no true Scotsman fallacy

Meanings

1. An informal fallacy in which one attempts to defend or protect an a posteriori claim from a falsifying counterexample by covertly modifying the initial claim, especially transforming it into a tautology by saying that any counterexamples are ipso facto not valid members of the class being described.

Etymology

Attributed to the English philosopher Antony Flew, from his 1966 book God & Philosophy: :In this ungracious move a brash generalization, such as No Scotsmen put sugar on their porridge, when faced with falsifying facts, is transformed while you wait into an impotent tautology: if ostensible Scotsmen put sugar on their porridge, then this is by itself sufficient to prove them not true Scotsmen.