Many arguments, especially, it must be said, on Internet discussion boards, turn into fruitless wrangling over the meaning of words. In particular, if a Christian uses the word "Christian" to imply a certain type of behaviour, someone will almost inevitably give an example of a so-called "Christian" who did the opposite. The Christian may very well protest that the example probably wasn't a true Christian. At this point his opponent triumphantly cries "No True Scotsman!" and the argument is lost.
"No True Scotsman" is the name coined by Anthony Flew for a fallacy which goes like this:
Obviously this is fallacious reasoning. Wikipedia has a very clear description of the fallacy as above and tellingly remarks "Christians are often charged with employing this fallacy when they say that no true Christian would do something. 'Christian' is used by such a widely disparate set of people that it has very little meaning when it comes to behaviour. If there is no one accepted definition of the subject, then the initial argument should be accepted as the definition for the discussion at hand." OK, it's a wiki, but it makes sense - you do have to take words in context.
In contrast, the atheism.about.com FAQ says this:
"Here is a real life example of how this fallacy can be used:
'4. Another good example is abortion, our government has such a small Christian influence that the courts have ruled it's ok to kill babies now. Typical. The people who support legalized abortion but claim to be Christians don't really follow Jesus - they have lost their way.'"
The FAQ continues by analysing the passage as follows:
"In an effort to argue that abortion is wrong, it is assumed that Christianity is inherently and automatically opposed to abortion (begging the question). In order to do this, it is further argued that no one who supports legalized abortion - for any reason - can really be a Christian (equivocation through an ad hoc redefinition of the term 'Christian')."
This analysis itself is actually a pretty typical abuse of the "No True Scotsman" concept. It gets off to a bad start with "In an effort to argue that abortion is wrong...". You do not need to look very closely to see that the author is not is arguing the case that abortion is anti-Christian, he takes that much for granted - a stance which may need justifying but isn't begging the question since proving it is not his objective here. He then proceeds to say that the country is totally non-Christian and that the abortion laws prove it. Finally he turns the finger of blame onto people who claim to be Christians but who don't follow Jesus.
It is, of course, the phrase "claim to be Christians" that gives atheists the excuse to cry "No True Scotsman!". It all depends what you mean by the word "Christian". Atheists tend to use it to mean "everything that calls itself 'Christian'" - a useful ploy as it makes Christianity an easy target. Christians use it to mean believers in their faith and followers of Jesus - which sounds suspiciously like "it means whatever we want it to mean". But, in fact, the "Christian" shades of meaning are well-known, unlike the meaning of "true Scotsman" which can only be guessed at when McDonald suddenly introduces it.
The argument could certainly have been put more formally by stating the assumptions. However, in casual writing it is up to the reader to make allowances and work out what the writer means. Of course, if the writing is so sloppy that it doesn't make any sense at all then the writer has failed but that is not the case here. Equally, if the argument is superficially convincing but a closer inspection shows it to be wrong then the critic is justified in complaining. However, if it says one thing and the critic pretends it is saying another and then condemns it for not doing so rigorously then the critic is doubly wrong and loses all credibility. The atheist's rant against the Christian article for the wrong reasons is a classic strawman, not atypical from that source, it has to be said.
Flew's original illustration is actually a bit hard on the speaker. In all probability he wasn't suggestion that Angus, the sugar eater, isn't a Scot, only that he isn't worthy of the name "Scotsman". However, the incident is just an illustration. We must give Flew the benefit of the doubt as being in the best position to know what was going on in the imaginary man's mind.
So, then, the primary error was to re-define "Scotsman" on the fly so it no longer simply means "a man from Scotland" as the naive listener might think, but now means something else known only to the speaker and, of course, to Flew. In comparison, when a Christian unwarily says "but they can't have been true Christians", the term "true" is used to indicate people who truly follow Jesus, people who don't merely call themselves by his name. There is no change of meaning - the writer, like Flew, thinks it is obvious what he means. As indeed it is to all but the author of the atheism.about.com FAQ. However, an argumentative atheist does not want the word "Christian" to mean "follower of Christ". Typically he would prefer a meaning which allows him to implicate Mother Theresa in burning heretics alive.
So, of course, there is such a thing as a True No True Scotsman Fallacy, but often the term is abused, creating a new fallacy in its own right. I call this fallacy "The No True Scotsman Fallacy Fallacy"
Imitation is the sincerest form of flattery and the above has spawned an attempted rebuttal in the shape of an article called The No True Scotsman Fallacy Fallacy Fallacy.
The author actually says little beyond repeating the claim that Christians regularly commit the NTSF, which I dispute for the reasons already given. In particular, there is a world of difference between
I suspect it started as a Title in Search of an Article.
Monty Python has nothing to fear.