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Logical Fallacies, Literacy Education Online; Informal Fallacies, Texas State University page on informal fallacies; Stephen's Guide to the Logical Fallacies (mirror) Visualization: Rhetological Fallacies, Information is Beautiful; Master List of Logical Fallacies, University of Texas at El Paso; Fallacies, Internet Encyclopedia of Philosophy
A formal fallacy, deductive fallacy, logical fallacy or non sequitur (Latin for "it does not follow") is a flaw in the structure of a deductive argument that renders the argument invalid. The flaw can be expressed in the standard system of logic. [1] Such an argument is always considered to be wrong.
For example, John might be going to work on Wednesday. In this case, the reasoning for John's going to work (because it is Wednesday) is unsound. The argument is only sound on Tuesdays (when John goes to work), but valid on every day of the week. A propositional argument using modus ponens is said to be deductive.
In logic and philosophy, a formal fallacy [a] is a pattern of reasoning rendered invalid by a flaw in its logical structure. Propositional logic , [ 2 ] for example, is concerned with the meanings of sentences and the relationships between them.
Deductive reasoning plays a central role in formal logic and mathematics. [1] In mathematics, it is used to prove mathematical theorems based on a set of premises, usually called axioms. For example, Peano arithmetic is based on a small set of axioms from which all essential properties of natural numbers can be inferred using deductive reasoning.
Invalid deductive arguments, which do not follow a rule of inference, are called formal fallacies. Rules of inference are definitory rules and contrast with strategic rules, which specify what inferences one needs to draw in order to arrive at an intended conclusion. Deductive reasoning contrasts with non-deductive or ampliative reasoning.
A fallacy in argumentation that targets the person making an argument rather than the argument itself. ad ignorantium A logical fallacy where a proposition is considered true because it has not been proven false or vice versa. ad infinitum An argument or process that is supposed to continue indefinitely, without ever reaching an end or conclusion.
Examples include most fields of science and aspects of personal knowledge. The terms originate from the analytic methods found in Organon , a collection of works by Aristotle . Prior analytics ( a priori ) is about deductive logic , which comes from definitions and first principles.