No true Scotsman


No true Scotsman, or appeal to purity, is an informal fallacy in which one attempts to protect a universal generalization 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, without reference to any specific objective rule: "no Scotsman would do such a thing"; 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.

Examples

Philosophy professor Bradley Dowden explains the fallacy as an "ad hoc rescue" of a refuted generalization 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 is a Scotsman and he puts sugar on his porridge."
Person A: "But no true Scotsman puts sugar on his porridge."
The essayist David P. Goldman, writing under his pseudonym "Spengler," compared distinguishing between "mature" democracies, which never start wars, and "emerging democracies", which may start them, with the "no true Scotsman" fallacy. Spengler alleges that political scientists have attempted to save the "US academic dogma" that democracies never start wars against other democracies from counterexamples by declaring any democracy which does indeed start a war against another democracy to be flawed, thus maintaining that no democracy starts a war against a fellow democracy.

Counterexamples

Arguments of this kind are not necessarily always faulty. For example:

Person A: "No pacifist stabs people to death."
Person B: "I know one who stabbed a couple of people to death."
Person A: "That guy was not a true pacifist."
In this case, the "No true Scotsman" analogy does not hold. The definition of pacifism includes opposition to violence and abstinence of its use in all situations. Stabbing people and pacifism are in a logical conflict. The logical conclusion is that the stabber was not really a pacifist.
It is also not wrong to say "no true vegetarian would eat steak", because the definition of being a vegetarian includes not eating meat.

Origin

The introduction of the term is attributed to British philosopher Antony Flew, because the term originally appeared in Flew's 1971 book An Introduction to Western Philosophy. In his 1975 book Thinking About Thinking, he wrote: