Logical Fallacy #17 - No True Scotsman

No true Scotsman is an informal fallacy, an ad hoc attempt to retain an unreasoned assertion.[1] When faced with a counterexample to a universal claim ("no Scotsman would do such a thing"), rather than denying the counterexample or rejecting the original universal claim, this fallacy modifies the subject of the assertion to exclude the specific case or others like it by rhetoric, without reference to any specific objective rule ("no true Scotsman would do such a thing"),[2] creating an implied tautology. It can also be used to create unnecessary requirements.

A simple rendition of the fallacy:[3]

Person A: "No Scotsman puts sugar on his porridge."

Person B: "I am Scottish, and I put sugar on my porridge."

"Person A: "Well, no true Scotsman puts sugar on his porridge."

A cited example of a political application of the fallacy was asserting that "no democracy starts a war", then distinguishing between mature or "true" democracies, which never start wars, and "emerging democracies", which may start them.[4]