Fallacies: Ad Hominem
Among fallacies, this is one of the easiest to recognize. We must be careful to guard against it, but also to be careful not to recognize it where it does not exist.
“Ad Hominem” is short for “argumentum ad hominem”, which is “argument against the person” in Latin. Basically, this means that you are arguing against the qualities of the person making the argument instead of the argument itself.
We do this out loud in argumentation, but it can also act as a bias. Imagine a politician whom you despise. Now imagine that they present an idea on a topic you hadn’t much considered. Your gut reaction is probably to hate the idea.
Let’s look, though, to a more classical example of ad hominem.
A: “Carl Carson says X.”
B: “Well, Carl Carson is a serial killer.” (so, X is false)
Let’s assume that Carl in the example above is a serial killer. That does not automatically mean that all of his beliefs are false. The two aren’t related. His moral failings don’t impact his judgment on unrelated issues. His knowledge of the price of gas, for example, isn’t impacted.
There are, however, cases where questioning the source is valid. Calling out bias or self-interest, for example.
A: “Carl Carson says that Chevy makes the best cars.”B: “Carl is the rep at the Chevy dealership where you are shopping. He just wants to make a sale.”
In this example, it is not necessarily the case that Chevy doesn’t make the best cars. It is, however, valid to call his motives for making the claim. Even if Carl believes the claim, he might have been influenced by his time working for Chevy. Rejecting this counterargument as “Ad Hominem” would not be valid in this case.
Let’s add the following sentence, however, to the previous discussion:
B: “Therefore you know Chevy cars are crap or someone else besides Carl would make that claim.”
At this point, B is committing Ad Hominem. Referencing a biased source may not be strong support for a claim, but having done so is hardly proof against it. The follow-up of “or someone else would have mentioned it” is also problematic, but could be best argued as “burden of proof” fallacy.
Handling
Like all fallacies, avoid using it and feel free to call it out if it is used against you.
The challenge here, really, is to practice noticing the argument from yourself and others. This can happen internally (as a bias) or externally. Keep in mind that the fact that someone uses Ad Hominem does not mean that their claim is false, but it should cause you to keep a watchful eye.
References
Both of these articles are a good read and each covers the topic better than I did myself.