No True Scotsman, or appeal to purity, is an informal fallacy in which one attempts to protect their generalized statement from a falsifying counterexample by excluding the counterexample improperly. Rather than abandoning the falsified universal generalization or providing evidence that would disqualify the falsifying counterexample, a slightly modified generalization is constructed ad-hoc to definitionally exclude the undesirable specific case and similar counterexamples by appeal to rhetoric. This rhetoric takes the form of emotionally charged but nonsubstantive purity platitudes such as "true", "pure", "genuine", "authentic", "real", etc.

Philosophy professor Bradley Dowden explains the fallacy as an "ad hoc rescue" of a refuted generalization attempt. The following is a simplified rendition of the fallacy: