No true Scotsman is a kind of informal fallacy in which one attempts to rescue a universal generalisation from counterexamples by changing the definition in an ad hoc fashion to exclude the counterexample. Rather than denying the counterexample or rejecting the original claim, this fallacy modifies the subject of the assertion to exclude the specific case or others like it by rhetoric i.e. those who perform that action are not part of our group and thus criticism of that action is not criticism of the group.
Philosophy professor Bradley Dowden explains the fallacy as an ‘ad hoc rescue’ of a refuted generalisation attempt. The following is a simplified rendition of the fallacy:
Person A: ‘No Scotsman puts sugar on his porridge.’
Person B: ‘But my uncle Angus likes sugar with his porridge.’
Person A: ‘Ah yes, but no true Scotsman puts sugar on his porridge.’
The introduction of the term is attributed to British philosopher Prof. Antony Flew, because the term originally appeared in Flew’s 1971 book An Introduction to Western Philosophy.
A practical example of this fallacy occurs when Marxists try to defend their regressive and unworkable ideology against the overwhelming evidence from the 20th century that almost every communist regime was brutally repressive; and most of them resulted in poverty for everybody except the communist party elite. ‘But they weren’t true communists’ they say. Yeah, right.
The Logical Place 