From (https://en.wikipedia.org/wiki/No_true_Scotsman)
No true Scotsman or appeal to purity is an informal fallacy in which one attempts to protect an initial a posteriori claim from a subsequent falsifying counterexample by then 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 to definitionally (as opposed to evidentially) exclude the undesirable counterexample.[4] The modification is usually identifiable by the use of non-substantive rhetoric such as “true”, “pure”, “genuine”, “authentic”, or “real”, which can be used to locate when the shift in meaning of the claim occurs.[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.”
