No true Scotsman

The "no true Scotsman" argument is an argument of the 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 behaviors 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, Christians are often charged with employing this fallacy when they say that no true Christian would do something. Christian is used by such a widely disparate set of people that it has very little meaning when it comes to behavior. 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.