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Re: universal truths and the universe (was Re: Three ideas)

From: Patrick J. Hayes <phayes@ihmc.org>
Date: Thu, 20 Jan 2022 05:35:17 +0000
To: "Peter F. Patel-Schneider" <pfpschneider@gmail.com>
CC: "public-rdf-star@w3.org" <public-rdf-star@w3.org>
Message-ID: <EBBA252C-CEC2-4ECC-AEBD-E2244C50E149@ihmc.org>

> On Jan 19, 2022, at 8:31 AM, Peter F. Patel-Schneider <pfpschneider@gmail.com> wrote:
> WARNING:  Thinking about things like this can rot your brain or, worse, turn you into a philosopher.  On the other hand, thinking too little about things like this has potentially worse outcomes. Thinking too much about how much to think about thigns like this will definately rot you brain; I speek from experience.  :-)

I entirely concur.

> Pat appears to be arguing that there are universal truths

Why 'universal'? I am using "truth" in the ordinary sense in which it is used in court proceedings, intelligence reports and responsible newspapers, as well as everyday life. In fact I dont think we are here arguing about truth as such, so much as about what kinds of sentence should be called true (or not).

> and that the universe, on which the notion of universal truths is based, has a single unbranching time line.

No, I am not assuming that. However, as we now know, the actual 1934 was in fact in the future of the actual 1885.

>   So it makes sense to have a statement like "Bette Davis played the part of a vulgar waitress in the move 'Of Human Bondage' in 1934" be a universal truth no matter what the current time is.

This terminology of "universal" is confusing (and misleading) becasue it suggests some special kind of truth superior to just plain truth. The argument I am using is quite simple. If (as I claim) truth of a well-formed non-indexical proposition is independent of time, and if we also insist that it makes sense to speak of a proposition being true 'at a time', then if it is true at any time it must be true at all times. For if it is true 'at t1' then it is (simply) true, and if it is true then it is true 'at t2'. The addition of a time to a time-independent assertion does not affect its truth, pretty much by definition. This argument has nothing to do with assumptions about the topology of time. 

> It is possible for the universe to not have an unbranching time line and still have universal truths.  It is just that many of these universal truths have to be more complex. 

If they are expressed in a tensed language, yes. But RDF Is not designed to be a tensed language.

> So instead of the above statement one might need "Bette Davis played the part of a vulgar waitress in the move 'Of Human Bondage' in 1934 at all time points after X" where X is a point in time.  For some X, even the one where the seance occurs (which is before 1934), this might be true and for other X it might be false. 

Are you talking about varieties of tense logic? Because this would be correct in some S4 logics if the sentence were posed as a future-tense statement in the logic. I tried to avoid tense by writing it in the 'simple present' which in English can be used timelessly, as in "Two plus two is four."

> But in either case each instance of this enhanced statement with X replaced by a time point has a universal truth. 


> (Yes, this is a bit sloppy.)
> The nature of time in the universe is unknown by any human at this time, as far as I know.  There could be a single unbranching time line. Time could be like a tree, where at any point there is a single past but multiple futures.  Time could be even more complex, where there are both multiple pasts and multiple futures. It is also possible that those things that we think of as unchangeable physical truths, and thus universal, are not really universal, perhaps needing to be stated about a point in time, or time-space, or even something more complex.  It is even possible, I think, that those things we think of as unchangeable mathematical or even logical truths are not really universal.

Well, I disagree, but we have now really entered the Halls of Philosophy. 
> Does this all matter for RDF?  Maybe.  What is certain is that some kinds of mixing of current truths and other truths can cause problems in RDF and in extensions of RDF.

Not just RDF, but for any proposed social-scale notation for humans and machines to communicate with one another with enough mutual clarity to support some inference machinery.


> peter
> PS:  See http://4.bp.blogspot.com/-CWM1zDcmWXs/TroD0VsX4WI/AAAAAAAAAVA/Jc5bN5xSTkc/s1600/ch930919.gif for an interesting universe where the laws of physics might have changed.
Received on Thursday, 20 January 2022 05:35:34 UTC

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