While attacking a person’s motivations does not, in itself, disprove their claim, there is the question of the impact of motivations on credibility. The discussion begins with a look at an inductive argument in which credibility is used to support a claim. This is, obviously enough, the classic argument from authority:
Premise 1: A makes claim C about Subject S.
Premise 2: A is an authority on subject S.
Conclusion: C is true.
The strength of this argument depends on the expertise of person A as well as such factors as any bias on the part of A and the consensus of other experts in the field. If the alleged authority’s expertise is sufficiently lacking, then a fallacious appeal to authority is committed. This is because the evidence fails to warrant the conclusion. It should be noted that even a non-fallacious argument of this sort is still relatively weak since the idea is that the claim is probably true because the authority probably has good reasons/evidence for the claim. While people often think this reasoning only applies to experts, such as doctors or scientists, in certain circumstances anyone could be considered a potential expert. Because of this, it might be preferable to have an argument from credibility rather than or in addition to the argument from authority.
One key factor in assessing the expert is their degree of bias in the matter. A person’s motivation to make a claim is relevant to this factor, since motivations can be biasing. For example, if the Democrats in the senate are motivated from vengeance to attack Kavanaugh, then they would be biased and thus lose a proportional degree of credibility. Not all motivations, obviously enough, damage credibility and the key question to ask about a motivation is whether it would unduly influence the person (in a way irrelevant to the truth) to make a claim.
If a person’s motivations are sufficiently biasing, then their credibility would be effectively eliminated. In such a case, an argument from authority based on them would be a fallacy. However, even in this situation it would not follow that their claim is false. To infer that a fallacy must have a false conclusion because it is a fallacy is itself a fallacy: the fallacy fallacy. As such, one way to look at this matter is that a person’s credibility can only drop to zero (no credibility). If such a person asserted a claim, this would provide zero support—leaving the support for or against the claim unchanged.
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It is, however, interesting to consider the possibility of negative credibility: credibility so bad that it serves as evidence against a claim. This would make it, in effect, an anti-authority argument which might have the following form:
Premise 1: A makes claim C about Subject S.
Premise 2: A has negative authority/credibility on subject S.
Conclusion: C is false.
Back when I was in graduate school, we joked about this sort of thing and called this sort of person “a reliable indicator of the false.” On the face of it, there do seem to be cases in which this would be plausible reasoning. For example, a person who is consistently wrong or dishonest about a subject would seem to provide evidence against claims they make in that area. There is also the idea that if credibility can support a claim, then negative credibility can undercut a claim and help refute it.
Returning to the Democrats, if their motivations are extremely wicked, then it could be argued that they have negative credibility and are thus probably wrong or lying when they make claims about Kavanaugh. While this would certainly be appealing to those who loath the Democrats, there is the question of whether the idea of negative credibility makes sense and whether the proposed argument is good logic.
One obvious concern arises when one considers how the argument from authority works. As noted above, the idea is that one inductive concludes that an expert’s claim is probably true because they have good reasons/evidence for the claim that they make. As such, the idea is that there is a good argument behind the argument from authority. As to why people would use the argument from authority rather than the argument(s) behind it, the obvious answer is that non-experts might not understand the argument. For example, a person who does not understand advanced physics would probably not get the arguments used by experts in physics. In the case of the “anti-authority” argument, while there could be a terrible argument behind the claim that the person is using, this would not serve as evidence against the claim. This would, after all, be just the fallacy fallacy. But what about just focusing on credibility?
If you believe claims I make about my past because you find me credible, this is not a matter of there being an argument behind my claim that you trust I am using to back my claim. Rather, you would believe me because of relevant facts about me—such as my relative honesty, the accuracy of my memory, and so on. Roughly put, you would accept the claim as true because I say so and I am probably right and telling the truth. If you doubt my claims, then you would be doubting my credibility—that I am dishonest, that my memory is flawed, and so on. That is, I am probably either lying or just wrong. Obviously enough, consideration of motivations enters into this assessment.
If you suspected I had a wicked motivation to lie about my past, then you would certainly suspect that what I was saying was thus untrue. After all, if I did not have that motivation, then I would presumably tell the truth. While it would certainly be tempting to consider this good reasoning, there is still an obvious problem—while a wicked motivation would give a person a reason to lie about certain things, this motivation does not indicate whether they are lying about a particular claim (and thus be evidence the claim is false). To illustrate, imagine I am dragged before congress to answer questions and that I am full of wicked motivations. Lindsay Graham asks me if I orchestrated the accusations against Kavanaugh and I, obviously enough, deny this charge. While Graham would think of me as lacking credibility, my wicked motivations and lack of credibility in no way prove or even support the claim that I orchestrated the accusations. Also, if Graham asked me if I was mind-controlling the late-night comedians to make them mock Kavanaugh, I would deny that as well. No matter how wicked my motivations, they would not serve as evidence for that claim. What would be needed, as always, is positive evidence for the claim. As such, while wicked motivations impact credibility, they do not serve to prove (or disprove) claims.