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marnixR
Post  Post subject: proof that something doesn't exist  |  Posted: Mon Sep 19, 2011 7:42 pm
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outside the field of mathematics, is it possible to prove that something doesn't exist ?

a bit like going into a darkened room looking for a black cat that you're not even sure is there - at what stage can you feel confident that the cat isn't there ?

an analogue case in astronomy was the case of the planet Vulcan, which was supposed to orbit the sun inside the orbit of mercury in order to explain irregularities in the latter's rotation

sightings were made from time to time but never confirmed, and in the end, with improvements in telescopes and search methods, the size of a body that could possibly reside in an intra-mercury orbit until shrank until it approached a size below which the body could no longer be described as a planet
as soon as the boundary conditions for the existence of such a body did no longer intersect with the boundary conditions of the definition of a planet, the case for the existence of Vulcan could be said to be disproven

in view of the above i'd have to give a qualified yes in answer to the question, but i'm not very confident that the methodology can be extended to a wider range of proofs of non-existence - the example may only highlight the possibility of proving the non-existence of a well-defined object in a confined space

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iNow
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Tue Sep 20, 2011 4:15 am
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AFAIK, no. You can only continually decrease the likelihood of existence, but not ever reach conclusive proof of non-existence. I'd love to hear from someone better versed in logic or philosophy than myself, though. Maybe Prometheus?

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Prometheus
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Tue Sep 20, 2011 10:15 am
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iNow wrote:
I'd love to hear from someone better versed in logic or philosophy than myself, though. Maybe Prometheus?


Well that cheered me up, haven't laughed so much for a while. Everything i know about philosophy could be found in a philosophy for dummies book.

Karl Popper is well known about talking about the falsification of a theory/hypothesis. You only need one confirmed (ie not measurement error) inverse instance to disprove a theory. But the theory was only ever an approximation of truth, so we change the theory to include the aberrant observation.

Popper spoke about verisimilitude where the truth content of a theory is measured on a continuum. Popper believed that veracity could only be spoke of in the context of this continuum (which he measured as the truth content minus the falsity content). Based on this idea of a continuum a theory could be very low on the scale, with little truth content. I guess Vulcan would be pretty low.

But if the 'truth' really is out there, then competing theories should be measured on the same scale (my thinking now). Therefore, if you can obtain a theory that is 100% certain then you can be 100% certain of the falsity of competing theories. But can we ever be 100% certain of our theories? If not neither can we ever be certain of something's non-existence.


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marnixR
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Tue Sep 20, 2011 4:51 pm
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i suppose the probability continuum makes sense
for the layman anything that approaches a high probability of being true is called true, or at least something akin to proven beyond reasonable doubt

presumably the exact probability where something is deemed to be true is a matter of personal preference

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Hermit
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Sun Sep 25, 2011 8:27 am
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My own thinking works by probabilities. This is pretty damn imprecise, since mostly I cannot give an exact probability, but as a mental tool it works for me.

For example ; if someone talks about homeopathy, my response is that the claim that it has therapeutic effect, apart from placebo, is so low in probability as to be false for all practical purposes.

I think that 'proving' something is wrong, is rare. So we have to settle for showing that something is highly improbable. If said probability is low enough, we can operate on the working assumption that it is close enough to disproved.


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iNow
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Sun Sep 25, 2011 1:57 pm
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Hermit wrote:
I think that 'proving' something is wrong, is rare. So we have to settle for showing that something is highly improbable. If said probability is low enough, we can operate on the working assumption that it is close enough to disproved.

Which works well because then we also leave ourselves open to correction based on new evidence should that ever appear.

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marnixR
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Sun Sep 25, 2011 7:12 pm
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Hermit wrote:
I think that 'proving' something is wrong, is rare.


the intention of my OP was more about proof of non-existence rather than about proof of something being wrong
although related in some way, they're really quite distinct notions

so this thread is more about : does the yeti exist ? does god ? do UFOs ? etc.

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Hermit
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Sun Sep 25, 2011 10:34 pm
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marnixR wrote:
does the yeti exist ? does god ? do UFOs ? etc.


All very improbable. None disproved, but close enough.

When you ask : "does God exist?', the question contains an imprecision that makes it almost meaningless. Whose god? What description? If we ask :"does the Christian God exist?" my response would be my estimate is about a trillion to one against. If we ask : "does any kind of being of sufficient power to be regarded as a deity exist?", then I have to consider possibilities such as an alien in another galaxy of that level of technological power, and the probability increases accordingly.


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marnixR
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Mon Sep 26, 2011 6:17 am
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mind you, the imprecision is also there for UFOs (unless you insist that they're spacecraft, in which case they're IFOs), the Loch Ness monster (something "big" - how big counts as big ?), etc. but i suppose god is in a category of his own

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KALSTER
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Mon Sep 26, 2011 11:47 am
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I guess the old adage applies: You can't prove a negative. In the case of gods and such, the mental probability scale thing works for me too. Though, once one part of the whole desired definition of a god is disproved (through logic arguments for example), that god becomes impossible.

When it comes to theories, I guess it is determined by how well they work for what they are intended. An example is the difference between Newton's laws of motion. They are perfectly fine at low speeds, but become increasingly inaccurate as the relevant velocity increases, though they could in a sense be regarded as the same theory now, since Einstein's reduces to Newton's at low speeds.

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