A logical fallacy in which the explanation for some phenomenon cannot be tested. Theories are tested by looking at their predictions and seeing if they hold up. If the explanation makes no predictions, or predicts things that would happen anyway, then it cannot be tested.

Example: "I won because of a good-luck curse." But when asked to try again, the response "It's a one-time only curse" so it can't be tested again.
Example: "I have good luck because i carry a good luck charm." But what is a good luck charm? Something that gives good luck. To test it, find another good luck charm and see if it gives good luck. But, how to tell what a good luck charm is? Something that gives good luck. But something that gives good luck always gives good luck, so there's no prediction.

To prove the fallacy, show that the explanation makes no predictions or that the predictions it makes can never be wrong.

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