## Debunking folk logic - "you can't prove a negative"

catweazel
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### Debunking folk logic - "you can't prove a negative"

If only I had a dollar for every time someone has said, "you can't prove a negative". The sad part is that the irony of the assertion is completely lost on its utterer. If it is true that you can't prove a negative then the assertion that you can't prove a negative is unprovable.

Well, it's very easy to prove a negative. One of the laws of logic is the law of non-contradiction. If something is false then it cannot also be true. This particular law can be derived using inference rules from an empty set. Yet another law of logic is the double negation law where not not p = p.

So, why do people insist on asserting "you can't prove a negative"? Perhaps it's because they need an excuse to continue to believe whatever it is they believe in the face of overwhelming evidence. There are people who will continue to believe X even when they know that X is fully false. People are strange things, and I do not have three legs.
"There is, ultimately, only one truth -- cogito, ergo sum -- everything else is an assumption." - Me, my swansong.

gm10
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### Re: Debunking folk logic - "you can't prove a negative"

I've only ever heard that statement in the context of religious beliefs, never in the context of logic.

In view of your previous thread I'm wondering if you want to provoke a discussion of quantum superposition though.
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DAMIEN1307
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### Re: Debunking folk logic - "you can't prove a negative"

The only thing i could say to this in my case was,

"For me, my marriage was a negative for 10 years...This is true...lol...all joking aside, my ex-wife and i still get along, long distance via phone...lol...and we both agree that we can prove it was a negative...lol...DAMIEN
ORDO AB CHAO

rene
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### Re: Debunking folk logic - "you can't prove a negative"

gm10 wrote:
Fri Nov 29, 2019 7:33 am
I've only ever heard that statement in the context of religious beliefs, never in the context of logic.
Oh, it happens. Or, well, something very closely related certainly does. I.e., referring to the mentioned law of non-contradiction, that law is in classical logic equivalent to the so-called law of the excluded middle, either P or !P, and that one is in fact denied (as an axiom, I should say) in intuitionistic logic, the basis of a mathematical school of thought known as constructivism. A primary characteristic of which, then, is that proof of existence by contradiction / reductio ad absurdum is deemed invalid; that said proof can only be obtained by having constructed that which supposedly exists.

Note; I definitely do not subscribe to constructivism --- but it is absolutely interesting how far you can in fact get on that basis. An interesting intellectual exercise if nothing else...

BigEasy
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### Re: Debunking folk logic - "you can't prove a negative"

( (2x2=5) is false ) is true
Windows assumes I'm stupid but Linux demands proof of it

Portreve
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### Re: Debunking folk logic - "you can't prove a negative"

Let's consider a specific situation.

I make the claim to catweazel that I don't speak Spanish. You can't really meaningfully prove that, because I can always fake ignorance.

On the other hand, I make the claim that I do speak Spanish. You can test that my making me demonstrate that I can function in Spanish.

I think the "can't prove a negative" thing is tightly tied to falsifiability. In that, it's no different than standard rigor associated with a scientific hypothesis.

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Know when what you're doing is trying to fit a square peg in a round hole, and STOP.

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lsemmens
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### Re: Debunking folk logic - "you can't prove a negative"

Of course you can prove a negative - just try and overdraw your bank account. See how long it takes them to increase that negative balance.
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GS3
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### Re: Debunking folk logic - "you can't prove a negative"

I understand "you cannot prove a negative" perfectly.

An affirmation can easily be proven with a single example. "Bipedal mammals exist." Proof: Pamela Anderson, Dolly Parton, etc. (Although I am afraid I am giving away my age.)

A negative assertion cannot be proven. "Alien civilizations do not exist." The only way to prove this would be to inspect the entire universe and confirm their non-existence.
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lsemmens
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### Re: Debunking folk logic - "you can't prove a negative"

I can easily prove a negative assertion. money does not exist in my wallet. I look in my wallet, and the moths fly out!
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murray
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### Re: Debunking folk logic - "you can't prove a negative"

lsemmens wrote:
Mon Dec 02, 2019 9:13 pm
I can easily prove a negative assertion. money does not exist in my wallet. I look in my wallet, and the moths fly out!
Ha ha! I have the same problem
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catweazel
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### Re: Debunking folk logic - "you can't prove a negative"

Portreve wrote:
Fri Nov 29, 2019 11:21 am
I think the "can't prove a negative" thing is tightly tied to falsifiability.
This statement is false.

"There is, ultimately, only one truth -- cogito, ergo sum -- everything else is an assumption." - Me, my swansong.

lsemmens
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### Re: Debunking folk logic - "you can't prove a negative"

catweazel wrote:
Mon Dec 02, 2019 11:14 pm
Portreve wrote:
Fri Nov 29, 2019 11:21 am
I think the "can't prove a negative" thing is tightly tied to falsifiability.
This statement is false.

Now, you're just being negative. or not!
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catweazel
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### Re: Debunking folk logic - "you can't prove a negative"

lsemmens wrote:
Mon Dec 02, 2019 11:50 pm
catweazel wrote:
Mon Dec 02, 2019 11:14 pm
Portreve wrote:
Fri Nov 29, 2019 11:21 am
I think the "can't prove a negative" thing is tightly tied to falsifiability.
This statement is false.

Now, you're just being negative. or not!
What do you suppose my reply means?

I think you misunderstood its meaning.
"There is, ultimately, only one truth -- cogito, ergo sum -- everything else is an assumption." - Me, my swansong.