Can't Prove a Negitive?

Great Debates
1

is there any truth at all to the statement “you can’t prove a negitive”?

odviously its untrue as written.

‘my name is not bob.’ can be proven by showing my name is andrew.

any proof of a possitive contains the disproof of the negitive.

likely people mean “you can’t prove a negitive over an unrestricted domain”. (as in: you can’t prove something doesn’t exist SOMEWHERE in the universe). so in that case they are useing some sort of shorthand for “you can’t prove a universal negitive”. but there is nothing special about negitives that make them harder to prove than possitives.

all cats have spines. is just as hard to prove as there are no spineless cats.

I don’t get it, why is “you can’t prove a negitive” taken as such strong truth. (interestingly, if true… you could NEVER EVER EVER prove it, ya know?)

its certainly possible to prove negitives, even of the totally unrestricted domain sort.

you can prove that there are no square circles, no lists of all lists, no unmoveable object AND unstopable force, no towns where every man who does not have the barber cut his hair cuts his own. ect.

am I missing something?

2

Of course you can, Bob.

3

The negative referred to is the negative case of existence of a particular thing.

There is no God.

Communism cannot work in an advanced industrial society.

A positive proposition can be demonstrated, by finding a single case where it is true.

Dogs can have teeth. Find a dog. Look in his mouth, if he has no teeth, find another dog. When you find one dog with teeth, you show the bite marks on your hand to the doubting debater. Q. E. D.

But finding seven million failed communist societies doesn’t prove that one cannot exist. It might discourage a lot of would be communists, but it isn’t proof.

In mathematics, negative propositions can be proven, but it usually much more involved than simply finding an example that demonstrates the possibility averred by a positive proposition.

If you think neutrinos might change from tau neutrinos to electron neutrinos, you can start looking for examples of radiation one could expect from the process whereby a neutrino changed from one state to another. If you find one, you have proven that such a thing is possible. If you look for a decade or so, and fail to find one, you have only proven that it is hard to find them.

In law, you can prove that someone was in a room, by finding physical evidence that only could have been left in the room by that person, like blood, or fingerprints. But if you don’t find the blood or fingerprints, it doesn’t prove that the person was not ever in that room. Even finding out where that person was at some particular time won’t prove that he wasn’t ever in that room.

If you claim that the District Attorney’s office planted your blood at the scene of a murder, they cannot prove that they did not, unless they can display evidence of every single bit of blood that you ever lost, at any time, which they might have been able to obtain. But who would believe a silly claim like that?

Tris

4

but thats not a special property of negitives. just of absolutes you have equal amounts of difficultys with saying ALL blah blah blah as you do saying NO blah blah blah.

5

Yes, proving an absolute is essentially similar to proving a negative. In fact, you can state any absolute as a negative of its denial. All dogs are green, there are no dogs which are not green.

Tris

6

It depends. You cannot prove a negative with induction, but you can use with deduction.

Induction: “There are no white crows.”
Deduction: “There are no even prime numbers other than 2.”

7

This post does not contain the letter Z

Damn!.. I thought I had it there.

8

A more reasonable formulation would be

“Can’t prove the non-existance of something”

9

I can prove the non-existence of the purple pixies that are endemic to my greenhouse.

10

Hypothesis: A negative cannot be proved.

Proof: D’oh! :smack:

11

Merlely becauise you are denominated “Andrew” does not mean you are not also Bob.

12

Name. Name. NAME.

13

the idea that we “can’t prove a negative” is as ill-formed as the the set of consequents the idea is supposed to prove.

people say “you can’t prove god doesn’t exist.” i say that’s because the concept of god’s existence is not properly defined. the problem of evil proves god doesn’t exist, for that particular god, and if the premises it is based on are accepted.

if you base your idea that “a negative can’t be proved” on the fact that we have to accept certain premises beforehand, you would agree that we can’t prove a damn thing. the principle of noncontradiction and the concept of truth are examples of things that are accepted beforehand in any proof.

what i think someone usually means when he says this is that the fact that you haven’t found something yet doesn’t mean it doesn’t exist. and i think they should be clear on that. because you can prove a negative.

14

You can prove that something doesn’t exist by showing that its supposed properties are logically inconsistent.

15

Yes, on existence claims. It’s a good question, and often confusing, because it needs elaboration. Scientists often say this. It’s important to note what they are referring to. You’ll also often hear James Randi saying he can’t prove negatives when it comes to testing the paranormal. He can’t prove that it doesn’t exist, but what he can show you, is how unlikely it is. Physicists will tell you the same thing, including the unlikelihood of perpetual motion machines. One poster just showed how in mathematics they can easily prove negatives. There are numerous scenarios in which negatives can be proved if you would like some more examples. One needs empirical data to go on. Two criteria has to be met in order for them to do this. A descriptive concept is required, something quantifiable. Secondly, one has to limit the domain of the search. With God, where would the search start? Theologians can’t even give us a descriptive concept of Him, let alone locate Him. With others, one might be able to come up with a good descriptive concept, but then it fails in limiting the domain of the search. Examples such as Bigfoot, the Loch Ness monster might meet the descriptive concept, but how would you go about limiting the domain of the search? It would be extremely problematic. Some are more easily tested. Paranormal claims and perpetual motions can be tested on a regular basis. The fact they haven’t found any evidence for it, still doesn’t mean that have proved that it doesn’t exist. But again, they have plenty of evidence to show the unlikeihood of it. The scientific community has good reasons of why they have adopted much of Hume’s approach about “extraordinary claims require extraordinary proof” and the “burden of proof lies with the one making the claim.”

JZ

16

Exactly. And that applies to all claims, existential or otherwise.

17

I don’t know about the Pixies, but I think I saw Bjork hiding behind your hydrangias.

18

Like, nonexistential? :confused:

19

Right. Claims like “2 is not an even number”.

20

You can’t prove I can’t prove a negitive.