The Ad Hominem Fallacy: what it is and how to avoid it. Let this be a guide for the next 4 years.

It's a bit more nuanced than that though. Technically, formal logic fallacies only apply to situations where formal logic is being applied.

So if there are two nobel prize winning physicists talking about dark matter, you would expect them to both argue based on factual and scientific merit. In that case, if one quotes a paper, and the other dismisses it because the author is an asshole, that would be ad hom. If one of them dismisses the paper because the author has been caught fabricating claims, that's still ad hom, but it's getting into a bit of a grey area - the scientists are both capable of evaluating merit based on content, but if the discussion is casual, they might not be in a position to immediately perform said evaluation.

On the other hand, if two random blokes are discussing black holes on the subway, and one person quotes an academic paper, and the other one quotes the time-cube website, it's perfectly reasonable to bring a relative assessment of source quality into the discussion. That's not ad hom, because the participants are not considered to be individually capable of assessing the factual merit of the sources, and the rhetorical mode is not deductive.

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