# Can you prove a negative? **URL:** \ **Category:** Great Debates\ **Created:** [February 3, 2010, 12:28am UTC](https://boards.straightdope.com/t/can-you-prove-a-negative/527458 "2010-02-03T00:28:37Z")\ **Posts on this page:** 5\ **Page:** 3
**Author:** ![JThunder](https://avatars.discourse-cdn.com/v4/letter/j/e95f7d/32.png) [@JThunder](https://boards.straightdope.com/u/JThunder)\ **Post date:** [February 5, 2010, 3:26am UTC](https://boards.straightdope.com/t/can-you-prove-a-negative/527458/41 "2010-02-05T03:26:33Z")
> [@Revenant\_Threshold](#): > > Apologies, perhaps a poor word choice on my part. The absurd conclusion certainly has to be obviously wrong, or contradictory, which is what makes it wrong. But it has to be a conclusion that follows from some part of the premise even as it rejects another part of it. It has to be an accurate absurdity in that it must be a correctly _determined_ piece of nonsense. And that’s where the problem lies, because you can’t guarantee the correctness of that determinative process. You’re using confused terminology and asking us to accept the resultant logic. That’s simply fallacious. > [@](#): > > It does, because without knowing them, we can’t compare them to the negative to know that they aren’t contradictory or otherwise supplantitive. Again, untrue, as demonstrated by the technique of _reductio ad absurdum_. Look, you’ve been starting with erroneous terminology and heaping confused logic on top of that. No wonder you’re drawing the conclusions that you do. ---
**Author:** ![JThunder](https://avatars.discourse-cdn.com/v4/letter/j/e95f7d/32.png) [@JThunder](https://boards.straightdope.com/u/JThunder)\ **Post date:** [February 5, 2010, 3:30am UTC](https://boards.straightdope.com/t/can-you-prove-a-negative/527458/42 "2010-02-05T03:30:05Z")
Excellent arguments all around, **FinnAgain**. > [@FinnAgain](#): > > And as I said, _some_ negatives can be proved, like “there is nothing in this box.” But “nowhere on earth is there a unicorn” cannot be proven. It’s a very, very safe assumption, but it is not proof. It’s the same reason why debates about faith often end up with some theists shifting the burden of proof and demanding that you disprove God. I’ve found that it usually cuts both ways. For example, here on the Dope, I’ve never seen any theist argue in the manner that you described, though it does happen in real life. On the SDMB, people are much more likely to say things like, “No miracles have ever, ever occurred. Not once.” or “There has never been a single example of prayer being answered.” When pressed to defend these claims, people say “But you can’t prove a negative! Don’t ask me to do so!” As I said, it cuts both ways, and it’s just as fallacious whether it’s used by theists or atheists. ---
**Author:** ![JThunder](https://avatars.discourse-cdn.com/v4/letter/j/e95f7d/32.png) [@JThunder](https://boards.straightdope.com/u/JThunder)\ **Post date:** [February 5, 2010, 3:32am UTC](https://boards.straightdope.com/t/can-you-prove-a-negative/527458/43 "2010-02-05T03:32:26Z")
> [@Voyager](#): > > The problem with this technique is that sometimes one person’s absurdity is another person’s quantum physics. Absurd things can be true. Only when you take “absurd” to include things that are counter-intuitive. That’s a legitimate use of the term; however, the technique of _reduction ad absurdum_ entails a much more specific meaning. It specifically requires that the results be self-contradictory, not simply counter-intuitive. ---
**Author:** ![Revenant\_Threshold](https://avatars.discourse-cdn.com/v4/letter/r/f1d935/32.png) [@Revenant\_Threshold](https://boards.straightdope.com/u/Revenant_Threshold)\ **Post date:** [February 5, 2010, 3:47am UTC](https://boards.straightdope.com/t/can-you-prove-a-negative/527458/44 "2010-02-05T03:47:33Z")
> [@JThunder](#): > > You’re using confused terminology and asking us to accept the resultant logic. That’s simply fallacious. Oddly enough, that’s actually an example of the point I was trying to make, though certainly I apologise for putting it so badly. An absurd conclusion doesn’t say anything about the premise unless it has some accurate connection, accurate development, from that premise. A conclusion that is absurd, yet has no connection at all to the premise, doesn’t say anything useful about that premise. It’s just an absurd statement. For example, if I take an argument at random, then tack on “All dogs are both entirely black and entirely white” as a conclusion, then that’s an absurd conclusion. But if it has no basis in the premises given, then that it is absurd has no affect on the validity of the premise. > [@](#): > > Look, you’ve been starting with erroneous terminology and heaping confused logic on top of that. No wonder you’re drawing the conclusions that you do. This would be another example. I claim that my conclusion is valid, but you’re pointing out that it isn’t because the process used to arrive at it is flawed. It’s the same problem; an absurd conclusion, if not properly and logically arrived at, says nothing about the initial premise. ---
**Author:** ![Sophistry\_and\_Illusion](https://avatars.discourse-cdn.com/v4/letter/s/b3f665/32.png) [@Sophistry\_and\_Illusion](https://boards.straightdope.com/u/Sophistry_and_Illusion)\ **Post date:** [February 5, 2010, 4:40pm UTC](https://boards.straightdope.com/t/can-you-prove-a-negative/527458/45 "2010-02-05T16:40:58Z")
> [@jtgain](#): > > It seems like we are selectively choosing what is easy and hard to prove. > > “Bob has never gotten a DWI” can be proven even though it is negative. Run a NCIS check or request criminal histories from all 50 states. > > Similarly “Bob always drives drunk” is impossible to prove, though it is an affirmative statement. You would need evidence of each and every time that Bob drove, and have a corresponding BAC test above .08. > [@FinnAgain](#): > > No, we’re not. Some positives are impossible prove, as are some negatives. When someone talks about the rhetorical fallacy of proving a negative, they’re generally talking about the impossibility of providing negative proof once the burden of proof has been shifted fallaciously from a positive claim that hasn’t been proven. > > That was the point. That some negatives can be impossible to prove and are used as part of the fallacy of shifting the burden of proof, while other negatives are easy to prove and/or unlikely to be used for burden-shifting. > Simply, the way that debates are often structured means that it’s much less likely that anybody would fallaciously shift the burden of proof from a negative to a positive that’s impossible to prove. I think there are two separate discussions in this thread, and people are talking past each other to some extent. First: I think \*\*FinnAgain \*\*is correct to point out that a lot of people use the claim about proving negative statements fallaciously, in that they think their opponents failure to prove a negative means that their own positive claim is proved. Second: But I also think that when people say “You can’t prove a negative” they mean something more general, outside the context of this particular rhetorical fallacy. They mean to claim there is something special about negative statements that makes them unprovable. And I think this is false for a number of reasons that have been enumerated above: (1) This assumes a sort of ‘infallibilism’ about knowledge, in that evidence can only count as proof if it makes a claim 100% certain and rules out any conceivable possibility of error (a standard of evidence that is not appropriate to empirical inquiry). (2) It ignores that the structural feature supposedly shared by negatives is actually shared by many, if not all, universal statements (as the examples by \*\*jtgain \*\*and \*\*Noone Special \*\*show). (3) It ignores that there are often general considerations that support negative claims and prevent one from having to look everywhere. When I say, “There are no real magicians,” I don’t have to have looked at every square inch of the world. My confidence isn’t based on my having looked everywhere and not found any. It’s based on an understanding of how the world works (There is tons of evidence that the world doesn’t work based on magic, so we know that there can’t be any real magicians.) [Previous page](https://boards.straightdope.com/t/can-you-prove-a-negative/527458.md?page=2)