No true Scotsman or appeal to purity is an informal fallacy in which one attempts to protect an a-posteriori claim from a falsifying counterexample by covertly modifying the initial claim. Rather than admitting error or providing evidence that would disqualify the falsifying counterexample, the claim is modified into an a-priori claim in order to definitionally exclude the undesirable counterexample. The modification is signalled by the use of non-substantive rhetoric such as “true”, “pure”, “genuine”, “authentic”, “real”, etc.
No True Scotsman