Sunday, December 9, 2007

You Can Prove a Negative

"A principle of folk logic is that you can’t prove a negative. Skeptics and scientists routinely concede the point in debates about the possible existence of everything from Big Foot and Loch Ness to aliens and even God. In a recent television interview on Comedy Central’s The Colbert Report, for example, Skeptic publisher Michael Shermer admitted as much when Stephen Colbert pressed him on the point when discussing Weapons of Mass Destruction, the comedian adding that once it is admitted that scientists cannot prove the nonexistence of a thing, then belief in anything is possible. [...] There is one big problem with this. Among professional logicians, guess how many think that you can’t prove a negative? That’s right, zero.


So why is it that people insist that you can’t prove a negative? I think it is the result of two things: (1) Disappointment that induction is not bulletproof, airtight, and infallible, and (2) A desperate desire to keep believing whatever one believes, even if all the evidence is against it. [...] Meaning: your argument against aliens is inductive, therefore not incontrovertible. Since I want to believe in aliens, I’m going to dismiss the argument no matter how overwhelming the evidence against aliens, and no matter how vanishingly small the chance of extraterrestrial abduction.
If we’re going to dismiss inductive arguments because they produce conclusions that are probable but not definite, then we are in deep manure. Despite its fallibility, induction is vital in every aspect of our lives, from the mundane to the most sophisticated science. Without induction we know basically nothing about the world apart from our own immediate perceptions. So we’d better keep induction, warts and all, and use it to form negative beliefs as well as positive ones.
You can prove a negative — at least as much as you can prove anything at all.

Steven D. Hales,, 7. December 2007


Gary McGath said...

The principle that you can't prove a negative isn't a "concession" -- it's a rejection of an invalid demand.

Without the burden-of-proof principle, any assertion whatsoever can be admitted, and additional arbitrary assertions can be added at will to support it. For example, I could claim that my cat can predict the future. If you challenge me for evidence, I can say that we just haven't learned to translate meowing accurately. Induction rests on the principle of burden of proof; without it, any claim at all can be put into doubt.

Granted, some people utter this principle as a license to spew out any arbitrary assertion which pops into their heads. But they aren't actually using the principle; if they did, they'd have to concede that claims of the Flying Spaghetti Monster, prophesying cats, and all the rest are just as valid as their own claim. What they're doing is special pleading. They're saying, "I'm making a demand which is in principle impossible to satisfy, therefore my reasoning is valid."

If proof were expected for the rejection of arbitrary assertions, then those who make them could exhaust their opponents by making any number of wild supporting claims.

Nick Barrowman said...

When it comes to using observational evidence to argue for existence (a positive claim) or non-existence (a negative claim), and if prove means to establish with certainty, then I don't agree with Steven Hales. You can't prove a negative, whereas you can prove a positive.

The burden of proof then rests upon those making claims of existence.