Reply #5: ...

[Fumesucker]

http://en.wikipedia.org/wiki/No_true_Scotsman

The term was advanced by philosopher Antony Flew in his 1975 book Thinking About Thinking: Do I sincerely want to be right?.<2>
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."
—Antony Flew, Thinking About Thinking

A simpler rendition would be:
Teacher: All Scotsmen enjoy haggis.
Student: My uncle is a Scotsman, and he doesn't like haggis!
Teacher: Well, all true Scotsmen like haggis.

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"