Fallacies Involving Credibility

We rely on information passed on by other people for much of what we know. The value of such evidence depends on the credibility of the source. When we accept a conclusion on the basis of someone's testimony, our reasoning can be diagrammed as follows:

X says p. Therefore, p is true.

If such an argument is to have any logical strength, two assumptions must be true.

First, X must be competent to speak on the subject. If p is a statement in some technical area, then X must have some expertise in that area. If it is a statement about some event, X must be someone who was in a position to know what happened. Second, X must be reporting what he or she knows objectively, without bias, distortion, or deceit. In other words, X must be someone who not only knows the truth, but who also tells the truth.

We examine two fallacies:

  • Appeal to authority (argumentum ad verecundiam)

    Using testimonial evidence for a proposition when the conditions for credibility are not satisfied, or the use of such evidence is inappropriate.

  • Ad hominem

    Using a negative trait of a speaker as evidence that his statement is false or his argument is weak.


Comprehension Questions

1 Identify the fallacy committed by the following argument.

Donít criticize my country for having the KGB. Your country has the CIA.

a) appeal to authority
b) tu quoque
c) poisoning the well
2 Identify the fallacy committed by the following argument.

Of course, the generals want a higher military budget. It's their jobs that are on the line.

a) appeal to authority
b) tu quoque
c) poisoning the well
3 Identify the fallacy committed by the following argument.

The new Tumbleweed truck was judged best by the NFL's Rookie of the Year, defensive lineman Crusher Rockman. You know it has to be tough!

a) appeal to authority
b) tu quoque
c) poisoning the well


Fallacies of Context

© Copyright 1998, W.W. Norton & Co.