# The "you cannot prove a negative" argument

#### stateofmind

##### seeker of lies
Valued Senior Member
I never really understood the line of reasoning behind this argument. Can someone who believes in it and uses it please explain it to me and some examples of how it's used?

I just realized that more people know it as the "you can't prove a negative" argument. Can whoever mods this forum change the quotes of the title to "you can't prove a negative"? Thanks.

I never really understood the line of reasoning behind this argument. Can someone who believes in it and uses it please explain it to me and some examples of how it's used?

It's not just something that people "believe in", it's an accepted fact.

Want an example, eh? OK, prove I'm not living in an alternate universe from you and have tapped into your Internet.

Okay, thanks for the example. If you could would you explain the reasoning that makes it an accepted fact?

take the tassie tiger as an example. its belived it went extint ages ago but there have been repeated unconfermed sightings of it in the tassie wilderness where the area is quite hard to access. Now either we find a living tassie tiger and its confermed to be still alive OR we dont. However the fact that we HAVENT found one isnt proof that it doesnt exist. Then there is the black swan effect, ie all swans in england are white and there for it was belived that it was impossable to have a black swan, however this doesnt seem to perterb all the black swans living in Australia (enough that the black swan is the symbol for SA)

I never really understood the line of reasoning behind this argument. Can someone who believes in it and uses it please explain it to me and some examples of how it's used?

This is a false argument.

You assume the positive and use reductio ad absurdum. This is how you can prove a negative.

I will prove it is false there exists a greatest integer. In other words, I will prove a negative.

For example, assume there exists a greatest integer, say G.

Yet, for any integer, x + 1 is an integer by the inductive hypothesis.
In addition, by the properties of integers, x + 1 > x.

So, take G + 1.

This is greater than the G, a contradiction.

This is a false argument.

You assume the positive and use reductio ad absurdum. This is how you can prove a negative.

I will prove it is false there exists a greatest integer. In other words, I will prove a negative.

For example, assume there exists a greatest integer, say G.

Yet, for any integer, x + 1 is an integer by the inductive hypothesis.
In addition, by the properties of integers, x + 1 > x.

So, take G + 1.

This is greater than the G, a contradiction.

Why is the theoretical "greatest number" considered a negative?

Why is the theoretical "greatest number" considered a negative?

I did not say it was.

I proved a negative. That is the point of the thread.

I did not say it was.

I proved a negative. That is the point of the thread.

What is the "negative" in the "you can't prove a negative" argument?

he is saying you can prove a neg NUMBER. Oviously this isnt what the thread is talking about but you cant help people here caught in there own "cleverness"

The "negative" in this argument seems to be anything that is outside the realm of commonly accepted science. Do you think this is a correct interpretation?

The "negative" in this argument seems to be anything that is outside the realm of commonly accepted science. Do you think this is a correct interpretation?

First off, pay NO attention to Jack. He's recognized here as using false logic and making blunders ALL over the place. Here's the way his logic works in the sense he used it:

His Statement: A baby elephant weighs more than it's mother.
His Challenge: Prove that is NOT true.

(Which of course, anyone with any sense can do.)

Now then... You're interpretation is close but not all inclusive. There are (and will always be) many, many scientific unknowns, guesses and theories.

And the way a theory is tested is referred to as the "falsification test." In other words, run experiments to see if the theory hold up under as many sets of conditions as you (or anyone) can create.

And to make that clear, use the elephant example above. Simply by weighing the mother and baby, you can show that that the theory (statement) is incorrect and must be discarded because it failed the test.

Also, many things CAN be proven correct under specific conditions and become accepted as "fitting in the realm of accepted science" - but along comes someone like Einstein who shows that Newton wasn't correct under ALL conditions.

First off, pay NO attention to Jack. He's recognized here as using false logic and making blunders ALL over the place. Here's the way his logic works in the sense he used it:

His Statement: A baby elephant weighs more than it's mother.
His Challenge: Prove that is NOT true.

(Which of course, anyone with any sense can do.)

Now then... You're interpretation is close but not all inclusive. There are (and will always be) many, many scientific unknowns, guesses and theories.

And the way a theory is tested is referred to as the "falsification test." In other words, run experiments to see if the theory hold up under as many sets of conditions as you (or anyone) can create.

And to make that clear, use the elephant example above. Simply by weighing the mother and baby, you can show that that the theory (statement) is incorrect and must be discarded because it failed the test.

Also, many things CAN be proven correct under specific conditions and become accepted as "fitting in the realm of accepted science" - but along comes someone like Einstein who shows that Newton wasn't correct under ALL conditions.

true, theories can be proven wrong but not right but things can be proven true but not impossable (black swan, tassie tiger ect)

I never really understood the line of reasoning behind this argument. Can someone who believes in it and uses it please explain it to me and some examples of how it's used?
This is not a logical fallacy and it really isn't true. It's also sort of an abbreviation for the statement that "You can't prove an unqualified, universal negative."

This would be a statement such as, "There are no purple roosters."

The problem the statement addresses is that since our knowledge is limited it is impossible to make such a statement definitively. There could be a purple rooster somewhere that we don't know about.

But you can prove a qualified universal negative such as, "There are no purple roosters in this room right now."

And you can prove an unqualified, particular negative, "Some roosters are not purple."

But there's still a problem in that you can prove some unqualified, universal negatives. Such as, "There are no square circles."

~Raithere

How can you test what isn't there?

StateofMind said:

I never really understood the line of reasoning behind this argument. Can someone who believes in it and uses it please explain it to me and some examples of how it's used?

Consider the argument over evolution and creationism. In order for Intelligent Design to be considered a science, it must seek a testable hypothesis to demonstrate the existence of the designer. That is, it must prove the existence of God, or Magrathea, or something like that.

How, though, can a scientist prove that what cannot be detected, perceived, sensed—ultimately, tested—doesn't exist? There is nothing to work with in attempting to prove the negative condition.

What it boils down to is the notion of affirmative evidence. Science doesn't regard God because it has absolutely nothing to work with. If, one day, an entity emerged claiming to be God, we could, theoretically, test that claim according to the criteria that describe God. But until then, there is nothing to test.

A lack of evidence may not be evidence of lack. But a lack of evidence is most certainly not evidence that something exists.

You can prove a specific negative a more general one you can't.

The "negative" part of this argument seems to be arbitrary. It would be just as hard to prove the positive of the examples given.

Prove that purple roosters do not exist.

Prove that purple roosters do exist.

Both of these statements would require the same investigation to prove or disprove them. Purple roosters may or may not exist somewhere.

Here's where I probably differ from a lot of people here. I contend that I don't know if purple roosters (naturally born that way) exist or not. How could I possibly know? I've only explored less than .0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001% of the universe. This seems to be the only objective stance on the matter unless I had seen a purple rooster by direct experience - then I would contend that purple roosters do exist.

The fallacy that I've experienced many times in discussions with people is that because there's no documented evidence for something then it follows that it probably does not exist.

The probability of a probability's correctness is proportionate to the extent of the understanding of the system the probability attempts to predict - try to wrap your head around that one

The "negative" part of this argument seems to be arbitrary. It would be just as hard to prove the positive of the examples given.
Prove that purple roosters do not exist.
Prove that purple roosters do exist.
Both of these statements would require the same investigation to prove or disprove them. Purple roosters may or may not exist somewhere.
Well, no. We only have to find one purple rooster to prove the affirmative while even if you looked everywhere on the planet there would still be a possibility, however improbable, that there is a purple rooster on another planet so you still wouldn't have proved the universal negative.

The fallacy that I've experienced many times in discussions with people is that because there's no documented evidence for something then it follows that it probably does not exist.
Typically the burden of proof is upon the side making the claim. So if someone is making a claim and cannot provide evidence of that claim it is reasonable to conclude the claim is false. The alternative is to assume that every claim is true, which is obviously problematic.

~Raithere

Typically the burden of proof is upon the side making the claim. So if someone is making a claim and cannot provide evidence of that claim it is reasonable to conclude the claim is false. The alternative is to assume that every claim is true, which is obviously problematic.
There's always the alternative of not assuming either way. This can be done in a variety of ways. Anything from saying

I consider it highly unlikely but I can't be 100% sure.

to

I don't know.

to

I'll wait and see.

to

I don't think it can be decided, now at least.

Etc.

Given that things that did not have evidence later did have evidence and are now considered true, I think it would be very odd to decide that everything is false unless it is proven true - or the weight of evidence indicates it is true.

There's always the alternative of not assuming either way. This can be done in a variety of ways. Anything from saying

I consider it highly unlikely but I can't be 100% sure.

to

I don't know.

to

I'll wait and see.

to

I don't think it can be decided, now at least.

Etc.

Given that things that did not have evidence later did have evidence and are now considered true, I think it would be very odd to decide that everything is false unless it is proven true - or the weight of evidence indicates it is true.

lol well said.