No true Scotsman or appeal to purity is an informal fallacy in which one attempts to protect an a posteriori claim from a falsifying counterexample by covertly modifying the initial claim.[1] [2] [3] Rather than admitting error or providing evidence that would disqualify the falsifying counterexample, the claim is modified into an a priori claim in order to definitionally exclude the undesirable counterexample. The modification is signalled by the use of non-substantive rhetoric such as "true", "pure", "genuine", "authentic", "real", etc.[2]
Philosophy professor Bradley Dowden explains the fallacy as an "ad hoc rescue" of a refuted generalization attempt.[1] The following is a simplified rendition of the fallacy:
The "no true Scotsman" fallacy is committed when the arguer satisfies the following conditions:[4] [3]
An appeal to purity is commonly associated with protecting a preferred group. Scottish national pride may be at stake if someone regularly considered to be Scottish commits a heinous crime. To protect people of Scottish heritage from a possible accusation of guilt by association, one may use this fallacy to deny that the group is associated with this undesirable member or action. "No Scotsman would do something so undesirable"; i.e., the people who would do such a thing are tautologically (definitionally) excluded from being part of our group such that they cannot serve as a counterexample to the group's good nature.[5]
The description of the fallacy in this form is attributed to British philosopher Antony Flew, who wrote, in his 1966 book God & Philosophy,
In his 1975 book Thinking About Thinking, Flew wrote:
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.[6]
Author Steven Pinker suggested that phrases like "no true Christian ever kills, no true communist state is repressive" exemplify the fallacy.[7]