LFotW: Ad hominem

An ad hominem argument is any that attempts to counter another’s claims or conclusions by attacking the person, rather than addressing the argument itself. Taken broadly, this fallacy can be any argument that a person’s claims is false because of a negative quality possessed by the person. True believers will often commit this fallacy by countering the arguments of skeptics by stating that skeptics are closed minded. Skeptics, on the other hand, may fall into the trap of dismissing the claims of UFO believers, for example, by stating that people who believe in UFO's are crazy or stupid.

Sometimes this fallacy can be more subtle. For example, undesired research results can be dismissed because the researchers have an apparent conflict of interest. While true conflicts are a legitimate concern, the use of even minor potential conflicts to dismiss research finding a-priori is little more than an ad hominem logical fallacy.

Subtype – Argument from prior error
This form of ad hominem logical fallacy argues that the current claims of a person or group are incorrect because of errors committed in the past. Creationists, for example, often point to fossil frauds, or the discovery of living coelacanth previously thought to be extinct to counter the current evidence for evolution.