no true Scotsman fallacy

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.

no true Scotsman fallacy (plural no true Scotsman fallacies)

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.

• https://www.logicalfallacies.org/no-true-scotsman.html
• no true Scotsman on Wikipedia.Wikipedia