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.[1][2][3] 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.[4] The modification is signalled by the use of non-substantive rhetoric such as "true", "pure", "genuine", "authentic", "real", etc.[2]
Philosophy professor Bradley Dowden explains the fallacy as an "ad hoc rescue" of a refuted generalization attempt.[1] The following is a simplified rendition of the fallacy:[5]
Person A: "No Scotsman puts sugar on his porridge."
Person B: "But my uncle Angus is a Scotsman and he puts sugar on his porridge."
Person A: "But no true Scotsman puts sugar on his porridge."
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