The story
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.
Etymology from Wiktionary, CC BY-SA 4.0.