No true Scotsman is a logical fallacy, an ad hoc attempt to retain an unreasoned assertion. When faced with a counterexample to a universal claim, rather than denying the counterexample or rejecting the original universal claim, this fallacy modifies the subject of assertion to tautologically exclude the specific case or others like it.
The term was advanced by philosopher Antony Flew in his 1975 book Thinking About Thinking: Do I Sincerely Want to Be Right?.
Imagine Hamish McDonald, a Scotsman, sitting down with his Glasgow Morning Herald and seeing an article about how the "Brighton 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 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."
Don't you just love that? Please leave a comment.