Ad hominem fallacies are among the most common logical fallacies, but they are also among the most misunderstood. Indeed, I often see people falsely accusing their opponent of committing an ad hominem fallacy. Therefore, I am going to explain how this fallacy actually works and give you some basic tools to identify it. There are two fundamental points that you need to understand, and I will elaborate on them throughout this post. First, in order for an argument to be ad hominem, it has to actually attack its opponent. Second, not all ad hominem attacks are ad hominem fallacies.

Defining “ad hominem”

“Ad hominem” is Latin for “to the man” or “to the person,” and it simply occurs anytime that you attack your opponent. This is very, very important. In order for an argument to be ad hominem it has to verbally assault its opponent. So, attacks against a person’s intelligence, race, gender, appearance, morality, etc. all count as ad hominem, but attacks against a person’s argument are not ad hominem (even if they are written in a hostile style). This seems very simple, and indeed it is, but somehow, people continuously mess this up and take criticisms of their arguments personally. For example, in a recent debate with someone who opposed GMOs, I asked them to show me their sources, and they responded with a link to an opinion piece written for an online news outlet. When I explained to them that their link was not a valid source of scientific information and was irrelevant to the debate, they responded by accusing me of committing an ad hominem fallacy.

Indeed, this is a frequent occurrence. I try really hard to avoid ad hominem attacks on this blog, and you will rarely see me call someone an “idiot,” “moron,” etc. Nevertheless, I constantly get accused of ad hominem fallacies, and I see this occurring on other pro-science pages as well. So I want to be explicitly clear about this: pointing out a problem in an opponent’s argument, asking for their sources, criticizing their sources, etc. does not count as ad hominem. You cannot accuse someone of an ad hominem fallacy just because they disagreed with you. Unless they actually attacked you or the authors of your sources, they did not make an ad hominem argument.

While we are going down this road, it’s worth mentioning that the same rules apply to “bullying,” “being rude,” etc. I cannot even begin to tell you how many times I have been accused of “bullying” just because I repeatedly asked someone for their sources and told them that I didn’t care about their opinions. Asking someone to back up their claims with facts and logic is neither rude nor offensive. So please stop being so thin-skinned. Further, you do not have the right to write/say anything without being subject to criticism or ridicule. Yes, you are entitled to an opinion, and yes, we should all be tolerant of other peoples beliefs, but when you are making a factual claim, you are neither expressing an opinion nor belief, and you should be held accountable for the accuracy of that claim. Imagine, for example, how ridiculous it would be if someone who didn’t accept gravity became upset when people ridiculed their absurd views. Even so, when you make factually incorrect statements about vaccines, evolution, climate change, etc. you aren’t expressing an opinion, you’re just wrong, and no one should be tolerant of your nonsense.

Not all ad hominem attacks are fallacies

Having now established what it means for something to be ad hominem, it is important to discuss what makes an ad hominem attack a fallacy. There are three basic uses of ad hominem assaults, only one of which is fallacious. The first is simply name calling for the sake of name calling, and the title to this post was intended to be a sarcastic example of this. It is ad hominem, and it’s certainly in bad taste (at least it would be if it wasn’t sarcasm), but it’s not actually a fallacy because it’s not being used as an argument. In other words, simply insulting someone isn’t enough to make something a fallacy. In order to be an ad hominem fallacy, you have to use an attack on a person as a means of attacking their argument. In the case of my sarcastic title, I got your attention by using hostile language, then proceeded to actually explain the logic of how these fallacies work; therefore, I did not commit an ad hominem fallacy (i.e., my insult was just an insult, not an argument).

The second use of ad hominem arguments is the really problematic one. It occurs when you are presented with an argument, and you respond by criticizing the person making the argument rather than the argument itself. For example, if someone is presented with an argument and simply responds with, “you’d have to be an utter moron to believe that,” then an ad hominen fallacy has been committed, because they simply attacked the people who accept the argument without ever addressing the argument itself. If, however, they said, “you’d have to be an utter moron to believe that because it commits the following logical fallacies (insert names of fallacies) and has been discredited by the following studies (insert citations)” then they would have not committed a fallacy. In other words, their comment is ad hominem, and it’s in bad taste, but it’s not a fallacy because they did not use the insult as their argument. Rather, they made a logical argument and explained the problems with their opponent’s view, and then they slapped an insult on there for no good reason. To be fallacy, the insult has to actually be part of your argument.

Amusingly, anti-scientists are generally the ones who I see committing ad hominem fallacies, even though they are also generally the ones who I see falsely accusing others of committing them. For example, the classic “shill gambit” is nearly always an ad hominem fallacy. I get accused of being a shill for Big Pharma or Big Ag almost daily ,when in reality, I receive absolutely no money from them because I support science not big industry (entertainingly no one seems bothered by the fact that I strongly oppose “Big Oil”). Nevertheless, people constantly respond to my posts with comments like, “what a shill” or “how much did Big Pharma pay you to write this?” These responses are ad hominem fallacies, because the people making them generally don’t follow up with logical criticisms of my arguments. Rather, they simply accuse me of being a shill then march off to declare victory to their fellow anti-scientists. In other words, their entire argument can be rephrased as, “you are wrong because you are a shill.” However, unless they can actually provide evidence that I am being paid off (which they can’t, since I’m not), this “argument” is fallacious because it attacks me, not my arguments.

This brings me to my final point and the third usage of ad hominem. There are situations in which you can attack the person instead of their argument without it being a fallacy. For example, let’s imaging a court room scenario where a key witness has identified the murderer, and the defense responds by providing evidence that the witness is a pathological liar. The defense’s argument is ad hominem because the attack is against the person not the person’s argument, but the attack is not fallacious because there is a serious question about this witness’s credibility. If the witness is truly a pathological liar, then they should not be trusted, and their testimony should be viewed as irrelevant. To be clear, the defense has to actually provide compelling evidence that the witness is a pathological liar in order for this argument to be valid. If they cannot back up that claim, then this argument is both an ad hominen fallacy and an ad hoc fallacy (as is the shill gambit).

Similarly, arguing that people like Vani Harri (aka the “Food Babe”), Sherri Tenpenny, Mercola, etc. shouldn’t be trusted because of the truly ludicrous claims that they have made is ad hominem, but it’s not fallacious because it raises serious and completely valid doubts about their credibility. For example, Vani Hari once argued that water crystallizes when you repeatedly say the words “Satan” or “Hitler” around it, and she was concerned that airplanes don’t contain 100% oxygen (the air you breathe is mostly nitrogen, btw). By making these claims, she has demonstrated a terrifying level of scientific ignorance and illiteracy, and she has made it completely clear that she doesn’t know what she is talking about. So, when someone says that you should not trust her because she has frequently been exceedingly wrong, they are making an ad hominem assault, but not an ad hominem fallacy, because her credibility truly is in question. To be clear, however, you do have to be careful when making this type of argument. The fact that her arguments have repeatedly been comically erroneous means that she shouldn’t be trusted, but it does not automatically mean that she is wrong. In other words, if you say, “she is wrong about X because she has repeatedly been wrong in the past,” then you have constructed a logically invalid argument because it is always possible (however unlikely) that she will eventually be right about something. You can, however, say, “she should not be trusted about X and cannot be used as a source because she has repeatedly been wrong in the past,” and there is nothing fallacious about that.

Conclusion

In summary, an argument is only ad hominem when it actually attacks someone. Simply explaining the problems with an argument or asking someone to provide sources does not not count as ad hominem, even if the explanation is given in hostile language. Further, even when an argument is ad hominem, it is only a fallacy if it is attacking the person instead of the person’s argument and if it is not merely pointing out a legitimate, relevant concern about someone’s credibility, morality, etc. So, to test whether or not an argument is an ad hominem fallacy simply follow the guide to the right

Note: originally, this article did not contain the flowchart and instead asked the following three questions:

Is the argument attacking someone? If #1 is “yes,” is the attack being used as the argument? If #2 is “yes,” does the attack raise a legitimate, relevant concern about the person’s credibility, morality, etc.?

However, that left out fallacies that are committed by using attacks as proofs, and I think that the actual diagram is easier to follow.

Other posts on logical fallacies