Topics Logic

Question A common discussion-killer is the declaration: "You can't prove a negative!" Immediately the conversation screeches to a halt and people turn to other topics. Is there really nothing more to be said? A: Fairies don't exist. B: You can't prove a negative. A: Okay, fair enough. So how do you like this pizza? Does it have to be this way?

Accepted:
May 27, 2009

Comments

I'm reminded of the exasperated Bertrand Russell faced with the young Wittgenstein: "He thinks that nothing empirical is knowable. I asked him to admit that there was not a rhinoceros in the room, but he wouldn't. I looked under all the desks without finding one but Wittgenstein remained unconvinced." It is Wittgenstein here who is being obtuse and in the grip of a silly theory. Of course we can establish empirical propositions both positive and negative – for example, that there are five desks in the room and no rhinoceroses.

By any sane standard, it is just plain false that you can't prove a negative, and that supposed "discussion-killer" should itself be promptly killed off.

Perhaps part of the problem is the word "prove", which also tends to get used when talking about such things as the existence of God. (No-one can prove that God exists, we're often told.) As our erstwhile leader, Alex George, has often pointed out, however, outside mathematics, one can rarely "prove" anything. So to be told in that sense that no-one can "prove" a negative is unhelpful. One can't "prove" a positive in that sense, either.

As Peter said, more or less.