In his 1975 book Thinking About Thinking, philosopher Anthony Flew outlined a form of argument that he dubbed the “No True Scotsman” fallacy:
Argument: “No Scotsman puts sugar on his porridge.”
Reply: “But my uncle Angus likes sugar with his porridge.”
Rebuttal: “Ah yes, but no true Scotsman puts sugar on his porridge.”
Read the whole thing.
(Via First Thoughts.)