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No true Scotsman is an informal fallacy, an ad hoc attempt to retain an unreasoned assertion. 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"). It can also be used to create unnecessary requirements.
A simple rendition of the fallacy:
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.
The use of the term was advanced by British philosopher Antony Flew:
Imagine Hamish McDonald, a Scotsman, sitting down with his Glasgow Morning Herald and seeing an article about how the "Brighton [(England)] Sex Maniac Strikes Again". Hamish is shocked and declares that "No Scotsman would do such a thing". The next day he sits down to read his Glasgow Morning Herald again; and, this time, finds an article about an Aberdeen [(Scotland)] man whose brutal actions make the Brighton sex maniac seem almost gentlemanly. This fact shows that Hamish was wrong in his opinion but is he going to admit this? Not likely. This time he says, "No true Scotsman would do such a thing".