No True Scotsman Informal Fallacy

The no true Scotsman fallacy is an informal fallacy. A fallacy is an example of bad reasoning. Formal fallacies deal with the structure and form of an argument. Arguments that use informal fallacies may have logically valid structure, but they still represent bad habits for thinking.

The no true Scotsman fallacy takes its name from an example like the following:

1. No true Scotsman would put sugar in his tea.

2. Peter puts sugar in his tea.

3. Therefore, Peter is not a true Scotsman.

Again, notice how this argument follows a logically valid form (modus tollens)

1. If someone is a true Scotsman, then he does not put sugar in his tea (If P, then ~Q)

2. Peter puts sugar in his tea (Q)

3. Therefore, Petere is no true Scotsman (~P)

So what is problematic, or fallacious about this argument? The argument relies on a very narrow definition of what a ‘true Scotsman’ is. To show the problem, imagine the following exchange:

Bob: All Californians love Arnold Schwarzenegger.

Jay: But I hate Arnold and I am a Californian!

Bob: Well, you’re not a true Californian, then!

In the above exchange, Bob has not given reasons to support his claim (All Californians love the Schwarz). Rather, Bob attempts to argue just by narrowing the definition of the subject about which he is talking. He's not giving reasons to support his claims; he's just trying to make his claim more narrow to make it sound legitimate.   

This informal fallacy is relevant because we should ask if this is all that Frankfurt is doing when he responds that Van Inwagen isn't talking about 'real' free will.