Fallacies are errors or tricks of reasoning. We call a fallacy an error of reasoning if it occurs accidentally; we call it a trick of reasoning if a speaker or writer uses it in order to deceive or manipulate his audience. Fallacies can be either formal or informal.
Whether a fallacy is an error or a trick, whether it is formal or informal, its use undercuts the validity and soundness of any argument. At the same time, fallacious reasoning can damage the credibility of the speaker/writer and improperly manipulate the emotions of the audience/reader.
Most formal fallacies are errors of logic: the conclusion doesn’t really “follow from” (is not supported by) the premises. Either the premises are untrue or the argument is invalid. Below is an example of an invalid deductive argument.
Premise: All black bears are omnivores.
Premise: All raccoons are omnivores.
Conclusion: All raccoons are black bears.
Bears are a subset of omnivores. Raccoons also are a subset of omnivores. But these two subsets do not overlap, and that fact makes the conclusion illogical. The argument is invalid—that is, the relationship between the premises doesn’t support the conclusion.
“Raccoons are black bears” is instantaneously recognizable as fallacious and may seem too silly to be worth bothering about. However, that and other forms of poor logic play out on a daily basis, and they have real world consequences. Below is an example of a fallacious argument:
Premise: All Arabs are Muslims.
Premise: All Iranians are Muslims.
Conclusion: All Iranians are Arabs.
This argument fails on two levels. First, the premises are untrue because although many Arabs and Iranians are Muslim, not all are. Second, the two ethnic groups are sets that do not overlap; nevertheless, the two groups are confounded because they (largely) share one quality in common. One only has to look at comments on the web to realize that the confusion is widespread and that it influences attitudes and opinions about U.S. foreign policy.
Informal fallacies take many forms and are widespread in everyday discourse. Very often they involve bringing irrelevant information into an argument or they are based on assumptions that, when examined, prove to be incorrect. Formal fallacies are created when the relationship between premises and conclusion does not hold up or when premises are unsound; informal fallacies are more dependent on the misuse of language and of evidence.
It is easy to find fairly well-accepted lists of informal fallacies, but that does not mean that it is always easy to spot them. Some moves are always fallacious; others represent ways of thinking that are sometimes valid and reasonable but which can also be misused is ways that make them fallacies.