Someone’s Right to Speak

When assessing an ad hominem argument, the first step is to determine whether its conclusion is about someone’s right to speak, about someone’s reliability, or about the truth, soundness, or strength of what is claimed. The second step is to determine whether its premises provide adequate justifica- tion for its conclusion. These steps will enable you to place the argument in the above table, but they will often not be easy or obvious. Although perjur- ers for hire almost always lie, most people exhibit some middling degree of reliability. When people are known for passing on rumors without checking their truth, this might be a reason to doubt what they say when they pass on yet another rumor (even if it is not a reason to believe that what they say is false). In assessing what they say, it would be best to look for additional evi- dence. If none is available, then we need to ask how often their testimony is true on matters of this kind. Only by careful inspection of individual cases can we determine the strength of such ad hominem arguments.

A related case occurs when someone is accused of inconsistency over time. If your neighbor says that the best time to prune roses is in the au- tumn, but then the next spring she tells you that the spring is the best time to prune roses, this inconsistency would give you some reason to doubt her expertise as a gardener. Maybe over the winter she got new information that changed her mind, but her reliability is in question until you have some ex- planation of why she would say different things at different times. If she wa- vers between contrary positions, no more than half of her views can be correct, so you have reason to ask your neighbor, “Why should I trust you?”