No true Scotsman is an informal fallacy, an ad hoc attempt to retain an unreasoned assertion. When faced with a counterexample to a universal claim, rather than denying the counterexample or rejecting the original universal 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.
Imagine Hamish McDonald, a Scotsman, sitting down with his Glasgow Morning Herald and seeing an article about how the "Brighton Sex Maniac Strikes Again". Hamish is shocked and declares that "No Scotsman would do such a thing". The next day he sits down to read his Glasgow Morning Herald again; and, this time, finds an article about an Aberdeen man whose brutal actions make the Brighton sex maniac seem almost gentlemanly. This fact shows that Hamish was wrong in his opinion but is he going to admit this? Not likely. This time he says, "No true Scotsman would do such a thing".
When the statement "all A are B" is qualified like this to exclude those A which are not B, this is a form of begging the question; the conclusion is assumed by the definition of "true A".
A simple rendition of the fallacy would be:
An example of a political application of the fallacy could be in asserting that "no democracy starts a war", then distinguishing between mature or "true" democracies, which never start wars, and "emerging democracies", which may start them. At issue is whether or not something labelled as an "emerging democracy" is actually a democracy or something in a different conceptual category.
A common mistake among people identifying Scotsman fallacies is to point out a fallacy has been made when the topic of the argument was actually clearly defined and never changed. For example:
This would be valid, because from the beginning "True Scotsmen" were defined as the topic of the argument, and it was given the characteristic of "those who drink ale". Of course, the topic must always be properly defined to avoid confusion or fallacious reasoning.
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