Fallacy

Fallacy (from Latin fallacia, “deceit,” from fallere, “to deceive”) — A mistake in reasoning that makes an argument invalid or persuasive without good reasons, although it seems correct. Its systematic study goes back to Aristotle, whose Sophistical Refutations (De Sophisticis Elenchis) catalogs thirteen kinds of apparent reasoning, six dependent on language (such as equivocation and amphiboly) and seven independent of it. In the Essay concerning Human Understanding (1689, IV.17), John Locke introduced the names ad verecundiam and ad ignorantiam and established ad hominem, which was already in use before him. In Fallacies (1970), C. L. Hamblin criticized the traditional treatment as superficial. A fallacy is not the same as a sophism, generally understood as a fallacy used with intent to deceive. Fallacies are divided into formal and informal.

Formal fallacies (an error in logical structure, independent of content):

Affirming the consequent: “If it rained, the street is wet; the street is wet; therefore it rained” (a water truck may have passed).
Denying the antecedent: “If it rained, the street is wet; it did not rain; therefore the street is not wet.”
Undistributed middle term (in the syllogism): “All dogs are mammals; all cats are mammals; therefore all dogs are cats.”
Four terms (quaternio terminorum): using one term in two senses within the same inference.

Informal fallacies (an error of content, relevance or language):

Ad hominem: attacking the person, not the argument.
Appeal to illegitimate authority: “A famous actor said the vaccine is dangerous.”
Straw man: refuting a distorted version of the opponent’s thesis.
False dilemma: “Either you are with us or against us.”
Begging the question (petitio principii): the conclusion is already assumed in the premises.
Appeal to ignorance: “No one has proved it does not exist, so it exists.”
False cause (post hoc ergo propter hoc): “After I changed shirts, the team won.”
Hasty generalization: concluding from too few cases.
Slippery slope: claiming without proof a chain of inevitable consequences.
Appeal to the people (ad populum): “Everyone believes it, so it is true.”
Naturalistic fallacy, treated in its own entry.

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