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A No True Scotsman (NTS) fallacy occurs when (1) it is asserted that no true member of a group they belong to would do a certain action or (2) a term is defined biasedly to allow easier use of the first form. Instead of acknowledging that some members of a group have undesirable characteristics, the fallacy tries to redefine the group to exclude them.
The fallacy does not occur in defining a group or label narrowly to begin with, but in narrowing it by excluding evidence that contradicts an initially broad definition.
The fallacy exists where a group without initially defined parameters is redefined arbitrarily:
P1: There exists a group X.
P2: Members of group X with quality Y are not members of group X.
C1: All members of group X have quality Y.
Most often, the fallacy occurs in a conversation:
A: "For all members in set S, quality P applies."
B: "Here's X. It's a member of set S, but P does not apply, invalidating your statement."
A: "No. X isn't a member of set S, because P does not apply to it."
Person A has committed a form of circular reasoning. The counterexample is Morton's-fork-ingly used to reinforce the original statement. Here is a stupidly obvious concrete example:
A: "All the marbles in this bag are blue."
B: "But this marble is red, and it's in the bag!"
A: "No. I just told you that all the marbles in this bag are blue. Since it's red, it can't actually be in the bag."
In practice, application of the Scotsman is far more subtle than this, but the line of thinking always boils down to a denialistic attitude towards counterexamples.
 Versus a definition
A NTS fallacy should be distinguished from definitions. A definition is a (valid) tautology, because Y was and is a requirement of membership in X:
P1: All members of group X have quality Y.
P2: Supposed members of group X that do not have quality Y are not members of X.
C1: All members of group X have quality Y.
For example, the fallacy does not apply if there is a clear and well understood definition of what membership in a group requires and it is that definition which is broken (e.g., "no honest man would lie" or "no theist can be atheist" and so on).
A NTS fallacy only occurs when the group is later redefined for no valid reason.
The term was coined by Antony Flew, who gave an example of a Scotsman who sees a newspaper article about a series of sex crimes taking place in Brighton, and responds that "no Scotsman would do such a thing." When later confronted with evidence of another Scotsman doing even worse acts, his response is that "no true Scotsman would do such a thing," thus disavowing membership in the group "Scotsman" to the criminal on the basis that the commission of the crime is evidence for not being a Scotsman. However, this is a fallacy as there is nothing in the definition of "Scotsman" which makes such acts impossible. The term "No True Scotsman" has since expanded to refer to anyone who attempts to disown or distance themselves from wayward members of a group by excluding them from it.
The True Scotsman fallacy can be used to uphold positive, neutral, or negative stereotypes by portraying people who do not conform to them as exceptions. An example would be claiming that feminists, being "well-educated and having strong opinions about gender issues," do not speak for "typical" women. It is also commonly used as a form of name-calling (i.e., Real men eat meat, True Americans don't question the existence of God, Real artists don't use a computer, True women stay in the kitchen instead of working, etc.)
 Prime examples
Phrases such as "un-American," "un-Christian", "un-Islamic" or "inhuman" are widely used in politics and media to distance oneself from a subject, defining them as outside the bounds of what the speaker considers to be truly 'American,' 'Christian', 'Muslim' or 'human' behaviour. These phrases strongly suggest the No True Scotsman fallacy, since the use, for example of "un-American" to describe specific political activities by some American citizens implies some special definition of "American" beyond mere nationality. (It is not a fallacy if such a special definition is consistently applied, though that doesn't make it a sensible definition.)
“”No man can ever be opposed to Christianity who knows what it really is.
With respect to religion, the fallacy is well used, often even overused. Religious apologists will repeatedly try to use the No True Scotsman argument to distance themselves from more extreme or fundamentalist groups, but this does not prevent such extremists actually being religious - they themselves would certainly argue otherwise. Moderate Muslim leaders, for example, are well known for declaring Islamic extremists as "not true Muslims" as Islam is a "religion of peace." Similarly, moderate Christians, such as those in Europe, are sometimes aghast when viewing their fundamentalist counterparts in the US, immediately declaring them "not true Christians," even though they believe in the same God and get their belief system from the same book. Many of these statements stating that the extremists are not true believers are often used as a reaction against Guilt by Association. The No True Scotsman fallacy can also run the other way when it comes to extremism. Extremists will make a religious statement and when someone points out that there are many believers who don't believe the extremist's viewpoint, the moderates are deemed to be not true believers (ie: Christians who support gay marriage are not "real Christians" or Muslims who support women's rights are not "real Muslims"). Modern pagans do it all the time, perhaps even more than other religions, due to the fact that there is no agreed-on orthodoxy for the whole group, with some well-established practices in one setting being considered unpalatable in others. Silver Ravenwolf, one of the best selling "leaders" of neopagans, has done this with multiple ancient, well-established practices.
It's a tricky business, as being a member of a religious group, to the minds of those involved, encompasses adhering to a certain standard of behavior. For example, charity can certainly be called an essentially Christian ethic, considering the emphasis that Jesus placed on it. The man himself would most definitely disavow the greedy and "What's mine is mine" mindset of many right-wingers who call themselves Christians. However, strictly speaking, a Christian is defined as "one who professes belief in the teachings of Jesus Christ"; there's no rule saying they have to do it right.
Atheists can also be guilty of the fallacy, such as someone hypothetically exclaiming that "true atheists" don't believe in ghosts or UFOs when such disbeliefs actually have nothing to do with an atheistic philosophy (although they can be linked by concepts of observational evidence and rationalism).
In recent years with the advent of the Tea Party Movement, the turn of phrase "Republican in name only" (RINO) has come into more prominence from people who want to disassociate from other people of the party. The trouble with this phrase is that it is not useful at all since its use tends to be a replacement for "Republican I don't agree with, despite being in the same party." People who use this phrase tend to throw it even if there is one issue the supposed RINO doesn't toe the party line on, such as being Pro-choice. Not to say this also doesn't exist on the other side as well, but it is rare for people to throw the accusation of DINO when the term Blue Dog Democrat is far less damning and more useful.
 See also
- A hilarious spin-cycle in and out of No True Scotsman
- A YouTube video on the fallacy
- And you never were a Scotsman, either!