As this is being written, Michael Cohen is testifying before the same congress he was convicted of lying to. This time he claims that he is telling the truth, despite that past lie. As would be expected, Democrats are focused on using Cohen’s testimony against Trump and the Republicans. As would also be expected, Republicans are claiming the hearing is a waste of time and are focusing on discrediting Cohen. As would be suspected, the public will tend to line up along ideological lines—the left accepting the negative things Cohen say about Trump, the right rushing to defend Trump by attacking Cohen.
Since Cohen lied to congress in defense of Donald Trump and has an impressive criminal record composed primarily of fraud and deceit, it is no surprise that the Republicans are focusing on these facts. The gist of their attack on him is that what he says about Trump should be rejected because he is a known liar. Interestingly, this approach can turn out to be an ad hominem fallacy or a legitimate assessment of credibility, depending on the reasoning being used.
The ad Hominem is a general category of fallacies in which a claim or argument is rejected based on some irrelevant fact about the author of or the person presenting the claim or argument. Typically, this fallacy involves two steps. First, an attack against the character of person making the claim, her circumstances, or her actions is made (or the character, circumstances, or actions of the person reporting the claim). Second, this attack is taken to be evidence against the claim or argument the person in question is making (or presenting). This type of “argument” has the following form:
Premise 1. Person A makes claim X.
Premise 2. Person B makes an attack on person A.
Conclusion. Therefore A’s claim is false.
The reason why an ad Hominem (of any kind) is a fallacy is that the character, circumstances, or actions of a person do not (in most cases) have a bearing on the truth or falsity of the claim being made (or the quality of the argument being made).
If the Republican argument is as follows, then it would be the ad hominem fallacy:
Premise 1. Michael Cohen claims Trump knew in advance about the WikiLeaks plan.
Premise 2. Michael Cohen is a liar.
Conclusion. Therefore, Trump did not know in advance about the WikiLeaks plan.
This is obviously bad logic, as is shown by the following example:
Premise 1. Michael Cohen said that he is married.
Premise 2. Michael Cohen is a liar.
Conclusion. Therefore, Michael Cohen is not married.
Or, to really drive the point home,
Premise 1. Michael Cohen said that 2+2=4.
Premise 2. Michael Cohen is a liar.
Conclusion. Therefore, 2+2 does not equal 4.
So, to conclude that a specific claim made by Cohen is false because Cohen is a liar and a convicted criminal would be to fall victim to this fallacy. As always, it must be noted that the conclusion of a fallacy can be true—to infer that a fallacy must have a false conclusion would be to fall victim to the fallacy fallacy. As such, the claims that Cohen makes must be assessed on their merits—the fact that he is a criminal and a liar is not evidence that any specific claim he makes is not true.
At this point one might be thinking the obvious—surely the fact that he is a liar and a criminal has some relevance! It does—these facts reduce his credibility. When it comes to assessing credibility, many factors come into play and these obviously include the honesty and character of the person. A person who is known to be a liar and to have committed crimes of deceit clearly has reduced credibility. This reduced credibility does not, as discussed above, prove that a specific claim made by the person is false. However, reduced credibility means that one has less reason to accept a claim made by the person.
Given that Cohen has been shown to have lied to benefit Trump and himself in the past, it is rational to consider that he might be lying now. Thus, it would be sensible to be skeptical about his claims. However, a lack of credibility does not count as evidence against a claim, it just means that one has reason to be concerned about accepting a claim based on the person’s credibility. In practical terms, it would be unwise to believe a claim solely based on Cohen’s testimony. However, his lack of credibility does not diminish or reduce the support provided by other evidence. For example, the evidence is overwhelming that Cohen paid Stormy Daniels $130,000 in hush money at the behest of Trump. The fact that Cohen is a liar and said this before congress does not make that claim false.
In sum, attacking Cohen does not prove his specific claims are false—they must be assessed on their own merits. However, Cohen’s track record of lying does reduce his credibility so his testimony alone does not suffice to establish a claim.