Proving a negative

[Nyarlathotep]

If this question seems silly please bear with me as I have had only a smattering of philosophy and formal logic in college. Further, I am really, really bored at work so my mind is wandering.

I have often heard the phrase that you cannot prove a negative. I have been thinking about that statment and I have come up with a hypotheical situation where you seemingly can.

Let's say that I wanted to prove that some animal is NOT a duck. As long as a certain set of characteristics that define a duck, I should be able to prove that the animal is not a duck by showing that it lacks one or more of the agreed on characteristics. Let's say we define a duck as follows:

1) the animal quacks

2) the animal is a bird

3) the animal has webbed feet

I know that non-ducks could have all three properties but for simplicity sake, lets' assume those three things constitute a duck.

Now if the animal I want to prove is not a duck is, for example, a wildebeast, it will have none of those characteristics. Would I have then, not proved that it not a duck? I wouldn't have proven what it is, only what it is not, so would that not be proving a negative?

Are there times when you CAN prove a negative or am I missing something?

[c4ts]

Even if you can prove that the positive is impossible, you still haven't proven a negative.

[gnome]

I would say there are times when you can prove a negative.

You can prove something is not a duck because we have very complete knowledge of what a duck is.

The phrase is most often used in situations of vastly incomplete information... for example, whether something exists. Can't prove something doesn't exist easily, unless its very existence creates a logical contradiction.

Also can't easily prove that something didn't happen. Something that fails to happen, by definition leaves no evidence behind to prove it.

So while it's stated a little more absolutely than it truly should, it's often true.

And it remains the case, in my opinion, that the burden of truth for a claim is the one that makes the positive claim.

[Dancing David]

The you can't prove a negative school is mainly a weak excuse for "I have no proof at all".

God does not exist. There is no evidence that god exists. Until the arrival of said evidence, there is no need to prove the negative, there is no positive to refute.

[Nyarlathotep]

Originally posted by gnome

The phrase is most often used in situations of vastly incomplete information... for example, whether something exists. Can't prove something doesn't exist easily, unless its very existence creates a logical contradiction.

Also can't easily prove that something didn't happen. Something that fails to happen, by definition leaves no evidence behind to prove it.



I see your point. The logic I thought of can only prove whether a particualr thing can be put into some well defined category, and that's pretty much all.

[Nyarlathotep]

Originally posted by c4ts
Even if you can prove that the positive is impossible, you still haven't proven a negative.

I don't follow that logic. If, for example, I had another wet of categories to prove that my wildebeast is in fact a wildebeast, wouldn't that prove it to be not a duck since it can't, by definition, be both a duck and a wildebeast at the same time?

[Paul C. Anagnostopoulos]

I can also prove that there are no ducks in my office.

I think we mean that you can't prove a negative in the general case. You cannot prove that no animals are ducks. You cannot prove there are no ducks in anyone's office.

~~ Paul

[ImpyTimpy]

You can't prove something is not there because a counter claim can always be made that when you weren't looking it appeared. For example, you could say there are no ducks in my office because I searched everywhere for one and could not find one. You're partly correct - you couldn't find one, it does not mean one does not exist in your office - it might be hiding from you.

That's why believers love this form of argument - prove it doesn't exist - you can't prove the negative. You can prove positive or show lack of evidence for positive but you can't prove negative.

:p

[Solitaire]

Originally posted by ImpyTimpy
That's why believers love this form of argument - prove it doesn't exist - you can't prove the negative. You can prove positive or show lack of evidence for positive but you can't prove negative.

Okay.
The square root of two is not a ratio of two numbers. (http://www.cut-the-knot.com/proofs/sq_root.shtml) :) P.S. Found a clearer page on this. (http://www.lancs.ac.uk/~mccaffea/maths/proofs/root2.htm)

[Dancing David]

I propose the p-duck, a duck in all ways except that you can't prove that it isn't there.
No matter how Paul may chose to prove that there are no p-ducks in his office, I will grant them powers to avoid detection.

I ahev some beany baby animals in my office, and a Tinky Winky but alas no ducks, I can not prove there are no p-ducks in my office.

[ImpyTimpy]

Blah, static system (mathematics) is not the living world. It is merely a representation of some it's aspects. We're not discussing static systems here but dynamic, living systems :p

Originally posted by Synchronicity

Okay.
The square root of two is not a ratio of two numbers. (http://www.cut-the-knot.com/proofs/sq_root.shtml) :)

[LW]

Originally posted by Dancing David


God does not exist. There is no evidence that god exists.

Except that there is evidence: a rather large number of people claim to have experienced god in a form or another. It is extremely weak non-compelling evidence but it is evidence.

[Gregory]

Hasn't it been proven that insects can't grow past a certain size? That would be a negative.

[chulbert]

When someone says you cannot prove a negative what they generally mean is that you can't prove a universal negative.

You can't prove something doesn't exist anywhere without omniscience.

[Yahweh]

Although you cannot logically and philosophically you cannot prove the phrase "A wildebeest is not a duck" you can the wording of the phrase and achieve your desired effect with "A wildebeest is a duck". That creates a logical contradiction and therefore "A wildebeest is a duck" is a false adage.

[Agammamon]

You can prove a negative if there is a logical inconsistency involved. For example "there are no square circles" can be proved, since the very definition of a circle prohibits it from being a square.

More topically, giving God the property of omnipotence means that he can (for example) create a rock that he can't lift. Well if he can't lift it he's not omnipotent. And if he can't create it . . .

The main problem with proving that something doesn't exist is that the things that its important to know about, one way or the other, have such fuzzy, changing definitions that its impossible to hold them down long enough to hammer the nails into the coffin.

[Yahweh]

"Tell me, if God is so powerful, can he microwave a burrito so hot that even he himself could not eat it?" - Homer Simpson

[Zep]

Originally posted by Nyarlathotep
I don't follow that logic. If, for example, I had another wet of categories to prove that my wildebeast is in fact a wildebeast, wouldn't that prove it to be not a duck since it can't, by definition, be both a duck and a wildebeast at the same time?
Maybe on THIS planet... But somewhere out there in the vast reaches of time and sapce (or even Planet X), there could quite easily be / have been some animal that meets both sets of your criteria, and is thus a wilde-duck. Hence, you have NOT proven your negative. :)

Zep

[Gregory]

Zep--That only works if whatever qualities you use to define "duck" and "wildebeast" happen to be compatable. For example, if you define ducks as having wings, and wildebeasts as being wingless, then clearly there cannot exist an animal that is both duck and wildebeast (this was Agammamon's point).

[Zep]

You are essentially saying that the two conditions are mutually exclusive. However that is being achieved by adjusting your criteria until that condition comes into being. "Proving a negative" is not a condition of mutual exclusivity, which perforce requires two criteria. Instead, the quest here is to do with singularities. You are usually asked to show that something - a single criterion - does NOT exist.

The classic proposition is that of the invisible pink unicorn - I make the claim that I own one, and you have to prove I don't. Regardless of the reality (or, indeed, the stupidity) of the situation, you can supply no logical proof that I DON'T own one! At home in my garage at the moment.

For example, you can say that there is no fossil history of it - I say that it is too rare to be found, or that scientists have been covering it up for centuries. You might ask to put your hand on it (if you can touch it then it exists, if not, it doesn't) - I say that us mere mortals cannot feel it even if they touch it. And so on ad nauseum. Whatever "test" you may have for proof of your negative, I can concoct a method, rational or otherwise, to make it fail.

Obviously, if the conditions were reversed - that I have to prove I DO have an invisible pink unicorn - then the onus falls on me to prove a positive. And that's what science is about.

Woo-woos like Victor Zammit use this "prove me wrong" tactic all the time. They make a grandiose claim - I see/hear dead people - and then say there is $1,000,000 in it if they are proved wrong. It's a no-lose situation for them because they don't need to provide proof of their claim, they just have to deny any disproofs as I did above.

Of course, their explanations do tend to stretch even the most jaded of imaginations after a while, but they are nothing if not irrationally dogged, these people!

Zep

[Agammamon]

If the definition of a unicorn includes its being visible, then invisible ones cannot exist. We aren't adjusting criteria here, we are just trying to pin down the attributes of the thing in question. If neither of us have a clear idea of what we're talking about then neither of us can prove our respective points.

[Paul C. Anagnostopoulos]

The definition of a unicorn does include its being visible. However, that is not the case for the hamster.

~~ Paul

[Zep]

Originally posted by Agammamon
If the definition of a unicorn includes its being visible, then invisible ones cannot exist.

We aren't adjusting criteria here, we are just trying to pin down the attributes of the thing in question. If neither of us have a clear idea of what we're talking about then neither of us can prove our respective points.
1. Incorrect. MY definition INCLUDES it being invisible, so it DOES exist but you just can't see it. These aren't mutually exclusive attributes. (I can play this game...)

2. Adjusting attributes is precisely the point, as far as woowoo believers go anyway. They constantly do this when confronted with solid proof that their current fad phenomenon is bunk.

Your last sentence sums them up neatly - if you can't DISprove their silly hypotheses for any reason then they MUST be true. And if you do, they will just change them to dodge away again.

Zep

[Chanileslie]

Originally posted by Nyarlathotep
If this question seems silly please bear with me as I have had only a smattering of philosophy and formal logic in college. Further, I am really, really bored at work so my mind is wandering.

I have often heard the phrase that you cannot prove a negative. I have been thinking about that statment and I have come up with a hypotheical situation where you seemingly can.

Let's say that I wanted to prove that some animal is NOT a duck. As long as a certain set of characteristics that define a duck, I should be able to prove that the animal is not a duck by showing that it lacks one or more of the agreed on characteristics. Let's say we define a duck as follows:

1) the animal quacks

2) the animal is a bird

3) the animal has webbed feet

I know that non-ducks could have all three properties but for simplicity sake, lets' assume those three things constitute a duck.

Now if the animal I want to prove is not a duck is, for example, a wildebeast, it will have none of those characteristics. Would I have then, not proved that it not a duck? I wouldn't have proven what it is, only what it is not, so would that not be proving a negative?

Are there times when you CAN prove a negative or am I missing something?

I think you are looking at this in the wrong way. You can "prove" (you know how I feel about that word which is bandied about much to freely by some!) that a certain animal is not another animal, but is that really the negative or are you actually eliminating the knowns.

You know that a wildebeast does not quack, is not a bird and has hooves not webbed feet. You have just used knowns to narrow down your category.

Now, you can't "prove" some unknown quantity (the negative) is not a duck because you don't have anything on which to base your comparison. Does the unknown quantity quack? I can't answer that because I haven't seen, heard or felt the unknown qunatity.

You can't prove a negative is the lazy man's way of saying, you can't prove something either exists or doesn't exist that you have had no contact or knowledge on because a duck can quack, is a bird and has webbed feet.

For example, if I walked up to you and said, prove that Jubusta Beast exists by comparing it with a duck? You have never seen, heard of nor had contact with a Jubusta Beast because I just made it up, but now it is up to you to prove that such an imaginary being exist or doesn't exist.

[Jethro]

It's really just a subset of the problem of being unable to prove anything outside of pure mathematics and logic. How do you know that the wildebeast of your example truly is a wildebeast? Are you sure you aren't hallucinating? Maybe it IS a wildebeast, but it is a heretofore unknown duck/wildebeast hybrid.

ANY observation can be reconciled with ANY hypothesis by using additional postulates. This, combined with the fact that absence of evidence is not evidence of absence, makes negatives especially difficult to dispense with.

[DialecticMaterialist]

Yes you can prove a negative, by showing a conflicting wolrdview to be more parsimonious or by direct falsification.

For example the claim "All doves are black", find one white dove and it's disproven. Very simple.

Or "The world was created in 6 days", prove evolution true and it's disproven. By proving evolution you have shown that the world "was not created in six days."

[ceo_esq]

It's certainly possible to prove a negative, although proving a negative historical proposition can be challenging. The thing to be wary is not proofs of negative propositions but negative proofs, as they can lead to what is sometimes called the Fallacy of the Negative Proof:It occurs whenever a historian declares that "there is no evidence that X is the case," and then proceeds to affirm or assume that not-X is the case. . . . [A] simple statement that "there is no evidence of X" means precisely what it says – no evidence. The only correct empirical procedure is to find affirmative evidence of not-X – which is often difficult, but never in my experience impossible.(Source: David Hackett Fischer, Historians' Fallacies: Toward a Logic of Historical Thought 47-48 (1970))

[ImpyTimpy]

But these examples are showing you disproving a positive, not proving a negative. Problem with proving a negative is that another possibility can always be offered to counter the proof.

Originally posted by DialecticMaterialist
Yes you can prove a negative, by showing a conflicting wolrdview to be more parsimonious or by direct falsification.

For example the claim "All doves are black", find one white dove and it's disproven. Very simple.

Or "The world was created in 6 days", prove evolution true and it's disproven. By proving evolution you have shown that the world "was not created in six days."

[TexasBEAST]

I never quite got the "you can't prove a negative" claim. It made my head hurt thinking about it. Instead, I approached it from the William Clifford "Ethics of Belief" direction. You have an obligation to positively prove a claim or assertion that you make that something is true. You must provide the testable evidence to support your claim, if you are going to make that claim. Now, if I am not making an assertion that something is true, I have no obligation to provide evidence or proof. My being skeptical or doubtful in the absence of any evidence to support your assertion does not obligate me to provide any evidence. I am right and entitled to be skeptical of your claim in the absence of proof of your claim. You are the one required to provide proof, since you were the one making an assertion or claim--not I.

Let's say that we both believe in physical, material reality. We look around us and believe in the natural universe that we are able to observe with our physical senses. OK, fine. Now, let's say you go beyond that and claim that there also exists a realm of supernatural reality, in which there is some entity that you call "God". You are making a positive assertion of the existence of something above and beyond and outside of that which we already have proof for and mutually believe in. Therefore, you have the burden of proving your assertion. I don't have any burden to prove the nonexistence of "God" because I am not making a positive assertion of such; I am only withholding belief in your positive claim until you are able to produce verifiable evidence supporting it.

My doubt is justified, as well. You should only believe a notion to the extent that you are able to support that notion with verifiable evidence. To the extent that you are not able to support that notion with verifiable evidence, my refusal to accept the notion as true (i.e. "believe") is completely justified.

The same idea is in effect when our courts assume that you are "not guilty" until proven otherwise. If the state is going to claim that you committed a crime, you don't have to prove that you didn't; they're supposed to prove that you did. The onus is on them, since they are the ones making the claim that something is true.

It's not necessarily that you can't prove a negative--it's that you don't have to, ethically.

[CSX2]

Well said TexasBEAST.

[Gregory]

But wouldn't the same logic that keeps you from proving a negative keep you from proving a positive, too? Let's say that I have a unicorn (a visible one) in my room, and I want to prove it. I take you to my room and show you my unicorn, but you claim that it's not a real unicorn; it's just a horse that's been surgically altered. I offer to bring in experts in the field (surgeons, taxidermists, whatever) to testify that this is not the case; you dismiss their testimony on the grounds that I'm obviously paying them off. And so on.

[TexasBEAST]

Originally posted by Gregory
But wouldn't the same logic that keeps you from proving a negative keep you from proving a positive, too? Let's say that I have a unicorn (a visible one) in my room, and I want to prove it. I take you to my room and show you my unicorn, but you claim that it's not a real unicorn; it's just a horse that's been surgically altered. I offer to bring in experts in the field (surgeons, taxidermists, whatever) to testify that this is not the case; you dismiss their testimony on the grounds that I'm obviously paying them off. And so on.
I don't follow. Certainly a person could be very skeptical with your claims of having a unicorn. They could even be so thorough about it that they made themselves a real pest.

But I don't see how that "keeps you from proving a positive, too". If you want people to believe your claim, you maintain the responsibility to endure all of that skepticism. Skepticism doesn't do away with the burden of proof.

[BillyJoe]

Originally posted by LW
Except that there is evidence: a rather large number of people claim to have experienced god in a form or another. It is extremely weak non-compelling evidence but it is evidence. Claims are not evidence.
You need evidence to back up your claim.

[Zep]

Originally posted by Gregory
But wouldn't the same logic that keeps you from proving a negative keep you from proving a positive, too? Let's say that I have a unicorn (a visible one) in my room, and I want to prove it. I take you to my room and show you my unicorn, but you claim that it's not a real unicorn; it's just a horse that's been surgically altered. I offer to bring in experts in the field (surgeons, taxidermists, whatever) to testify that this is not the case; you dismiss their testimony on the grounds that I'm obviously paying them off. And so on. This may be so, but the situation is that you have made a claim, and have provided what you believe is compelling and positive proof. Your proof is now up for consideration, and subject to testing (which you accede to, as above). This is good, but it is still only proving a positive.

The problem with proving a negative would be like if you claimed that you used to have a unicorn in your room but it isn't there now. How would we ever be able to test that a unicorn was not there? Try to find unicorn sh*t swept under the bed perhaps? Your room looks just the same if a unicorn was there and is gone, or if the unicorn was never there. Ergo it is not possible to prove that negative.

Zep

[DialecticMaterialist]

But these examples are showing you disproving a positive, not proving a negative. Problem with proving a negative is that another possibility can always be offered to counter the proof.

And what is the difference? Either way I am showing a claim to be false. How is proving creationism to be false not proving a negative? Sure I may have to prop up an alternative and conflicting model to do it, but it is still being disproven none the less, albeit indirectly. That's in fact what I have in mind when I speak of "proving a negative though." In all fairness what else could proving a negative mean?

When I speak of "proving a negative" I mean "disproving a claim". I can do this by proving its opposite. Or showing the claim to be at odds with certain standards of evidence.

Lastly I imagine yes anyone could show how the negative is still possibly true, even given all the evidence against it, but that misses the point. I am talking about probability here, not mere possibility. So resorting to mere possibility is insufficient at refuting my claims. (Notice, people can resort to mere possibility when confronting a positive too i.e. evidence for evolution is illusory or coincidental).

The only absolute negatives I would say are proven are those of logic, math and other basic axioms.

[MRC_Hans]

Disproving a positive is not the same as proving a negative.

Example: I claim to be able to prove the existence of God, because he speaks to me in a booming voice in my basement. You come to my basement, and indeed there is a booming voice there claiming to be God, but a closer investigation reveals speakers and a tape-recorder. You have now disproved my evidence of God, but you have not proved the non-existence of God.

The statement "you cannot prove a negative" is not an ultimative statement, it is a principle of debate. As has already been pointed out, the idea is: Whoever makes a claim has the burden of proof.

In this case the "not a duck" example from the first post is an example where a negative can be proven, by proving a positive that excludes the original claim. However, it is still a fallacy of debate, because whoever made the "it is a duck" claim should have been required to prove it (and would have failed).

Hans

[DialecticMaterialist]

Disproving a positive is not the same as proving a negative.

How are they different? Claim "No circle has four sides"...how is that an unporven negative?

What do you actually mean by proving a negative anyways?

Example: I claim to be able to prove the existence of God, because he speaks to me in a booming voice in my basement. You come to my basement, and indeed there is a booming voice there claiming to be God, but a closer investigation reveals speakers and a tape-recorder. You have now disproved my evidence of God, but you have not proved the non-existence of God.

No that is just not accepting a non sequitur. Of course one could not presume just because the man didn't hear God's voice that there is no God....that wouldn't be merely trying to prove a negative but disproving another positive claim.

The claims "God speaks to me" and "God exists" are two different postive claims.

The statement "you cannot prove a negative" is not an ultimative statement, it is a principle of debate. As has already been pointed out, the idea is: Whoever makes a claim has the burden of proof.

Not necessarily. Holocaust believers, and paleontologists who believe that dinosaurs existed, are not given the burden of proof, those who doubt them are. The burden of proof goes to the side not yet established, not the side merely making a claim.

In this case the "not a duck" example from the first post is an example where a negative can be proven, by proving a positive that excludes the original claim. However, it is still a fallacy of debate, because whoever made the "it is a duck" claim should have been required to prove it (and would have failed).

Unless it is already established (considered default by current evidence) in which case it would be doubters job to prove their claim.

It is on such a basis that holocaust, and evolution deniers: AIDS and Moon landing heretics; have the burden of proof thrown on them, not other people to prove their claim every time they make it.

But let me ask you, since you have stated that "disproving a claim" is not "proving a negative" or showing something to provisonally not exist is likewise (though that's what I imagine me and other mean by proving a negative) then what do you actually mean by proving a negative?

Your God example seemed to indicate a non sequitur. But isn't that a straw man?

[Patricio Elicer]

Originally posted by Gregory
Hasn't it been proven that insects can't grow past a certain size? That would be a negative. As gnome said above, I think that the claim "you can't prove a negative" only applies in the absense of information on the matter being discussed.

When there's information, you're not proving a negative, but a positive. In your insect's case, what you are indeed proving is the positive fact that insects grow up to a certain size.

In his opening example, Nyarlathotep is proving that ducks exist, that they have distinctive characteristics, and that everyone can see those facts. As a consequence of this positive proof, he can deduce that the other animal is not a duck.

BTW, welcome to the forums Gregory :)

[DialecticMaterialist]

As gnome said above, I think that the claim "you can't prove a negative" only applies in the absense of information on the matter being discussed.

I think that is a somewhat unreasonable definition here. It simply equates "proving a negative" to an argument from ignorance. To me and people who actually say you can prove a negative it means proving something does not exist, does not happen or a claim is false. It does not mean "assuming where we have no information." That's a straw man.

[MRC_Hans]

Originally posted by DialecticMaterialist

(I said: Disproving a positive is not the same as proving a negative)

How are they different? Claim "No circle has four sides"...how is that an unproven negative?

I tried to give an (possibly silly) example. Let me be more specific: The dismantling of one claim does not have general value. I'll try another (silly) example: I claim that unicorns exist, and my proof is that I have one in my garage. So you show up at my address, where you will find that I don't even have a garage, let alone a unicorn. You have now dismantled my proof, but you have not disproven, in general, the existence of unicorns, and you cannot be expected to do so; the burden of proof is on me.

--- On the four-sided circle: There is a difference here; a circle is a defined geometrical figure. Having four sides falls outside this definition. Thus any figure with four sides is not a circle. This is a positive proof.


What do you actually mean by proving a negative anyways?

Again, as above, it means that I can't make a claim and require you to either accept it or disprove it. It means you cannot be required to disprove an unsubstantiated claim.

No that is just not accepting a non sequitur. Of course one could not presume just because the man didn't hear God's voice that there is no God....that wouldn't be merely trying to prove a negative but disproving another positive claim.

The claims "God speaks to me" and "God exists" are two different postive claims.

Mmmm, well, but obviously connected. IF God REALLY speaks to me, it sorta follows that he exists. God's speach would be strong evidence of His existence. ---But it was a silly example.


Not necessarily. Holocaust believers, and paleontologists who believe that dinosaurs existed, are not given the burden of proof, those who doubt them are. The burden of proof goes to the side not yet established, not the side merely making a claim.

Excuse me, but this is blatant nonsense. Holocaust and dinosaurs are thoroughly backed by solid physical evidence. Once you have backed a claim or theory with solid evidence, whoever wants to reject the theory is required to dismantle that evidence.

Unless it is already established (considered default by current evidence) in which case it would be doubters job to prove their claim.

It is on such a basis that holocaust, and evolution deniers: AIDS and Moon landing heretics; have the burden of proof thrown on them, not other people to prove their claim every time they make it.

All the examples you give are established, or backed, by a solid body of evidence. Therefore, the burden of proof now rests on whoever wants to reject them.

But let me ask you, since you have stated that "disproving a claim" is not "proving a negative" or showing something to provisonally not exist is likewise (though that's what I imagine me and other mean by proving a negative) then what do you actually mean by proving a negative?

Lessee.. Let's take another claim... Nessie!

I claim that Lake Loch Ness in Scotland houses one or several large marine creatures. My claim is based on anecdotical evidence, which cannot be used as proof of anything, so my claim is an unsubstantiated claim.

You cannot disprove my claim, however. That is proving a negative. Even if you were to pump Loch Ness dry (a daunting task, btw), and thus prove that it does not hold a monster, I could just say: "Well, but it WAS there."

Now, however, I produce evidence for my claim; I give you a nice photo of a big animal with a small head on a long neck, swimming in the water. The surrounding landscape is evidently the banks of Loch Ness. My claim is now a substantiated claim.

You now have the possibility of dismantling my evidence. Assume you have an expert study the photo, and she soon discovers it to be faked. My evidence has been dismantled. You have not proven a negative (because Nessie might still exist), but I'm back to having an unsubstantiated claim, and the burden of evidence.

Your God example seemed to indicate a non sequitur. But isn't that a straw man?

No, my example was meant to demonstrate that dismantling a piece of evidence is not the same as proving a negative.

Hans

Edited for typos.

[Martin]

Originally posted by Patricio Elicer
When there's information, you're not proving a negative, but a positive. In your insect's case, what you are indeed proving is the positive fact that insects grow up to a certain sizeNo, showing that insects do not grow beyond a certain size would indeed be proving a negative. Of course, it's not possible. Just because we haven't seen any insects grow beyond a certain size doesn't mean the next one won't. Behold, the limitations of deduction ;)

[DialecticMaterialist]

I tried to give an (possibly silly) example. Let me be more specific: The dismantling of one claim does not have general value. I'll try another (silly) example: I claim that unicorns exist, and my proof is that I have one in my garage. So you show up at my address, where you will find that I don't even have a garage, let alone a unicorn. You have now dismantled my proof, but you have not disproven, in general, the existence of unicorns, and you cannot be expected to do so; the burden of proof is on me.

Ok and if we just stopped there you would be right. To be more precise I cannot absolutely prove there are no unicorns. Just like I can't absolutely prove there are horses. I really can't absolutely prove very much.

But lets get into probability. In a way I have shown that there are no unicorns because I have made the claim superfluous and less probable then the alternative worldview which does not include the existence of unicorns.

In other words the claim "Unicorns do not exist" has been proven provisonally

--- On the four-sided circle: There is a difference here; a circle is a defined geometrical figure. Having four sides falls outside this definition. Thus any figure with four sides is not a circle. This is a positive proof.

Yes but it is also proving a negative: Circles cannot have four sides. How I prove the negative is irrelevant.


Again, as above, it means that I can't make a claim and require you to either accept it or disprove it. It means you cannot be required to disprove an unsubstantiated claim.

That is burden of proof then and that is not what I, nor anybody who thinks you can prove a negative means.



Mmmm, well, but obviously connected. IF God REALLY speaks to me, it sorta follows that he exists. God's speach would be strong evidence of His existence. ---But it was a silly example.

Well if Bill Cosby speaks to me, that would prove he exists. But that claim alone has little bearing on the general issue of whether Cosby exists or not...



Excuse me, but this is blatant nonsense. Holocaust and dinosaurs are thoroughly backed by solid physical evidence. Once you have backed a claim or theory with solid evidence, whoever wants to reject the theory is required to dismantle that evidence.

Or come up with evidence of their own because that side is now established. I don't see how you can call my claims "nonsense" because you have just repeated what I said: the burden of proof is on the side not yet established.

(By established btw I mean supported by evidence).

Hence Holocaust deniers when they claim "the idea of the holocaust is a sham" have the burden of proof, even if they are not making a positive claim.


All the examples you give are established, or backed, by a solid body of evidence. Therefore, the burden of proof now rests on whoever wants to reject them.

Yes, exaclty my point.



Lessee.. Let's take another claim... Nessie!

I claim that Lake Loch Ness in Scotland houses one or several large marine creatures. My claim is based on anecdotical evidence, which cannot be used as proof of anything, so my claim is an unsubstantiated claim.

Ok sorry to nick pick here, but testimony can count as evidence depending on what kind of claim we are dealing with.

For example my friend says "I ate waffles for breakfast" I count his testimony as good enough for me to believe.

But if my friend says "I drank a galon of oil for breakfast" I may raise my eyebrow. It all really depends on how much the claim diverges from background knowledge or parsimony.

You cannot disprove my claim, however. That is proving a negative. Even if you were to pump Loch Ness dry (a daunting task, btw), and thus prove that it does not hold a monster, I could just say: "Well, but it WAS there."

Yes but at this point how probable is your claim? I mean given how superfluous your claim is, and how certain facts make it improbable, I have to say at this point your claim is likely to be false. And since almost all proof is a matter of mere probability, I have thus disproven your claim for all practical purposes.

Sure maybe the monster was there and mysteriously disapeared. Maybe evolution is wrong and the earth was created in six days. Yes maybe, but not very likely. And proof is about probability.

When I make a postive claim for example "African exists" there is also a *possibility* I may be wrong. Africa may be an illusion or its existence made up as part of a government conspiracy. It's possible, but we don't say "you can't prove a positive" because of that.

Now, however, I produce evidence for my claim; I give you a nice photo of a big animal with a small head on a long neck, swimming in the water. The surrounding landscape is evidently the banks of Loch Ness. My claim is now a substantiated claim.

You now have the possibility of dismantling my evidence. Assume you have an expert study the photo, and she soon discovers it to be faked. My evidence has been dismantled. You have not proven a negative (because Nessie might still exist), but I'm back to having an unsubstantiated claim, and the burden of evidence.

Yes but your argument is basically saying that since something is merely possible, the claim cannot be considered false or have evidence against it. And that is simply not how proof or evidence works, it works on the basis of probability. Hence matters of mere possibility become irrelevant.

No, my example was meant to demonstrate that dismantling a piece of evidence is not the same as proving a negative.

You're right there, dismantling a single piece of evidence by itself is not. But showing the claim to be improbable, by either putting it at odds directly with something we know to be true or showing the claim to be superfluous, is.

[Patricio Elicer]

Originally posted by Martinm
No, showing that insects do not grow beyond a certain size would indeed be proving a negative. Yes, but only because we have positive information about the size of insects. My point in this case is that a negative can be proven only because we have proven a positive fact before.

Of course, it's not possible. Just because we haven't seen any insects grow beyond a certain size doesn't mean the next one won't. Behold, the limitations of deductionAccording to that logic, we couldn't prove a positive either. All we could do is to show a present state. I see a perfect symmetry in the argumentation here.

[MRC_Hans]

Originally posted by DialecticMaterialist

Ok and if we just stopped there you would be right. To be more precise I cannot absolutely prove there are no unicorns. Just like I can't absolutely prove there are horses. I really can't absolutely prove very much.

Mmm, right you cannot "prove the nonexistence of unicorns". But more importantly, you cannot, in a debate, be required to prove the nonexistence of a postulate. The one who makes the claim must provide the evidence. Horses? What then are those things outside my window? I da ngive you very compelling evidence for the existence of a creature that fits the definition for "horse".

But lets get into probability. In a way I have shown that there are no unicorns because I have made the claim superfluous and less probable then the alternative worldview which does not include the existence of unicorns.

Well, probability is quite another thing (and the abuse of probability analysis could merit a thread all by itself, heheh). Based on available evidence, the probability of the existence of unicorns is quite slim.

In other words the claim "Unicorns do not exist" has been proven provisonally

Mmm, to be very strict, you should say that "unicorns are entirely hypothetical". Depending on the definition of unicorns, of course; if it is just a horse-like creature with a single horn, they might exist (or have existed), although we have no evidence. OTOH, if you include their alleged magical properties in the definition, then I would agree that they are outside our (solidly backed by evidence) world-view.

Yes but it is also proving a negative: Circles cannot have four sides. How I prove the negative is irrelevant.

OK, then we can say that if the claim falls outside a given definition, we are able to disprove it conclusively. I agree this is proving a negative, albeit a special case.


That is burden of proof then and that is not what I, nor anybody who thinks you can prove a negative means.

In the rules for debate, this is one of the meanings of it. Btw, speaking for "anybody" is another fallacy :D


Well if Bill Cosby speaks to me, that would prove he exists. But that claim alone has little bearing on the general issue of whether Cosby exists or not...

If you can PROVE that Bill Cosy speaks to you, it has to be rather compelling evidence that he exists.

Or come up with evidence of their own because that side is now established. I don't see how you can call my claims "nonsense" because you have just repeated what I said: the burden of proof is on the side not yet established.

(By established btw I mean supported by evidence).

Hence Holocaust deniers when they claim "the idea of the holocaust is a sham" have the burden of proof, even if they are not making a positive claim.

OK; then we agree. I must have misunderstood your wording.

Yes, exaclty my point.


Ok sorry to nick pick here, but testimony can count as evidence depending on what kind of claim we are dealing with.

For example my friend says "I ate waffles for breakfast" I count his testimony as good enough for me to believe.

But if my friend says "I drank a galon of oil for breakfast" I may raise my eyebrow. It all really depends on how much the claim diverges from background knowledge or parsimony.

This is about probability. You decide to put faith in a probable claim, and disbelieve an improbably one. This is quite sensible, butformally the two claims are not different. And one trick used by people who make outrageous claims is to make them sound more probable than they are (or trying to ake the opposing claim seem more improbable than it is, e.g. abiogenesis).

Yes but at this point how probable is your claim? I mean given how superfluous your claim is, and how certain facts make it improbable, I have to say at this point your claim is likely to be false. And since almost all proof is a matter of mere probability, I have thus disproven your claim for all practical purposes.

Sure maybe the monster was there and mysteriously disapeared. Maybe evolution is wrong and the earth was created in six days. Yes maybe, but not very likely. And proof is about probability.

When I make a postive claim for example "African exists" there is also a *possibility* I may be wrong. Africa may be an illusion or its existence made up as part of a government conspiracy. It's possible, but we don't say "you can't prove a positive" because of that.

Yes but your argument is basically saying that since something is merely possible, the claim cannot be considered false or have evidence against it. And that is simply not how proof or evidence works, it works on the basis of probability. Hence matters of mere possibility become irrelevant.

You're right there, dismantling a single piece of evidence by itself is not. But showing the claim to be improbable, by either putting it at odds directly with something we know to be true or showing the claim to be superfluous, is.

I agree that there is much difference between formal argument and arguing about actual phenomenon. Actually that's what makes it all fun ;)

Hans

[DialecticMaterialist]

Mmm, right you cannot "prove the nonexistence of unicorns". But more importantly, you cannot, in a debate, be required to prove the nonexistence of a postulate. The one who makes the claim must provide the evidence.

Like I said not always, paleontogists and Halocaust believers again vs. doubters/deniers. Also what does it matter if it is required? We are arguing whether or not you *can* prove a negative.


Horses? What then are those things outside my window?

Illusions. ;)


I da ngive you very compelling evidence for the existence of a creature that fits the definition for "horse".

Sure but how do you know its not an illusion, or hallucination, or robots that look like horses?

Or maybe you are just making it all up?

See just like when dealing with a "negative" claim, I can come up with a lot of ad hoc in the face of positive as well.


Mmm, to be very strict, you should say that "unicorns are entirely hypothetical".

Not even that: conjecture. But the chances of this conjecture are very slim, and hence very likely false.


Depending on the definition of unicorns, of course; if it is just a horse-like creature with a single horn, they might exist (or have existed), although we have no evidence.

True as might dragons, but again I speak of probability. Epistemic probability.



In the rules for debate, this is one of the meanings of it. Btw, speaking for "anybody" is another fallacy

How is it another fallacy? You just have to use basic reasoning to guess at what another is likely to do: agree or disagree. Of course I *may* be wrong, but I *may* also be able to breath underwater.



This is about probability. You decide to put faith in a probable claim,

How is going by evidence an act of faith? I always thought faith consisted in doing the opposite.


and disbelieve an improbably one. This is quite sensible, butformally the two claims are not different. And one trick used by people who make outrageous claims is to make them sound more probable than they are (or trying to ake the opposing claim seem more improbable than it is, e.g. abiogenesis).


Yes true, but that's why you have to show how their arguments are weak, invalid or their premises are untrue. Just because something can be abused does not necessarily mean it should not be used.



I agree that there is much difference between formal argument and arguing about actual phenomenon. Actually that's what makes it all fun

I'm not quite sure on the division between formal argument and arguing over actual phenomenon. :)