“No true Scotsman” is “a kind of ad hoc rescue of one’s generalization in which the reasoner re-characterizes the situation solely in order to escape refutation of the generalization.”1
Example:
Smith: All Scotsmen are loyal and brave.
Jones: But McDougal over there is a Scotsman, and he was arrested by his commanding officer for running from the enemy.
Smith: Well, if that’s right, it just shows that McDougal wasn’t a TRUE Scotsman.
Now, once or twice I’ve been accused of this fallacy–of “redefining” terms–when discoursing on the differences between Christians and Muslims, Christianity and Islam. The problems my interlocutors have had is that I “defined” Christians and Muslims by the standards set forth by the founders and documenters of both Christianity and Islam. Hmmm, that would seem to me to be fair, not fallacious.
When someone claiming to be a Christian acts against the teachings of Christ and the Apostles (say, the Papal legate, the Abbot of Citeaux Arnaud Amalric commanding that the inhabitants of Béziers be massacred), that would seem to very legitimately impeach that person’s claim to be Christian or to be acting in the name of the Founder of Christianity, would it not?
When someone claiming to be a Muslim acts in accordance with the life and teachings of Mohamed (say, mass murder, rape and enslavement of those who disagree with the teachings of Islam, as Mohamed’s first “victory”–the massacre of the Banu Quraysh Jews–and his explicit teachings demand), one would legitimately consider that person to be a legitimate follower of Mohamed. OTOH, “peaceful” Muslims violate both the commandments of Mohamed and disrespect his life example.
Based on the life and teachings of these two men, and the explicit commands they left their followers, reason would dictate one evaluate those claiming to be their followers based on whether or not they actually do follow those they claim to follow.