It is most commonly used to refer specifically to the ad hominem as abusive, sexist, racist, or argumentum ad personam, which consists of criticizing or attacking the person who proposed the argument (personal attack) in an attempt to discredit the argument. It is also used when an opponent is unable to find fault with an argument, yet for various reasons, the opponent disagrees with it.
Other common subtypes of the ad hominem include the ad hominem circumstantial, or ad hominem circumstantiae, an attack which is directed at the circumstances or situation of the arguer; and the ad hominem tu quoque, which objects to an argument by characterizing the arguer as acting or arguing in accordance with the view that he is arguing against.
Ad hominem arguments are always invalid in syllogistic logic, since the truth value of premises is taken as given, and the validity of a logical inference is independent of the person making the inference. However, ad hominem arguments are rarely presented as formal syllogisms, and their assessment lies in the domain of informal logic and the theory of evidence. The theory of evidence depends to a large degree on assessments of the credibility of witnesses, including eyewitness evidence and expert witness evidence. Evidence that a purported eyewitness is unreliable, or has a motive for lying, or that a purported expert witness lacks the claimed expertise can play a major role in making judgements from evidence.
Argumentum ad hominem is the inverse of argumentum ad verecundiam, in which the arguer bases the truth value of an assertion on the authority, knowledge or position of the person asserting it. Hence, while an ad hominem argument may make an assertion less compelling, by showing that the person making the assertion does not have the authority, knowledge or position they claim, or has made mistaken assertions on similar topics in the past, it cannot provide an infallible counterargument.
It does not include arguments posed by a person that contradict the person's actions.
A (fallacious) ad hominem argument has the basic form:
Ad hominem is one of the best known of the logical and systematic fallacies usually enumerated in introductory logic and critical thinking textbooks. Both the fallacy itself, and accusations of having committed it, are often brandished in actual discourse (see also Argument from fallacy). As a technique of rhetoric, it is powerful and used often because of the natural inclination of the human brain to recognize patterns.
The first premise is called a 'factual claim' and is the pivot point of much debate. The contention is referred to as an 'inferential claim' and represents the reasoning process. There are two types of inferential claim, explicit and implicit. The fallacy does not represent a valid form of reasoning because even if you accept both co-premises, that does not guarantee the truthfulness of the contention. This can also be thought of as the argument having an un-stated co-premise. In this example, the un-stated co-premise "everything that A claims is false" has been included, and the argument is therefore now a valid one. However in the ad hominem fallacy the un-stated co-premise is always false, thereby maintaining the fallacy. Note that this does not imply that the contention "eugenics is a bad idea" is false, merely un-supported by the pattern of reasoning below it.
This argument would generally be accepted as reasonable, as regards personal evidence, on the premise that criminals are likely to lie to protect each other. On the other hand, it is a valid example of ad hominem if the person making the claim is doing so on the basis of evidence independent of their own credibility.
In general, ad hominem criticism of evidence cannot prove the negative of the proposition being claimed:
Assuming the premise is correct, Paula's evidence is valueless, but the umpire may nonetheless have made the right call.
On the other hand, where the person taking a position seeks to convince us by a claim of authority, or personal observation, observation of their circumstances may reduce the evidentiary weight of the claims, sometimes to zero.
Mandy Rice-Davies's famous testimony, during the Profumo Affair, "Well, he would [say that], wouldn't he?", is an example of a valid circumstantial argument. Her point is that since a man in a prominent position, accused of an affair with a callgirl, would deny the claim whether it was true or false, his denial, in itself, carries little evidential weight against the claim of an affair. Note, however, that this argument is valid only insofar as it devalues the denial; it does not bolster the original claim. To construe evidentiary invalidation of the denial as evidentiary validation of the original claim is fallacious (on several different bases, including that of argumentum ad hominem); however likely the man in question would be to deny an affair that did in fact happen, he could only be more likely to deny an affair that never did.
Guilt by association can sometimes also be a type of ad hominem fallacy, if the argument attacks a person because of the similarity between the views of someone making an argument and other proponents of the argument.
This form of the argument is as follows:
This fallacy can also take another form:
A similar tactic may be employed to encourage someone to renounce an opinion, or force them to choose between renouncing an opinion or admitting membership in a group. For example:
Guilt by association may be combined with ad hominem abusive. For example:
A reductio ad Hitlerum argument can be seen as an example of a "guilt by association" fallacy, since it attacks a viewpoint simply because it was supposedly espoused by Adolf Hitler, as if it is impossible that such a man could have held any viewpoint that is correct.
As with regular ad hominem arguments, not all cases of inverse ad hominem are fallacious. Consider the following:
Here the arguer is not suggesting we accept Ludmila's argument, but her testimony. Her being an honest person is relevant to the truth of the conclusion (that he took the bag), just as her having bad eyesight (a regular case of ad hominem) would give reason not to believe her. However, the last part of the argument is false even if the premise is true, since having never told a lie before does not mean she isn't now.
Appeal to authority is a type of inverse ad hominem argument.