Logical fallacy/No true Scotsman

The "No true Scotsman" argument is an argument of the form:

Argument: "No Scotsman puts sugar on his porridge."
Reply: "But my friend Angus likes sugar with his porridge."
Rebuttal: "Ah yes, but no true Scotsman puts sugar on his porridge."

This form of argument is a fallacy when the behavior ("putting sugar on porridge") is not a definitional part of the label ("Scotsman"). Some behaviors are actually contradictory to the label; "no true vegetarian would prefer a beefsteak to a salad" is not fallacious.

Atrocities of religions are often discussed away using this fallacy.

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/Talk