No true Scotsman is a term coined by Antony Flew in his 1975 book Thinking About Thinking. It refers to an argument which takes this form:

Argument: "No Scotsman puts sugar on his porridge."

Reply: "But my friend Angus likes sugar with his porridge."

Rebuttal: "Ah yes, but no true Scotsman puts sugar on his porridge."

This form of argument is a fallacy if the predicate ("putting sugar on porridge") is not actually contradictory to the accepted definition of the subject ("Scotsman"), or if the definition of the subject is silently adjusted after the fact to make the rebuttal work.

Some behaviours are actually contradictory to the label; "no true vegetarian would eat a beef steak" is not fallacious because it follows from the accepted definition of "vegetarian".

In particular, members of religious faiths are often charged with employing this fallacy when they say that no true Christian would do something. The term "Christian" is used by such a widely disparate set of people that it has very little meaning when it comes to behaviour. If there is no one accepted definition of the subject, then the initial argument should be accepted as the definition for the discussion at hand.

It is also a common fallacy in politics, in which critics may condemn their colleagues as not being "true" liberals or conservatives simply because they occasionally disagree on certain matters of policy.