);
- interval I2 Meets interval J2; and point P2 Meets J2 (or
equivalently, point P2 Finishes I2);
- interval I3 Meets interval J3; and interval I3 Meets point P3 (or
equivalently, P3 Starts J3);
- interval I4 Meets point P4, and point P4 Meets interval J4.
In it important to note that, in case (1), (2) and (3), there is not
any time element, neither interval/moment nor point, that stands
between the two successive intervals, while in case (4) there IS a
time point which connects two intervals.
As I argued in my former message, all the above cases can be
accommodated by the single time theory (model) -- all these
scenarios may (but not necessarily) appear somewhere over the time
lines (even if the time itself is further characterised as linear)
without the need of any futher specifications. They are not conflict
with each other.
| You refer to an example by Galton where a Green light and a Red
light both switch On at the same time. This is somewhat
counterintuitive - I would have thought that one goes Off when the
other one goes On - but that doesn't matter.
| Yeah, this doesn't really matter. Actually, the facts that one light,
e.g., Green light, is On before P and then switched Off at P, and the
other, the Red light, is Off before P and then switch On at P can be
just expressed as: Holds(GreenOn, I) and Holds(RedOff, P), leaving
the real question still as how to express the state of the two lights
at the switching point P. If both lights are asserted as "On" at P,
the corresponding axioms will be:
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Holds(GreenOn, I)
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Holds(GreenOn, P)
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Holds(GreenOff, J)
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Holds(RedOff, I)
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Holds(RedOn, P)
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Holds(RedOn, J)
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Meets(I, P)
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Meets(P, J)
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Similar treatments apply to the case where both the lights are
asserted as "Off" at P.
As for the case where no information about the Switching point is
given at all, i.e., none of the two lights is asserted as On or Off
at the switching point P, the axioms wil be as simple as:
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Holds(GreenOn, I)
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Holds(GreenOff, J)
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Holds(RedOff, I)
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Holds(RedOn, J)
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Meets(I, J)
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Therefore, the only remaining case is that at the switching point
P, one light is known as On (or similarly, Off), while there is no
assersion as to the state of the other light at P. This is actually
similar to Galton's example, and can be treated by the same approach
(see below).
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You propose the following scenario description for the case where we
have decided to consider the Green light to be On at the dividing
instant, and we have decided to keep that open for the Red light:
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Holds(GreenOff, I2)
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Holds(GreenOn, P)
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Holds(GreenOn, J2)
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Holds(RedOff, I1)
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Holds(RedOn, J1)
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Meets(I2, P)
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Meets(P, J2)
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Meets(I1, J1)
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I1+J1 = I2+P+J2
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Actually, these axioms do not indicate that the two lights switch at
the same time. Let's assume that such a statement has been added,
otherwise the point with the example is lost. Now, in every model
for these axioms it must be determined whether P is included in I1
or in J1. (Or, if you disagree, what would a model be like where P
is neither included in I1 nor in J1?) Suppose P is included in I1.
Then, as long as timepoints and intervals are related along the
lines of Pat's core theory, you can't avoid the conclusion that the
interval I1 ends with P, and hence that the Red light is Off at the
dividing point. Similarly, if P is included in I2, it must be that
I2 begins with P, and that the Red light is On at the dividing
point. Therefore, in each of the models there is the kind of
choice that you called "arbitrary" with respect to whether the Red
light is to be considered On or Off.
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After I had sent my former message containing the above solutions, I
found I should include an additional "axiom", that is:
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Duration(I1) = Duration(I2)
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which implies that Duration(J1) = Duration(J2). Remember that
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Duration(P+J2) = Duration(J2)
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therefore the following axioms:
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Holds(GreenOff, I2)
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Holds(GreenOn, P)
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Holds(GreenOn, J2)
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Holds(RedOff, I1)
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Holds(RedOn, J1)
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Meets(I2, P)
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Meets(P, J2)
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Meets(I1, J1)
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I1+J1 = I2+P+J2
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Duration(I1) = Duration(I2)
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together express the specified example, showing that:
(a) "GreenOff" Meets "GreenOn", i.e., Meets(I2, P+J2) ,
"RedOff" Meets "Red"On, i.e., Meets(I1, J1) ;
(b) Since, together with the rest axioms, axiom I1+J1 = I2+P+J2
and axiom Duration(I1) = Duration(I2) indicate that the Green light
and the Red light are both Off before the switching point P, and are
both On after P. Therefore, both the Green light and the Red light
are switched at the same time point P
(c) The switching point P satisfies the "GreenOn" property which is
specified as P Starts the GreenOn interval (i.e., P+J2 ), where there
is nothing specified as whether the switching point P Finishes the
RedOff interval (i.e., I1) or Starts the RedOn interval (i.e., J1).
| My two examples come out in similar ways. For example A, you write:
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| 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 necessarily imply that there
must not be any such point, especially if one insists that "at a
moment (point?) when it (the ball) reaches the top of its
trajectory, he (Jim) turns the switch".
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But if (in a particular model) such a point exists for the clocktime
where Jim turned the switch, then it must be determined (in that
same model) whether the switch is on or off at that point, and you
have your Dividing Instant Problem back again.
For example B, you write:
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| I don't agree with the claim that "a point both exists and does
not exist at the clocktime whent he winner finishes his last cone
and the bell rings". Again, I think this claim was reached by
means of confusing two cases, that is, the case that an interval
"Meets" a point, and the case that an interval was "Finished-by" a
point.
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Not really. If you wish to avoid a dividing instant situation by
using a punctuated time domain (for each of the models, so that
there is no dividing instant problem in any of the models), then you
must exclude models where that timepoint is present. It can't be
present explicitly, and it can't be present implicitly by being the
ending or beginning of an interval, because in all of those cases
you end up assigning the truthvalue that you considered arbitrary.
The only way of complying is to have two successive open intervals
without any point between them. (That is, an interval not ending in
a point, and a subsequent Meeting interval not beginning in a
point). However, this in turn contradicts the assumption that the
Bell rings, since it was assumed the Bell rings at (time)points.
Therefore, the only possible models are those where the Bell rings
without the cones having been finished, and you obtain the
conclusion I indicated.
The bottom line is, therefore, that it is futile to try to impose
noncommitment for dividing instants on the level of the models and
by using nonstandard time domains such as "punctuated time". In
those cases where we wish to express that we don't know or don't
care whether a certain proposition is true or false at a point of
change, it's sufficient to use the multiple models approach while
admitting "standard" time (integers or reals, by preference). Then
we don't need any theory of time at all besides high-school or (at
most) college math.
All of this presumes of course standard two-valued logic, where
models can only assign the truth-value true or false. You may obtain
another perspective by going to e.g. three-valued logic, where
everything can be undetermined besides true or false. But, as H.C.
Andersen once said, that is another story.
| I feel my explanations/arguments about the time theroy (model) in the
first part of this message and the above revised demostration of the
two-lights exmple also apply to Erik's questions/arguments and his
examples A and B, since as Erik observed, they come out in similar
ways.
Jixin
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