The ad hominem fallacy is a class of fallacies which is not only common but also commonly misunderstood. Many people assume that any personal attack is an ad hominem argument, but that isn't true. Some attacks aren't ad hominem fallacies, and some ad hominem fallacies aren't clear insults.
What the concept argument ad hominem means is "argument to the man," although it is also translated as "argument against the man." Instead of criticizing what a person says and the arguments they are offering, what we have instead is a criticism of where the arguments are coming from (the person). This is not necessarily relevant to the validity of what is said - thus, it is a Fallacy of Relevance.
The general form this argument takes is:
1. There is something objectionable about person X. Therefore, person X's claim is false.
Types of Ad Hominem Fallacy
This fallacy can be separated into five different types:
- Abusive ad hominem: The most common and well-known type of ad hominem fallacy is just a simple insult and is called the abusive ad hominem. It occurs when a person has given up attempting to persuade a person or an audience about the reasonableness of a position and is now resorting to mere personal attacks.
- Tu quoque (two wrongs don't make a right): An ad hominem fallacy which does not attack a person for random, unrelated things, but instead attacks them for some perceived fault in how they have presented their case is often called tu quoque, which means "you too." It often occurs when a person is attacked for doing what they are arguing against.
- Circumstantial ad hominem: Dismissing an argument by attacking an entire class of people who presumably accept that argument is called the circumstantial ad hominem. The name is derived from the fact that it addresses the circumstances of those who hold the position in question.
- Genetic fallacy: Attacking the origins for the position someone is proposing instead of the person or the argument is called the genetic fallacy because it is based on the idea that the original source of an idea is a sound basis for evaluating its truth or reasonableness.
- Poisoning the well: A preemptive attack on a person which questions their character is called poisoning the well and is an attempt to make the target appear bad before they even have a chance to say anything.
All of these different types of ad hominem argument are fairly similar and in some cases can appear almost identical. Because this category involves fallacies of relevance, the ad hominem argument is a fallacy when the comments are directed against some aspect about a person which is irrelevant to the topic at hand.
Valid Ad Hominem Arguments
It is important, however, to remember that an argumentum ad hominem is not always a fallacy! Not everything about a person is irrelevant to every possible topic or any possible argument that they might make. Sometimes it is entirely legitimate to bring up a person's expertise in some subject as a reason to be skeptical, and perhaps even dismissive, of their opinions about it.
2. George is not a biologist and has no training in biology. Therefore, his opinions about what is or is not possible with regards to evolutionary biology do not have a lot of credibility.
The above argument rests upon the assumption that, if a person is going to make credible assertions about what is or is not possible for evolutionary biology, then they really should have some training in biology - preferably a degree and perhaps some practical experience.
Now, to be fair pointing out the lack of training or knowledge does not qualify as an automatic reason for declaring their opinion to be false. If nothing else, it's at least possible that they have made a guess by random chance. When contrasted with the conclusions offered by a person who does have relevant training and knowledge, however, we have a sound basis for not accepting the first person's statements.
This type of valid ad hominem argument is therefore in some ways the reverse of a valid appeal to authority argument.