The multitude of names given to informal fallacies can be more confusing than helpful, but nevertheless, the names of logical fallacies are important terms of art in any kind of argumentative writing or speech. Informal logical fallacies can be classified in different ways, but it is common to put them into groups such as fallacies of relevance, weak induction, ambiguity, and presumption.
Fallacies of relevance.
Slippery slope, red herring, and straw person are fallacies that change the issue under discussion to something that is easier to attack. An ad hominem argument attacks the person, rather than the issue. The fact that someone is untrustworthy, for example, does not guarantee that the conclusion of their argument is wrong or that their argument is invalid. The question of whether or not such information is relevant can, however, be rather subtle, because a person's trustworthiness would legitimately lead to questioning what they say. "Tu quoque" means "you too" and is a fallacious argument used in a debating situation to try to undermine the criticism of an opponent, but without actually presenting evidence in one's own defense. Appeal to the popularity of an idea (ad populum), to force, and to pity are further examples of arguments that present irrelevant evidence and draw attention away from the issue being debated.
Fallacies of weak induction.
Premises may present relevant information without justifying the claim made in the conclusion. A hasty generalization, for example, makes use of either too few cases or unrepresentative cases to make a broad claim. An appeal to ignorance attempts to justify a positive claim by rejecting the evidence on the other side as insufficient. To say, for example, that there is no intelligent life except on Earth, because there is no evidence of such life, is too strong a claim. However, the burden of proof is generally on those who make a positive claim, so it would be legitimate to say that there is no reason to believe in extraterrestrial intelligence. Appeal to authority is also a fallacy of weak induction. Although people constantly rely on information given to them by others, the opinions of experts alone are insufficient to justify a controversial opinion. Experts must have some reasons by which they were convinced of their beliefs and these should be communicated to others.
Fallacies of ambiguity.
The fallacy of equivocation describes an argument in which a word is used in two different senses. Such an argument can be thought of as formally invalid, but since only subtle changes of meaning are misleading, this fallacy is considered to be informal. Amphiboly is the name given to an argument that relies on ambiguity, but involving the grammar of the sentence rather than the meaning of an individual word. Composition and division are a pair of fallacies, in which an illicit inference is made from the properties of individuals in a class to the class itself, or from the class to the individuals. No one would be taken in by an argument that the concept "mammal" must be hairy because all mammals are hairy, but arguments that have seemed very compelling to many people may be of the same form. For example, some versions of the argument from design argue that the order and purpose found in every object in the universe implies that the universe as a whole has a purpose and a designer.
Fallacies of presumption.
Using the conclusion of the argument as a premise or otherwise assuming what is being claimed in the conclusion is called "begging the question." Circular reasoning appears to be formally valid, since the conclusion really does follow from the premises (P, therefore P). However, if it is assumed that P is controversial and therefore requires some kind of justification, it is clear that a circular argument will not advance the discussion. "That begs the question" is a phrase that is changing its meaning, at least in colloquial usage, to simply mean "that raises the question." This dubious usage loses any connection to the idea of logical fallacy, since the phrase is no longer being used to evaluate an argument. Begging the question is usually classified as an informal fallacy because it relies on tricking the reader into not noticing that the subject of controversy is being assumed.
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Fearnside, W. Ward. Fallacy: The Counterfeit of Argument. Englewood Cliffs, N.J.: Prentice-Hall, 1959.
Hurley, Patrick J. A Concise Introduction to Logic. Belmont, Calif.: Wadsworth, 1997.
David J. Stump