Random Thoughts Thread

[Montmorency]

Two of the most deeply-misunderstood logical fallacies are the "Ad Hominem" and the "No True Scotsman".


1. Ad Hominem

Calling someone a name does not automatically have any bearing whatsoever on one's arguments. Unless one explicitly predicates those arguments on some individual possessing some (presumably bad) trait(s), in which case the user is open to attack just in case it can be shown that the ad hominem is actually false or invalid.

You are a killer, therefore the political views you hold must be wrong.

Fallacy?

You are a killer, therefore you must hold wrong political views.

Fallacy?

You are a killer, therefore you love Hitler

Fallacy?

It is perfectly possible, though likely false, that membership in the set of killers should in itself entail membership in the set of Hitler-lovers. Thus the statement is not fallacious by the Ad Hominem.

Bonus: The same standards of relevance and possibility apply to the "Tu Quoque" fallacy, another oft-misunderstood fallacy.


2. No True Scotsman

This fallacy is one of equivocal or circular post-hoc reasoning. It does not comprehensively invalidate statements of the form "No 'legitimate' X..." or "An entity A can not be of Set X if it is a member of Set Y".

A: Is he a Scotsman? No Scotsman can jump.

B: He is a Scotsman based on multiple independent criteria, yet he can jump.

A: In that case those independent criteria must be wrong or irrelevant, as no true Scotsman can jump.

Fallacy?

A: Is he a Scotsman? No Scotsman can jump.

B: Yes, and he can jump.

A: In that case, he can't be a true Scotsman, as no true Scotsman can jump.

Fallacy?

In this case, the burden is on B to elucidate other factors that determine or preclude membership in the set of Scotsmen. Otherwise, all we have is B's unsupported affirmation of some entity's membership in both the set of Scotsmen and the set of people who can jump.

To make it even clearer, here's a fallacious argument obviously based on equivocation:

A: Is he a Scotsman? No Scotsman can jump.

B: Yes, and he can jump.

A: But he just beat that black guy in basketball, so clearly he can jump, and therefore is not a true Scotsman.

Ultimately, it would probably be best if in almost-all cases people just stopped referencing fallacies in their discussions at all.