No True Scotsman

Political parties and other interest groups need to have principles, and a person who claims to be a member of that party or group while denying the central principles that define the group is surely mistaken or confused. For example, one could hardly be an atheist while believing that Vishnu is the deity responsible for sustaining and supporting the universe. It is reasonable to demand some standards of behavior or belief, and there is no fallacy in saying that no true atheist worships Vishnu.

The No True Scotsman fallacy mimics this demand for standards, but it attempts to create (by definition) membership criteria that are not the defining criteria we normally expect. The fallacy is typically used in one of two ways. It can be used to try to enforce conformity and orthodoxy within a particular group, and it can also be used by people outside the group to "define" the group in negative ways.

When the No True Scotsman fallacy is used in the second of these ways it can bear a strong resemblance to the Straw Man fallacy, i.e. creating a misrepresentation of an opponent's view or of the characteristics of typical members. The fallacy also bears some resemblance to the fallacy of Equivocation, since the term at issue - "Scotsman," for example - shifts it meaning: a Scotsman is a member of a particular ethnic group, but a true Scotsman is a member of that ethnic group who doesn't put brown sugar on his porridge.

Despite these other elements, I have classified the No True Scotsman as a deductive circularity, since it seems to me that the central error in the fallacy is the use of mere stipulation to disallow counterexamples that what would otherwise refute the arguer's stated position.