An oldie but goodie.
From Professor Steven Hales, Bloomsburg University:
A principle of folk logic is that one can’t prove a negative. Dr. Nelson L. Price, a Georgia minister, writes on his website that ‘one of the laws of logic is that you can’t prove a negative.’ Julian Noble, a physicist at the University of Virginia, agrees, writing in his ‘Electric Blanket of Doom’ talk that ‘we can’t prove a negative proposition.’ University of California at Berkeley Professor of Epidemiology Patricia Buffler asserts that ‘The reality is that we can never prove the negative, we can never prove the lack of effect, we can never prove that something is safe.’ A quick search on Google or Lexis-Nexis will give a mountain of similar examples.
But there is one big, fat problem with all this. Among professional logicians, guess how many think that you can’t prove a negative? That’s right: zero. Yes, Virginia, you can prove a negative, and it’s easy, too. For one thing, a real, actual law of logic is a negative, namely the law of non-contradiction. This law states that that a proposition cannot be both true and not true. Nothing is both true and false. Furthermore, you can prove this law. It can be formally derived from the empty set using provably valid rules of inference. (I’ll spare you the boring details). One of the laws of logic is a provable negative. Wait… this means we’ve just proven that it is not the case that one of the laws of logic is that you can’t prove a negative. So we’ve proven yet another negative! In fact, ‘you can’t prove a negative’ is a negative - so if you could prove it true, it wouldn’t be true! Uh-oh.
Not only that, but any claim can be expressed as a negative, thanks to the rule of double negation. This rule states that any proposition P is logically equivalent to not-not-P. So pick anything you think you can prove. Think you can prove your own existence? At least to your own satisfaction? Then, using the exact same reasoning, plus the little step of double negation, you can prove that you aren’t nonexistent. Congratulations, you’ve just proven a negative. The beautiful part is that you can do this trick with absolutely any proposition whatsoever. Prove P is true and you can prove that P is not false.
Some people seem to think that you can’t prove a specific sort of negative claim, namely that a thing does not exist. So it is impossible to prove that Santa Claus, unicorns, the Loch Ness Monster, God, pink elephants, WMD in Iraq, and Bigfoot don’t exist. Of course, this rather depends on what one has in mind by ‘prove.’ Can you construct a valid deductive argument with all true premises that yields the conclusion that there are no unicorns? Sure. Here’s one, using the valid inference procedure of modus tollens:
1. If unicorns had existed, then there is evidence in the fossil record.Someone might object that that was a bit too fast - after all, I didn’t prove that the two premises were true. I just asserted that they were true. Well, that’s right. However, it would be a grievous mistake to insist that someone prove all the premises of any argument they might give. Here’s why. The only way to prove, say, that there is no evidence of unicorns in the fossil record, is by giving an argument to that conclusion. Of course one would then have to prove the premises of that argument by giving further arguments, and then prove the premises of those further arguments, ad infinitum. Which premises we should take on credit and which need payment up front is a matter of long and involved debate among epistemologists. But one thing is certain: if proving things requires that an infinite number of premises get proved first, we’re not going to prove much of anything at all, positive or negative.
2. There is no evidence of unicorns in the fossil record.
3. Therefore, unicorns never existed.
Maybe people mean that no inductive argument will conclusively, indubitably prove a negative proposition beyond all shadow of a doubt....MORE (4 page PDF)
