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.” [Brighton is not part of Scotland.] 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. [Aberdeen is part of Scotland.] 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.”
—Antony Flew, Thinking About Thinking: Do I sincerely want to be right? (1975)
Consider the following examples:
Bob: “All Australians like Vegemite!”
Jim: “But I’m an Australian. I don’t like Vegemite.”
Bob: “Well you’re not a true Australian then. All true Australians like Vegemite!”
Or
Jill: “Catholics are against abortion under any circumstances”
Mary: “I’m a Catholic, but I think abortion is acceptable in cases of rape or incest.”
Jill: “You’re not a true catholic then. If you were you would not accept abortion ever.”
This sort of reasoning is known as Ad Hoc or as a fallacy: Ad Hoc Rescue. In each case Hamish, Bob and Jill modify their assertion to address the objection raised against it. Ad Hoc reasoning is not inherently fallacious, but in the case of the ‘True Scotsman’ fallacy, and the examples which follow, the modification doesn’t address the real objection. It is dismissive of the objection.
For this reason the fallacy could be considered one of relevance: does my nationality have any relevance to my propensity for violence, or does my taste for yeast spread have any relevance to my national identity? In the third example the link between religious membership and a position of morality is perhaps more relevant than in the preceding examples.
It could also be considered an example of an ambiguity. What precisely is meant by a ‘Scotsman’ as opposed to a ‘true Scotsman’, or an ‘Australian’ to a ‘true Australian’?
It is also likely to be considered presumption. Clearly Flew’s original example is a circular argument. Hamish seems to be saying ‘no Scotsman could do horrible violence, because anyone doing horrible violence is not a true Scotsman’. This is akin to the claim ‘no A is a B because if it is a B it is not an A’. By linking the definition of what it is to be a Scotsman (or an Australian, or a Catholic, or whatever) to the truth of the conclusion a circular argument is constructed.
Mr. Melican's VCE Philosophy 