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Mental
Logical Fallacies
A logical fallacy is faulty reasoning in an argument that can look well-reasoned if no one notices the flaw.
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The idea
A fallacy is the use of invalid or otherwise faulty reasoning in building an argument. A fallacy can look well-reasoned if no one notices the flaw, which is what makes it persuasive.
Fallacies are usually divided into two kinds. A formal fallacy is a flaw in the structure of a deductive argument that makes the argument invalid. Affirming the consequent is one example: "If it rained, the street is wet. The street is wet, so it rained." The street could be wet for another reason. An informal fallacy comes from an error in an argument's content or context rather than its form, so an argument can be formally valid and still be fallacious. Well-known informal fallacies include the ad hominem attack, the false dilemma, begging the question and the appeal to ignorance.
People sometimes use fallacies on purpose, to manipulate or persuade by deception. They also slip into them by accident, through carelessness, cognitive or social biases, ignorance, or the limits of language. Vague or ambiguous wording is a common source of informal fallacies. A fallacy does not always lead to a false conclusion: the conclusion may happen to be true even though the reasoning behind it is invalid.
Check yourself
Question 1 of 2
Which of these is a formal fallacy?
Composition
what it's made of
- Formal fallacies — Flaws in the structure of a deductive argument that make it invalid.
- Informal fallacies — Errors in an argument's content or context rather than its logical form.
- Fallacies of ambiguity — Informal fallacies rooted in ambiguous or vague language.
- Fallacies of presumption — Informal fallacies that rest on unjustified assumptions.
- Fallacies of relevance — Informal fallacies whose reasons do not actually bear on the conclusion.
Connection
how it relates
Under Logic and Reasoning
Change
how and why it evolves
- Past
- The term fallacy entered the Western intellectual tradition through the Aristotelian work De Sophisticis Elenchis. Traditionally, a great number of informal fallacies were later identified and named.
- Now
- There is still no general agreement on how to group fallacies into categories. Newer approaches, such as the dialogical and epistemic approaches, do not count some traditional informal fallacies as fallacious in every case.
- Next
- As more arguing happens online and in fast-moving media, teaching people to spot fallacies will likely remain an important part of learning to think critically.