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)
testimonial evidence for a proposition when the conditions for
credibility are not satisfied, or the use of such evidence is
- Ad hominem
Using a negative trait of a speaker as evidence that
his statement is false or his argument is weak.