The fallacy takes one of the following two alternative forms, where the person committing the fallacy is labelled “Tom”:

        Tom: No A are B.
        Dick: Here is an example of A that is B.
        Tom: Well, no true A are B.

        Tom: All A are B.
        Dick: Here is an example of A that is not B.
        Tom: Well, all true A are B.

Note 1:

The term was coined by philosopher Antony Flew in his 1975 book Thinking About Thinking. Here is the canonical example given in the book:

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.”

Note 2:

The fallacy is similar to begging the question (episode 23) but with the difference that the conclusion is a modification of the subject of the initial claim so as to arbitrarily exclude a counterexample. Begging the question assumes the premise in the conclusion but does not modify it, and does not necessarily involve any counterexamples.

Note 3:

The fallacy is also similar to the overwhelming exception fallacy (discussed in the previous episode) insofar as both fallacies are committed in response to a counterexample but with the following difference: The overwhelming exception fallacy responds to the counterexample by acknowledging it by stating that the initial claim is “except for” the counterexample, and creating the false impression that the counterexample is insignificant. By contrast, the no true scotsman fallacy restates the initial claim in a way that fallaciously denies that the subject of the counterexample is a subject of the claim.

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