This fallacy occurs when someone rescues a generalization from counter-evidence by introducing a spurious appeal to purity. The classic example can be found on Wikipedia:
Person A: “No Scotsman puts sugar on his porridge.”
Person B: “But my uncle Angus is a Scotsman and he puts sugar on his porridge.”
Person A: “But no true Scotsman puts sugar on his porridge.”
The problem of course is that the existence of an actual Scotsman who does in fact put sugar on his porridge should be sufficient evidence to disprove the initial claim. Instead, the speaker (Party A) dismisses the counter-evidence on the basis that it doesn’t really count because the party in question isn’t TRULY a member of the class in question. Significantly, what makes the difference in this case is nothing but the fact that the party in question is not acting in a manner consistent with the generalization.
We get this fallacy from the philosopher, Anthony Flew, and it reflects some of the themes of his larger body of work. The problem he was getting at here is a question of meaning. If counter-evidence doesn’t count, so to speak, then generalizations like those offered at the outset of such an argument don’t really tell us anything after all. What starts off as a false statement ends up as a tautology, which is not really an improvement.
The accusation that someone has committed this fallacy usually arises in the context of discussions about group membership and the people in those groups. Atheists and Christians will often accuse each other of producing the fallacy when talking about who does or doesn’t belong in either category. National identity will also give rise to the concern, and any other source of significant identity can certainly produce candidates for arguments which might be thought to commit this fallacy. There is no inherent reason to restrict this label to discussions of people, however. Notions like ‘true love’ can produce the problem just as easily as discussions about who is or isn’t a real Christian, a genuine atheist, or a true American. So can ideas about genres or music, books, or film.
People also tend to focus on instances in which a positive value is idealized and then defended from counter-evidence, but it can also be used to blunt criticism (e.g. “So&So isn’t really a criminal; it was just a misdemeanor.>)
The fallacy occurs whenever a spurious distinction is introduced in defense of a generalization.