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You can't prove a negative : why, logically?

Posted: Tue Jan 28, 2014 5:14 pm UTC
by Mambo4
"You can't prove a negative" is a commonly used phrase in skeptical discussions.
Googling the phrase usually ends up with the context of James Randi (or similar) responding to the question: " can you prove <paranormal thing> doesn't exist? "

but I haven't seen a concise logical argument against the possibility of proving a negative, or if there is some academic idea in the discipline of logic that makes this claim.

Re: You can't prove a negative : why, logically?

Posted: Tue Jan 28, 2014 6:07 pm UTC
by SecondTalon
You can. Evidence of Absence, for example.

The context of Woobusters like Randi is usually "prove ghosts don't exist anywhere and never have and never will" sort of thing, which is an incredible amount of things to prove. Randi will be asked to prove that psychic powers do not exist. Randi cannot test everyone everywhere and determine that no one is psychic (not to mention testing of animals that may or may not possess the ability to see into the future), but he can test individuals who claim to be psychic and prove that they are not.

Using the set PeopleWhoClaimPsychicPowersAndAreWillingToTestThemAsPerRandiAndThePsychic'sAgreedUponParameters, a subset of PeopleWhoClaimPsychicPowersAndAreWillingToTestThem, which is a subset of PeopleWhoClaimPsychicPowers which is a subset of EveryoneCurrentlyAlive.... Randi has proven that none of them have psychic powers.

But.. we can totally prove that I do not have a cup of lava on my desk. For one, my office isn't melting.

Re: You can't prove a negative : why, logically?

Posted: Tue Jan 28, 2014 7:10 pm UTC
by pseudoidiot
Maybe it's a very special cup.

Re: You can't prove a negative : why, logically?

Posted: Tue Jan 28, 2014 7:38 pm UTC
by SecondTalon
That prevents the air from being heated? Then it's not a cup, it's a container.

Re: You can't prove a negative : why, logically?

Posted: Tue Jan 28, 2014 7:52 pm UTC
by Yoshisummons
But what if it's a very well insulated container, and is also invisible?

Re: You can't prove a negative : why, logically?

Posted: Tue Jan 28, 2014 8:15 pm UTC
by SecondTalon
A cup has defined characteristics, such as being tangible and visible, as well as having an open top and all that. Lava also has characteristics, for example it produces light and heat.

The premise isn't "There's some magical container on my desk".. the premise is "There's a cup..."

There is no cup of lava on my desk.

And if we're using LessWrong as a source here.. then let's look at what the RationalWiki has to say on it.

Re: You can't prove a negative : why, logically?

Posted: Tue Jan 28, 2014 10:03 pm UTC
by Noc
A better way to put it is probably "you can't prove a never." Or "an impossible," I guess.

...which is true! Technically. Statistically, no matter how much data you acquire, it won't be statistically significant against an infinite span of time, because...infinity. And experiments only tell us how something behaves under certain conditions, and you can't prove that those conditions are universal, or that they aren't subject to change in ways you aren't aware of. (See: the "It worked for me, I have no idea why it isn't working now!" phenomenon.)

But you can provide pretty compelling evidence that something is pretty unlikely. If I don't see a cup on my desk, I can be pretty sure that there isn't a cup on my desk, because there's a large body of evidence to suggest that when there's a cup on my desk, I can almost always see it. I can't rule out the possibility that there can never be a cup on my desk that I won't see -- and if it turns out that oh man! There was totally a cup there and I didn't see it? Then that's totally a thing that can be further investigated.

But given existing evidence about my visual ability, lack of severe neurological damage, the general visibility of cups and the lack of visual obstructions on my desk, I can generally make a pretty solid claim as to the presence or absence of a cup on my desk at a specific point in time, provided I am present to give my desk a quick look over. And you'd need to compellingly demonstrate the ability of a cup to hide in plain sight on my desk in order for me to start treating the potential presence of invisible cups as a reasonable thing.

Similarly, we can't prove that it's impossible for psychic powers to exist, but we can assemble fairly decent evidence that any given claim of psychic powers is probably going to be false.

Re: You can't prove a negative : why, logically?

Posted: Wed Jan 29, 2014 12:18 am UTC
by phlip
The missing words are basically: "You can't empirically prove a negative existential". That is, you typically can't use Science to prove "X doesn't exist". Any empirical evidence you can collect for such a statement is also consistent with "X exists, but has yet to be observed"... there's no evidence you can get that specifically discounts that possibility. "X can't exist" you can get to, in cases where X being even theoretically possible to exist is inconsistent with evidence, but a mere "X doesn't exist" is much harder to reach.

Eventually you just have to rely on Occam's razor, that "X exists but is, like, super rare you guys" just becomes too far-fetched, but that's not really a proof per se. At least, not in the sense that the sort of people who say "prove that ghosts don't exist" would accept.

Re: You can't prove a negative : why, logically?

Posted: Wed Jan 29, 2014 12:45 pm UTC
by Chen
SecondTalon wrote:A cup has defined characteristics, such as being tangible and visible, as well as having an open top and all that. Lava also has characteristics, for example it produces light and heat.


Well a sippy cup with sufficiently small opening made out of a sufficiently strong and non-conducting material might allow you to keep a cup of lava on your desk without burning stuff around it (though I wouldn't put anything directly over the small hole).

Re: You can't prove a negative : why, logically?

Posted: Wed Jan 29, 2014 4:04 pm UTC
by SecondTalon
That would still heat my office to uncomfortable levels.

My office is currently cold.

Re: You can't prove a negative : why, logically?

Posted: Wed Jan 29, 2014 5:21 pm UTC
by Chen
SecondTalon wrote:That would still heat my office to uncomfortable levels.

My office is currently cold.


I wonder how much heat energy you actually would have in a cup of lava. Sounds like a decent "What if?" question.

Re: You can't prove a negative : why, logically?

Posted: Wed Jan 29, 2014 5:37 pm UTC
by SecondTalon
Lava is 700 to 1,200 °C (1,292 to 2,190 °F). So.. I'd imagine quite a bit. More than enough to heat my 10x10x10 office cube.

Re: You can't prove a negative : why, logically?

Posted: Wed Jan 29, 2014 7:43 pm UTC
by Schrollini
SecondTalon wrote:Lava is 700 to 1,200 °C (1,292 to 2,190 °F). So.. I'd imagine quite a bit. More than enough to heat my 10x10x10 office cube.

Perhaps not as much as you might think. From Eugene Robertson's hit USGS report, Thermal Properties of Rocks, we know that the specific heat varies with composition and temperature, but 1000 J/kg K is a good ballpark figure. So a 1 kg cup of lava (300 - 500 ml) at 1000 K above room temperature has some 106 J of excess thermal energy.

A million joules sounds like a lot, but it's only 0.28 kWh. In other words, it's the energy put out by a 100 W light bulb in a little less than three hours. I don't know how quickly your cup of lava will radiate that heat away. Unless your cup is very well insulated, I imagine that it'll be more than 100 W at first, so keep it away from paper and other inflammable materials. But if you insulate it well enough to make it through an 8 hour day, it'll be giving off 35 W on average, which isn't a lot.

The obvious solution: More lava!

Re: You can't prove a negative : why, logically?

Posted: Thu Jan 30, 2014 9:25 am UTC
by bigglesworth
Or a little heater in the bottom of your lavamug.

If you mix it with coffee you could enjoy lavajava.

Or if you mix it with kava root, you could chill out with some lavakava.

Or if you want some real pep, you could drink kavajavalava.

Re: You can't prove a negative : why, logically?

Posted: Thu Jan 30, 2014 2:44 pm UTC
by LucasBrown
Or you could mix it with pastry and honey to get baklava.

There's a joke about balaclava in there somewhere, but I can't figure out how to structure it...

Re: You can't prove a negative : why, logically?

Posted: Thu Jan 30, 2014 6:07 pm UTC
by setzer777
SecondTalon wrote:That would still heat my office to uncomfortable levels.

My office is currently cold.


But can you prove that there's no hitherto unknown force preventing the definitional effects of lava from extending from the location of the lava to your sense organs?

Re: You can't prove a negative : why, logically?

Posted: Thu Jan 30, 2014 6:53 pm UTC
by SecondTalon
Only as far as arriving at the conclusion that I either exist as a fleshy body in the world I perceive OR I'm some sort of brain-inna-jar or some other sort of other hallucinating individual. And that is a completely different question.

So either there is no cup of lava, OR I'm a brain in a jar and it doesn't matter because the cup is simulated anyway.

And asking me to prove I exist is, quite frankly, the largest goalpost move we've seen yet.

Re: You can't prove a negative : why, logically?

Posted: Thu Jan 30, 2014 6:55 pm UTC
by setzer777
SecondTalon wrote:Only as far as arriving at the conclusion that I either exist as a fleshy body in the world I perceive OR I'm some sort of brain-inna-jar or some other sort of other hallucinating individual. And that is a completely different question.

So either there is no cup of lava, OR I'm a brain in a jar and it doesn't matter because the cup is simulated anyway.


What if there's a particle you don't know about that's exceptionally good at absorbing light/heat, but only in the range the lava is emitting it? What if there's a spacetime distortion shunting the heat far away from your office?

Re: You can't prove a negative : why, logically?

Posted: Thu Jan 30, 2014 7:07 pm UTC
by SecondTalon
Then both would be detectable by other means. If there was something shunting heat, it would be shunting heat and my office would be colder than normal. It's not.

And I'm sure the fluorescent lighting, lava, and my lighter share enough of the visible light spectrum that something that affects one would also affect the other two. The two I know exist are operating in normal parameters.

Really, if you're going that nutty, it's no longer up to me to prove it doesn't exist - that's now the given, and it's up to you to prove it does exist.

Re: You can't prove a negative : why, logically?

Posted: Thu Jan 30, 2014 7:10 pm UTC
by setzer777
I agree in terms of burden of proof. I just in mean in terms of why people say "you can't prove a negative". There's always hypothetical properties/interactions you're potentially unaware of.

Re: You can't prove a negative : why, logically?

Posted: Thu Jan 30, 2014 7:15 pm UTC
by SecondTalon
There's always hypothetical properties/interactions you're potentially unaware of.
Which makes proof impossible altogether. We cannot prove the Earth revolves around the Sun as it might actually revolve around Artax, Demonbeast of Rhthal who currently slumbers in the center of Sol, but will one day rise and resume his terrible flight throughout the universe, taking the planets with him. Because Magic. We can't even prove we exist to anyone else, as all of the evidence we offer could just be cleverness by the simulation up to and including the simulation rewriting the laws of physics to make it fit (lol what a dumbass, believing that light has a constant speed relative to all observers and that it's as simple as Energy equaling mass at the speed of light squared - ONLY IN THE SIMULATION DIPSHIT!).

I have plenty of evidence telling me no cup of lava exists in my office. I cannot see it, i cannot feel it, and I cannot measure any effects that it should be giving off. It doesn't exist - proved as well as anything can be proved.

Re: You can't prove a negative : why, logically?

Posted: Thu Jan 30, 2014 7:21 pm UTC
by bigglesworth
SecondTalon wrote:And asking me to prove I exist is, quite frankly, the largest goalpost move we've seen yet.
"In order to persuade me that paying taxes is acceptable, first you'll have to prove that you and everyone else actually exists"

Re: You can't prove a negative : why, logically?

Posted: Thu Jan 30, 2014 7:33 pm UTC
by setzer777
SecondTalon wrote:I have plenty of evidence telling me no cup of lava exists in my office. I cannot see it, i cannot feel it, and I cannot measure any effects that it should be giving off. It doesn't exist - proved as well as anything can be proved.


I dunno, with no controlled oversight it's technically only anecdotal eyewitness testimony, right?

Re: You can't prove a negative : why, logically?

Posted: Thu Jan 30, 2014 9:38 pm UTC
by PeteP
setzer777 wrote:
SexyTalon wrote:I have plenty of evidence telling me no cup of lava exists in my office. I cannot see it, i cannot feel it, and I cannot measure any effects that it should be giving off. It doesn't exist - proved as well as anything can be proved.


I dunno, with no controlled oversight it's technically only anecdotal eyewitness testimony, right?

I think a reasonable first test is taking a baseball bat and flailing around wildly. If your flailing is sufficiently random (and strong enough to break obstructions) and you do it long enough you should hit the cup if you do it long enough. Now It might be surrounded by a field which enables the cup to gently float away when it is hit, but it's only the first step after all. There is also the issue that the cup might have been there for a few minutes when ST posted and was subsequently removed.

Anyway this discussion is essentially invoking never before seen magic effects. But that isn't all that different from some of the discussion which lead to someone making the "you can't prove a negative" statement. "We have no reason to believe X exists nor any mechanism by which X could exist" "But can you prove that it does not exist?" "You can't prove a negative."

Re: You can't prove a negative : why, logically?

Posted: Thu Jan 30, 2014 9:39 pm UTC
by PeteP
setzer777 wrote:
SexyTalon wrote:I have plenty of evidence telling me no cup of lava exists in my office. I cannot see it, i cannot feel it, and I cannot measure any effects that it should be giving off. It doesn't exist - proved as well as anything can be proved.


I dunno, with no controlled oversight it's technically only anecdotal eyewitness testimony, right?

I think a reasonable first test is taking a baseball bat and flailing around wildly. If your flailing is sufficiently random (and strong enough to break obstructions) and you do it long enough you should hit the cup if you do it long enough. Now It might be surrounded by a field which enables the cup to gently float away when it is hit, but it's only the first step after all. There is also the issue that the cup might have been there for a few minutes when ST posted and was subsequently removed.

Anyway this discussion is essentially invoking never before seen magic effects. But that isn't all that different from some of the discussion which lead to someone making the "you can't prove a negative" statement. "We have no reason to believe X exists nor any mechanism by which X could exist" "But can you prove that it does not exist?" "You can't prove a negative."
But as phlip said "You can't empirically prove a negative existential" would be more precise.

Re: You can't prove a negative : why, logically?

Posted: Fri Jan 31, 2014 10:23 pm UTC
by Mambo4
phlip wrote:The missing words are basically: "You can't empirically prove a negative existential". That is, you typically can't use Science to prove "X doesn't exist". Any empirical evidence you can collect for such a statement is also consistent with "X exists, but has yet to be observed"... there's no evidence you can get that specifically discounts that possibility. "X can't exist" you can get to, in cases where X being even theoretically possible to exist is inconsistent with evidence, but a mere "X doesn't exist" is much harder to reach.


thank you , that's a reasonably concise, thought-experiment-free explanation.

Although, following the Rational Wiki link, "You can't prove a negative" is called out as psuedo logic.

A common saying in pseudologic is "You can't prove a negative." That saying is not true. An absence of something can be proved in various ways, e.g., by a reductio ad absurdum or by proving something else that is inconsistent with the presence of that something. For example, in law, a party may have the burden of proving nonreceipt of certain correspondence and may bear that burden of proof (at least by a preponderance of the evidence) by introducing into evidence a docket record in which the correspondence would have been noted. In mathematics, there are plenty of proofs of negative propositions, such as "there is no largest prime number"[1] or "there is no rational square root of 2".[2] One might also note that the saying itself is a negative.


So, Phlip's qualification ("You can't empirically prove a negative existential") make sense