What does the phrase “you can’t prove a negative” actually mean?
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Ok, how would you prove that there isn’t a teapot in orbit around the moon.
Show your work.
Imagine you order something online and it doesn’t arrive. So you send a complaint to the site and they ask you for a photo as proof
What would the proof even be? Nothing, because you can’t really prove a negative. The only way to prove a negative is to prove a positive that directly contradicts it
You can prove that ‘this mug isn’t green’ by proving the positive that it is in fact red
If I asked you to prove you had blown your nose, you could show me a used tissue, video of you blowing your nose, or someone who saw you blow your nose could tell me that you did.
If I asked you to prove you didn’t blow your nose, showing me an empty trash can wouldn’t prove there weren’t used tissues there before, you would need video of every second of your life to show you had never blown it, or someone would need to have seen your nose at all times to prove you hadn’t.
Imagine I say you owe me ten grand, you deny it. I take you to court.
The court will demand that I prove you owe me the money, because it would be ridiculous to demand you to prove that you don’t owe me.
In order to prove “There are no purple unicorns in space” you would have to search all of space simultaneously, which is impossible.
If I want to prove something, for example that Keanu Reeves has ever uttered the word “rice” in Turkish, that’s relatively simple. All I need to do to prove it is provide video evidence.
If I want to prove the negative of the above statement it’s basically impossible. I would need to have constant 24/7 video and audio coverage of his mouth from the moment he was born and hire tens of thousands of people to carefully watch and listen to the footage to ensure he never whispered the word. Such footage doesn’t exist.
So it’s basically impossible to prove beyond a shadow of a doubt that something *didn’t* happen. It’s possible to make an educated guess about the likelihood of it – has Reeves ever been to Türkiye? Is there evidence of him attempting to order at restaurants in the native language when he travels? Has he ever had a Turkish girlfriend? etc. – but “probably true” or “probably false” isn’t **proven**.
You did it, didn’t you?
When you order something online that doesn’t arrive & you get in touch about a refund/replacement & they ask you for proof that you didn’t receive your package.
How are you supposed to prove that?
Okay, hold on, let me just take a picture of _nothing_ & I’ll send it right over 🤷♂️
Proving a positive requires nothing more than an example.
> Blue cars exist.
> *Oh really? Prove it.*
> Look to your left.
> *Oh.*
On the other hand, proving a negative requires you to demonstrate the impossibility of the corresponding positive. Proving negatives is generally only possible in conceptual contexts such as mathematics, strictly controlled systems or limited size data sets.
The most famous example of the unprovable negative is “God doesn’t exist”. Anyone can just define the nature of God around your argument.
A provable negative would be something like “It didn’t rain in Washington this week” because you are limiting your dataset to a manageable size.
Let’s say we’re sitting in a restaurant and you get up to go to the bathroom. You’re gone for one minute, and then you return.
When you sit down, I ask you to prove to me that you *didn’t* drop down and do pushups in there before coming back out.
That would be proving a negative. There is no evidence you can provide to me that *proves* you did **not** do pushups in the bathroom.
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