An argument advanced by some defenders of Judge Kavanaugh is that Dr. Blasey’s claim that he assaulted her should be rejected because the Democrats stand to gain from it. In reply, some defenders of Dr. Blasey contend that Kavanaugh’s claim is untrue because the Republicans stand to gain if he is telling the truth. This provides an important real-world context in which to discuss the philosophical method of parity of reasoning as well as the relevance of bias in assessing claims.
The basic idea between the method of parity of reasoning is that if the structure of logic of one argument is good (or bad), then an argument with the same logical structure will thus (by analogy) be good (or bad). In the case of deductive logic, if an argument is valid (such that if all the premises are true, then the conclusion must be true), then any argument of the same form is also valid. Likewise, if a deductive argument is invalid (such that all the premises could be true while the conclusion is false), then any argument of the same form is also invalid. To illustrate, consider the following valid argument:
Premise 1: If today is Thursday, then tomorrow is Friday.
Premise 2: Today is Thursday.
Conclusion: Tomorrow is Friday.
Since the argument is valid, any argument using the same reasoning will also be valid—regardless of the content. This argument is, obviously enough, modus ponens (also known as affirming the antecedent). It has the following form:
Premise 1: If P, then Q.
Premise 2: P
Thus, deductive arguments are ideal for using the method of parity of reasoning—in fact, this method of analogy is a standard tool for assessing whether an argument is valid or invalid. The main downside is that using this method does require already knowing the relevant valid and invalid argument forms.
In the case of inductive logic (arguments in which true premises are supposed to make it likely that the conclusion is true), the matter is more complicated. This is because two inductive arguments could have the same logical form, yet one could be strong and the other weak. This is because the strength of an inductive argument is not just a matter of the logical structure, but also a matter of the specific standards used to assess inductive reasoning. Consider, for example, an argument by analogy:
Premise 1: X has properties P, Q, R.
Premise 2: Y has properties P, Q, R.
Premise 3: X has property Z.
Conclusion: Y has property Z.
Strong and weak arguments of this sort have the same form; they differ in the strength of the reasoning based on how similar X and Y are in relevant ways. As such, the strength of an inductive argument cannot be assessed just by considering the structure of the reasoning. I now turn to one of the arguments used to argue against the claim made by Dr. Blasey.
One of the arguments being deployed against the Democrats is this one:
Premise 1: The Democrats claim that Blasey’s claim (that she was assaulted by Kavanaugh) is true.
Premise 2: The Democrats have a political interest in blocking Kavanaugh.
Conclusion: Blasey’s claim is false.
While this reasoning will probably appeal to those who oppose the Democrats, it is important to consider the form of the argument and not just whether the claims are appealing to one’s ideology. Put in the abstract, the argument has the following form:
Premise 1: Party P claims C about subject S.
Premise 2: Party P has a political interest in subject S.
Conclusion: Claim C is false.
If this logic is good, then the following argument against the Republicans would, by parity of reasoning, also be good logic:
Premise 1: The Republicans claim that Blasey’s claim is false.
Premise 2: The Republicans have a political interest in getting Kavanaugh through.
Conclusion: It is false that Blasey’s claim is false (that is, it is true).
While those who dislike the Republicans might find this argument appealing, Republicans would certainly reject it. But, if the logic works against Democrats, then it should also work just as well against Republicans. Logic is, after all, without partisan bias.
Fortunately for both parties, this inductive argument is rather weak. Even if a party has an interest in a claim being false it does not follow that it is false. To conclude that interest in a claim (that is, having a stake in the matter) disproves a person’s claim is a classic ad hominem fallacy. The specific version is often called the circumstantial ad hominin, because the claim is discredited because of the source’s circumstances (in this case, having a stake in the matter). To illustrate, imagine that Sam owns a small bookstore and argues that a tax break for small business owners would be good for the economy. While it is sensible to consider his bias, it does not follow that these tax breaks would not be good for the economy.
Interest (having a stake in a matter) is, however, relevant to assessing the credibility of a source. If a party has an interest in a claim, then they have less credibility in this matter than a similar source that is not impacted by this biasing factor. Disproving a claim requires more than just showing interest or bias in the matter, it requires directly assessing the claim. As such, the fact that the Democrats would benefit from Blasey’s accusation being true does not prove it is not true. Likewise, the fact that the Republicans would benefit from it being true that Kavanaugh is innocent does not prove he assaulted Blasey.