A fallacy of relevance is one of several different types of fallacies in which an argument is either supported or refuted based on information that is actually irrelevant with regard to the argument being made. One of the most common fallacies of relevance is the use of ad hominem arguments, in which the character of a person making an argument is attacked as a way to discredit the argument without any actual points about the argument itself being made. A fallacy of relevance can also include different types of appeals, such as an appeal to authority or an appeal to ignorance.
The primary aspect of a fallacy of relevance is that someone attempts to support or discredit an argument based on information irrelevant to that argument. An ad hominem attack or argument is one of the simplest and most obvious ways in which this type of fallacy is committed. Perhaps “Bob,” for example, is arguing that convicted felons should be allowed to have the right to vote in the US. An ad hominem attack against Bob could include the fact that he is a convicted felon, probably with details about his crime and other information about his character meant to make him look bad. None of this information has anything to do with whether felons should be able to vote, however, so a fallacy of relevance has been committed.
Other common forms of fallacy of relevance include various appeals that are often made in an argument. An appeal to authority, for example, is an attempt to strengthen an argument based on the authority of the person who made it. This is the opposite of an ad hominem attack, and is used to suggest that the merits of the person making an argument should strengthen the argument itself. Since the person making a statement has no actual impact on the factual accuracy of a statement, however, this is also a fallacy of relevance.
An appeal to ignorance is also a common type of fallacy of relevance, in which someone supports his or her argument by stating that it has not been disproven. This type of argument is often used for claims that are currently difficult or impossible to fully disprove. An appeal to ignorance would be the argument that “Since no one has proven that life does not exist elsewhere in the universe, life must then exist elsewhere in the universe.” This is a fallacy of relevance, however, since it shifts the burden of proof away from the argument itself and onto the opposing viewpoint, which is irrelevant to the specific argument being made.