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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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marnixR
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Mon Sep 26, 2011 4:55 pm
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KALSTER wrote:
I guess the old adage applies: You can't prove a negative.


that was just my point when starting this thread : there are a few special case instances where you can prove a negative,as when something well-defined is or isn't present in a limited, well-defined space
otherwise all bets are off, and the best you can hope for is to substitute absolute proof by establishing very low probability

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padren
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Thu Sep 29, 2011 8:23 pm

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marnixR wrote:
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


There is always the corollary implications: I could claim "No one has proven that Loch Ness contains no monster" and you could rebuff with "But if it does contain a monster of that size, what flora/fauna would be needed to support it?" and and examination of whether the ecology of the lake is similar to lakes that are known to not contain monsters or if the ecology appears to deviate due to unknown factors.

If the ecology does not appear that different than other lakes, I would either have to challenge the assertion that it's impact would be noticeable (which may or may not be supported by major scientific disciplines) or change my overall assertion to a narrower definition. I could say "But if Nessy is a Ghost... she wouldn't have to eat!" but I've literally abandoned any definition of the Nessy being a flesh and blood monster. I could say "She could have unique monster DNA that lets her eat very little" to maintain most of my original assertion, but then I am adding even less probable assertions to explain how it all avoids conflict with well established data.


Can you prove Ether doesn't exist?

Probably not - but if that particular asserted definition of ether carries implications that conflict with all observed astronomical data... it really detracts so heavily from the assertion's credibility it may as well be dismissed.


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marnixR
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Thu Sep 29, 2011 9:02 pm
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for the same reason i can't prove that some portion of the moon is NOT made of green cheese - it's just that the right portions haven't been sampled yet

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padren
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Thu Sep 29, 2011 10:01 pm

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marnixR wrote:
for the same reason i can't prove that some portion of the moon is NOT made of green cheese - it's just that the right portions haven't been sampled yet


And yet such an assertion however difficult to disprove carries a lot of implications - that either green cheese can naturally form on the moon (which has serious implications for our understanding of chemistry, biology, and the environmental conditions on the moon) or that some powerful unobserved (and probably intelligent) force made the moon partly out of green cheese.

Even if you can't prove that cheese doesn't exist in the makeup of the moon, the issue is largely moot when the implications can tell us so much more about the assertion.


You could theorize that just about anything exists beyond the event horizon of a black hole and it would truly be "undisprovable" but when an expert actually does produce such a theory regarding black holes it's how consistently the implications of the theory fit with existing and future observations that makes viable or not.


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marnixR
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Fri Sep 30, 2011 6:22 am
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padren wrote:
Even if you can't prove that cheese doesn't exist in the makeup of the moon, the issue is largely moot when the implications can tell us so much more about the assertion.


ok, playing devil's advocate here : you could say that the moon being made of green cheese contradicts our CURRENT knowledge of how things work - future work may well reveal that green cheese is the natural outcome of processes we don't know yet

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padren
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Fri Sep 30, 2011 9:45 pm

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marnixR wrote:
padren wrote:
Even if you can't prove that cheese doesn't exist in the makeup of the moon, the issue is largely moot when the implications can tell us so much more about the assertion.


ok, playing devil's advocate here : you could say that the moon being made of green cheese contradicts our CURRENT knowledge of how things work - future work may well reveal that green cheese is the natural outcome of processes we don't know yet


Which is a fair counterpoint - and it still stands that you can't "prove" the assertion is 100% false. However, it does set the bar where the assertion is now complicated with hypothetical "sneaky processes" that would fundamentally change our understanding of a wide body of knowledge.

The main point being that - while many theories that include "aspects that are unprovable and/or undisprovable" they are not all weaker for it. Theories that model behavior either inside black holes or at the quantum level may fit that description, yet provide clearly defined implications for what we can observe... while others breakdown and either reduce scope of the claim to make it "impact everything else less" or add complications that make it far less viable.


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marnixR
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Mon Oct 03, 2011 6:46 am
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padren wrote:
However, it does set the bar where the assertion is now complicated with hypothetical "sneaky processes" that would fundamentally change our understanding of a wide body of knowledge.


agreed - i'd go as far as calling it the "epicycle effect" : if you have to add caveats and special reasoning to uphold a theory, chances are that it will turn out to be wrong

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DrRocket
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Sun Oct 09, 2011 8:44 am
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marnixR wrote:
KALSTER wrote:
I guess the old adage applies: You can't prove a negative.


that was just my point when starting this thread : there are a few special case instances where you can prove a negative,as when something well-defined is or isn't present in a limited, well-defined space
otherwise all bets are off, and the best you can hope for is to substitute absolute proof by establishing very low probability


There is no such thing as absolute proof in science. Proof applies to mathematics, but mathematics is not science. Science relies on evidence, not proof.

In fact, negative evidence is about as close to proof as one has in science. One valid piece of empirical evidence is all that is needed to falsify a scientific theory, though doubt can always remain as to whether or not the evidence is valid. The current controversy over "superliminal neutrons" is a good example -- if the data analysis is finally accepted as valid it will overturn special relativity in a big way.

One can "prove a negative" about as easily as one can prove a positive. Generally this is done by showing that the proposition to be shown false contradicts a proposition that is accepted as true. In fact to prove that "A is false" you need only prove that "not A is true".

With regard to something like "proving that unicorns do not exist", you are addressing the wrong issue. In science there is no need to assert or deny the existence of anything unless and until that something has a potential interaction with things hat are accepted as existing. There is no evidence that unicorns have ever existed, and there are no verifiable unexplained phenomena that would require unicorns for an explanation. Thus there is no reason to posit the existence of unicorns. Unicorns are irrelevant.

A similar situation exists in physics. Contrary to popular belief, special relativity did not disprove the existence of an aether. Rather it made an aether irrelevant. All observed phenomena can be explained without need for an aether, hence the aether was discarded. It might interest you to know that there is a perfectly valid physical theory, the Lorentz Ether theory, that includes an aether and that is experimentally indistinguishable from special relativity. However, in that theory the aether is not detectable and the theory is rather clumsy, so it is not used. There is no reason to use it, since it does nothing that special relativity does not. It is equally possible, and equally pointless, to devise a theory in which all observed phenomena involve the actions of pink pixies who mimic the known laws of physics. The existence of these pixies cannot be disproved, but they too are irrelevant.

If you let philosophers into the room they can debate whether or not a horse exists. I'm pretty sure that it does, and people with sore butts from riding it agree. But all sorts of idiocy can come out when philosophers start talking ontology and epistemology.

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mississippichem
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Sun Oct 09, 2011 1:46 pm
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A great example of scientific "negative logic" comes from chemical kinetics, the study of reaction mechanisms. No one has ever proved a mechanism, even for simple reactions. We can only know which mechanisms are not occurring. After knocking out five or so reasonable mechanisms it is often possible to tweak experimental parameters and rule out the remaining few. We are left with a proposed mechanism that completely obeys the laws of thermodynamics and mechanics and we have a lot of data to support it. However because molecules are so small and electrons have such a probabilistic behavior, we can never "prove" that a reaction happens in said manner.


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DrRocket
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Sun Oct 09, 2011 7:22 pm
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mississippichem wrote:
A great example of scientific "negative logic" comes from chemical kinetics, the study of reaction mechanisms. No one has ever proved a mechanism, even for simple reactions. We can only know which mechanisms are not occurring. After knocking out five or so reasonable mechanisms it is often possible to tweak experimental parameters and rule out the remaining few. We are left with a proposed mechanism that completely obeys the laws of thermodynamics and mechanics and we have a lot of data to support it. However because molecules are so small and electrons have such a probabilistic behavior, we can never "prove" that a reaction happens in said manner.


Excellent example.

In truth these mechanisms are grounded in the atomic theory and it has necver been "proved" that atoms exist. It is just that the model works so well, and that there is no contrary evidence whatever, so that atoms are accepted as a matter of course in modern physics and chemistry.

Somewhat more problematic are quarks. But even in that the case the evidence from the model, quantum chromodynamics, is sufficiently strong that there is no serious challenge to the existence of quarks at this time.

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x(x-y)
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Sun Oct 09, 2011 7:33 pm
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DrRocket wrote:
In truth these mechanisms are grounded in the atomic theory and it has necver been "proved" that atoms exist. It is just that the model works so well, and that there is no contrary evidence whatever, so that atoms are accepted as a matter of course in modern physics and chemistry.


You most likely know more about this subject than me, but instruments such as STMs and SEMs provide very much solid observational evidence for the existence of atoms in the form that our theories have predicted them. But, I suppose you could say that we cannot know the exact nature of atoms due to the Heisenberg Uncertainty Principle applied to electrons- i.e. they don't necessarily orbit the nucleus.

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DrRocket
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Sun Oct 09, 2011 8:49 pm
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x(x-y) wrote:
You most likely know more about this subject than me, but instruments such as STMs and SEMs provide very much solid observational evidence for the existence of atoms in the form that our theories have predicted them. But, I suppose you could say that we cannot know the exact nature of atoms due to the Heisenberg Uncertainty Principle applied to electrons- i.e. they don't necessarily orbit the nucleus.


They show "blobs" about where you would expect atoms in some crystal structures. That is certainly consistent with atomic theory, but does not demonstrate internal atomic structure, or provide anything like "absolute proof" of the atomic hypothesis. What makes the atomic hypothesis so widely accepted is quite simply the utility and predictive power of the mathematical model that goes with it.

That ought not be surprising. At the atomic and sub-atomic level our understanding is derived almost entirely from our mathematical models. Reality, whatever that may be, at that scale is so different from everyday experience that ordinary language and concepts simply fail.

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logic
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Sun Oct 23, 2011 12:45 pm

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marnixR wrote:
outside the field of mathematics, is it possible to prove that something doesn't exist ?

if you want a qualified answer you must qualify your question: what is something?
In general the answer is no.
mathematics is logic. you need logics: the famous example of unicorn refers to form.
if you define your "something" you may need physics
Hope that answered your question :)


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marnixR
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Sun Oct 23, 2011 7:21 pm
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logic wrote:
if you want a qualified answer you must qualify your question: what is something?


not easy to give a specific answer to that - my aim was at things that have a physical reality, ranging from coelacanths to nessie, from planet X to Pluto, from X-rays to ether
don't know if there's a generic name for that sort of entity

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logic
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Mon Oct 24, 2011 8:11 am

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marnixR wrote:
my aim was at things that have a physical reality, ranging from coelacanths to nessie

Then the answer is no


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marnixR
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Mon Oct 24, 2011 4:21 pm
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maybe there's no general solution, but in special cases there could be a solution - my earlier mentioned example of the postulated planet Vulcan is a case in point : through thorough investigation you are able reduce the maximum size of any object that can exist inside the orbit of Mercury to such an extent that it no longer qualifies as being a planet

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Hermit
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Tue Oct 25, 2011 12:37 am
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DrRocket wrote:
They show "blobs" about where you would expect atoms in some crystal structures.


While you are correct that this does not constitute proof, and is merely consistent with the theory, I think you are still not giving enough credit here.

The 'blobs' constitute very powerful evidence. This, along with the mass of other evidence shows that the atomic theory is a very, very strong model of reality. The probability of something destroying this model is very very low.

In layman speak, this means that the atomic theory is almost certainly correct.


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marnixR
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Tue Oct 25, 2011 6:14 am
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Hermit wrote:
In layman speak, this means that the atomic theory is almost certainly correct.


or that any competing theory would have to have a good explanation for those blobs

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marnixR
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Mon Oct 31, 2011 8:01 pm
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just finished reading "Darwin's lost world" by Martin Brasier, where at one point the topic of the absence of easily recognisable fossils in the Precambrium comes up, something that has been explained in a variety of ways in the past century and a half

in Charles Lyell's time the story was that the fossils were there, but just hadn't been found - the classic "absence of evidence is not evidence of absence", quite understandable given that geology in the early 19th century was still full of gaps

nowadays, there's not too many who hold this point of view, to the point that most would agree with JBS Haldane's famous quip, when asked what would disprove evolution, "fossil rabbits in the Precambrian"

so at what stage did absence of evidence become evidence of absence ? or were there other factors involved that made it first highly unlikely and then virtually impossible for complex life to have existed in the precambrian ?

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marnixR
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Tue Nov 08, 2011 7:50 pm
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thought i'd quote A.C. Grayling from his book "Thinking of Answers" - i hope that quoting a 2-page extract falls under proper use, because at least here's a qualified philosopher who hopefully can clarify the subject better than i've done so far :

The claim that negatives cannot be proved is beloved of theists who resist the assaults of sceptics by asserting that the non-existence of God cannot be proved. By this they hope to persuade themselves and others that at least the possibility remains open that a supernatural agency exists; and this is enough for them; from there they make the inflationary move from alleged mere possibility to not eating meat on Fridays or covering their womenfolk in black from head to toe.

They are however wrong both about not being able to prove a negative, and about not being able to prove that supernatural agencies exist and are active in the universe. Seeing why requires a brief refresher on the nature of proof.

Proof in a formal deductive system consists in deriving a conclusion from premises by rules. Formal derivations are literally explications, in the sense that all the information that constitutes the conclusion is already in the premises, so a derivation is in fact merely a rearrangement. All men are mortal, 'Socrates is a man, therefore Socrates is mortal' is a valid and sound deduction, and there is no information in the conclusion that was not already in the premises; the conclusion is a rearrangement of the premises' informational content.
Thus there is no logical novelty in the conclusion, though there might be and often is psychological novelty in the sense that the conclusion can seem unobvious or even surprising because the information constituting it was so dispersed among the premises. This is shown by the story of the Duke and the Priest; while giving a house party the Duke went downstairs to fetch more wine, leaving the Priest to entertain the guests. The Priest did so by saying that the first person to whom he ever gave confession was a murderer of an especially horrible
kind. His fellow guests shuddered accordingly. On returning, the Duke clapped the Priest on the shoulder affectionately and said, 'We've known one another for ages; I was the first ever person whose confession he heard.' The guests all promptly left; for here was psychological novelry aplenty.

Demonstrative proof, as just explained, is watertight and conclusive. It is a mechanical matter; computers do it best. Change the rules or axioms of a formal system, and you change the results. Such proof is only to be found in mathematics and logic.

Proof in all other spheres of reasoning consists in adducing evidence of the kind and in the quantiry that makes it irrational, absurd, irresponsible or even lunatic to reject the conclusion thus being supported. This is proof in the scientific and commonsense meaning. The definitive illustration of what this means, especially for the use that theists would like to make of the myth that you cannot prove a negative, is Carl Sagan's dragon-in-the-garage story which is surely too well known to require rehearsing, and if not then it should be; read it at: http://www.users.qwest.net/~jcosta3/article_dragon.htm.

No self-respecting theist would go so far as to claim that 'you cannot prove the non-existence of a god' entails 'a god exists'. As mentioned, their aim is merely to leave open the possibility that such a being might exist. But Sagan's dragon dashes even this hope. For one can show that it is absurd, irrational, intellectually irresponsible or even lunatic to believe that fairies, goblins, the Norse gods, the Hindu gods, the gods of early Judaism (yes, there were several), and so endlessly on, 'might exist'. It would compound the felony a million-fold to grant this and yet insist that one's own (Christian or Muslim, say) deity 'nevertheless' exists or might exist.

For a simple case of proving a negative, by the way, consider how you prove the absence of pennies in a piggy-bank. You break it open and look inside: it is empty. On what grounds would you assert nevertheless that there might possibly still be pennies in there, only you cannot see or hear or feel or spend them?


my problem is this though : in the case of his simple example, only a fool would deny the possibility of proving a negative - but what of more complex issues, where the search space is less confined and/or not very well defined ?

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tridimity
Post  Post subject: Re: proof that something doesn't exist  |  Posted: Tue Jan 24, 2012 12:03 pm

Joined: Fri Aug 12, 2011 5:55 pm
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Once you rule out the impossible, whatever remains, however improbable, must be true


How to prove the non-existence of an object.

Fermilab's attempt to confirm or refute the existence of the Higgs Boson springs to mind. The team of scientists working on this have already excluded the possibility of the Higgs existing within a certain range of masses, through experimental observation. There remains a window of masses in which the Higgs might exist (114-137 GeV/c2), and as you all most probably are aware, the team are working hard to confirm or refute its existence within this window. If the Higgs is not observed here, then for all intents and purposes the Higgs may be considered as non-existent.

I agree with many of the sentiments already posted, insofar as we must be pragmatic about these things, on a scale of probabilities*. The greatest probability of the non-existence of any object can be secured by establishing parameters for the object that can then be investigated by sensual observation.

*That is, unless one wishes to turn into a philosopher of the most pedantic order, incapable of believing in anything and consequently unable to muster the will power to get out of bed in a morning, since one might not exist!

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