non sequitur

In logic and philosophy, a formal fallacy is a pattern of reasoning with a flaw in its logical structure (the logical relationship between the premises and the conclusion). A formal fallacy is contrasted with an informal fallacy. A formal fallacy must have an invalid logical form and thus be unsound. An informal fallacy, however, may have a valid logical form and yet be unsound because one or more premises are false. An argument can be both a formal fallacy and an informal fallacy. In everyday conversation, the term logical fallacy usually refers to a formal fallacy. While "the logical argument is a non sequitur" is synonymous with "the logical argument is invalid", the term non sequitur typically refers to those types of invalid arguments which do not constitute formal fallacies covered by particular terms (e.g., affirming the consequent). In other words, in practice, "non sequitur" refers to an unnamed formal fallacy.

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