No True Fallacy

A note on the "No True Scotsman" fallacy:

This fallacy consists of making an assertion about a group such as "No Scotsman would cheat at cards" and then defending the truth of the statement by reverse applying the claim to membership on the group. Thus:

"No Scotsman would cheat at cards!"
"On the contrary, Angus is a Scotsman and he was caught cheating at cards."
"In that case, Angus is No True Scotsman, for no true Scotsman would cheat at cards!"

It's important to realize that this is only a fallacious line of reasoning _if_ there is not logical between the original group and the claim made about it. For instance, there is no logical connection between being a Scotsman and refraining from cheating at cards.

However, if there is a logical connection between the group and the characteristic applied to it, no fallacy is incurred. For instance, "No good parents lock their child in a room and starve him for days." The phrase "good parent" does not identify some random demographic group, which might contain both some people who do imprison and starve their children and some who don't. Rather, 'good parent' is a group which definitionally would not contain such parents. If you imprison and starve your children, then you aren't a good parent.

Of course, at times the terms themselves will be in dispute. For instance, you'll often see the following exchange played out in the political realm:

"No devout Catholic can support abortion."

"But I'm a devout Catholic, and I support abortion!"

In a case such as this, what is at issue is the definition of "devout Catholic". It's not that the first person is indulging in the 'No True Scotsman' fallacy, but rather that the two people have different definitions of the term "devout Catholic". One person means "identifies as Catholic and feels strongly about it in some sense" while the other means "believe all the things that the Catholic Church teaches".