No true Scotsman

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.

Example:

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."

An appeal to purity is commonly associated with protecting a preferred group. Scottish national pride may be at stake if someone regularly considered to be Scottish commits a heinous crime. To protect people of Scottish heritage from a possible accusation of guilt by association, one may use this fallacy to deny that the group is associated with this undesirable member or action. "No true Scotsman would do something so undesirable"; i.e., the people who would do such a thing are tautologically (definitionally) excluded from being part of our group such that they cannot serve as a counterexample to the group's good nature.