You Can Prove a Negative
Can't prove a negative? Sure you can!
One reason that some people suppose science and reason are incapable of establishing beyond reasonable doubt that certain supernatural claims—for example, that fairies or angels or spirit beings exist—are false, is that they assume you can't prove a negative. Indeed this is widely supposed to be some sort of "law of logic."
For example, Georgia minister Dr. Nelson L. Price asserts on his website that "one of the laws of logic is that you can't prove a negative." If Price is correct and this is indeed a law of logic, then of course it immediately follows that we can't prove that there are no fairies, angels, or spirit beings, or, indeed, that there is no god. We will have established that the nonexistence of God is indeed beyond the ability of reason and/or science to establish!
The fact is, however, that this supposed "law of logic" is no such thing. As Steven D. Hales points in his paper "You Can Prove a Negative," "You can't prove a negative" is a principle of folk logic, not actual logic.
Notice, for a start, that "You cannot prove a negative" is itself a negative. So, if it were true, it would itself be unprovable. Notice that any claim can be transformed into a negative by a little rephrasing—most obviously, by negating the claim and then negating it again. "I exist" is logically equivalent to "I do not not exist," which is a negative. Yet here is a negative it seems I might perhaps be able to prove (in the style of Descartes—I think, therefore I do not not exist!)
Of course, those who say "You can't prove a negative" will insist that I have misunderstood their point. As Hales notes, when people say, "You can't prove a negative," what they really mean is that you cannot prove that something does not exist. If this point were correct, it would apply not just to supernatural beings lying beyond the cosmic veil but also to things that might be supposed to exist on this side of the veil, such as unicorns, Martians, rabbits with twenty heads, and so on. We would not be able to prove the nonexistence of any of these things either...
Let's sum up. If "you can't prove a negative" means you can't prove beyond reasonable doubt that certain things don't exist, then the claim is just false. We prove the nonexistence of things on a regular basis. If, on the other hand, "you can't prove a negative" means you cannot prove beyond all possible doubt that something does not exist, well, that may, arguably, be true. But so what? That point is irrelevant so far as defending beliefs in supernatural entities against the charge that science and/or reason have established beyond reasonable doubt that they don't exist.
