Exposing PseudoAstronomy

November 17, 2009

Logical Fallacies: Ad hominem Follow-Up

Ad hominems and Assessing a Person’s Veracity

One thing I left out of my discussion of the ad hominem logical fallacy is that of assessing whether someone is actually being truthful or whether they may be trusted given their level of expertise with an issue.

While an ad hominem is still an ad hominem if it attacks a person rather than a claim, it can still speak towards the level of whether you should actually believe what someone claims.

For example, one of my secret pleasures is to watch Judge Judy court cases. Often during those cases, litigants – both defendants and plaintiffs – will try to bring up ad hominem attacks against their opponent in order to try to undermine the other’s case. Such as, “But he does drugs [so you shouldn’t trust him].”

While these are blatant ad hominems and the Judge often either ignores them or tells the person to just answer the question that was asked and to leave out commentary, sometimes very relevant ad hominems are allowed to stand and are explored. These are often of the circumstantial ad hominem variety. For example, if the plaintiff is suing for vandalism and brings up that the defendant has been prosecuted before for unrelated vandalism. While that does not provide any evidence as to whether the person did it this time, it does speak to the person’s character and a Judge can and often does consider that. Likewise, in federal and civil cases in “real” courts, cases will often hinge simply upon whether a jury believes one person or the other, and that is done based upon an analysis of their character as opposed to hard evidence about the actual circumstances.

Final Thoughts

While an ad hominem is still an ad hominem and does not speak at all to the actual claims or evidence that is presented, and hence it cannot be used to say whether or not that evidence is valid, they can be used and often are to asses whether the person’s claims actually should be looked at. Think of it as a “first pass:” If someone often lies about a topic, then they are unlikely to be believed about the next claim they make, regardless of whether that claim is true. For example, Rich Orman on the Dogma Free America podcast recently stated that Scientologists lie so often that if they said the sun rises in the east, he would start looking for it to come up in the west.



  1. Science, in a way, uses this. If you’ve been shown to have fudged your data, no one will look at your data in future. That Korean stem cell doc Hwang comes to mind. There’s a lot of trust that goes on in science and sometimes the ultimate punishment is losing that trust forever.

    Comment by Karl — November 25, 2009 @ 6:52 am | Reply

    • Good example, Karl. Same thing happens in astronomy. In my particular sub-sub-sub-sub-…-sub specialty of crater research, there’s one particular person who ignores all evidence that he is incorrect, and he continues to insist that he’s the correct one on a particular issue of secondary cratering, including throwing out completely wrong arguments such as, “It violates conservation of energy!” (which secondary cratering does not). The one time I saw him give a talk at a conference, I knew enough background and read enough of his inflammatory abstract to bring popcorn. I was not disappointed.

      But, no one takes him seriously any more even if he makes other claims. It’s a complete ad hominem, that we don’t look at his new claims, but that’s the price you pay when, for well over a decade, you simply are too set in your ways to examine contrary evidence.

      Comment by astrostu206265 — November 26, 2009 @ 8:35 am | Reply

  2. So when you ask for references but do not respond to them when they are given can we all just start to ignore you on the assumption that you do not have the ‘level of expertise’ required to discuss a topic with any insight?

    Comment by Generator — November 26, 2009 @ 5:18 am | Reply

    • Could you please reply to the post where this is relevant in the ongoing Comments discussion rather than in a separate post where people have no idea to what you are referring?

      Comment by astrostu206265 — November 26, 2009 @ 8:32 am | Reply

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