"Logical fallacy" redirects here. For an argument problematic for any reason, see Fallacy.
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Faulty deductive reasoning due to a logical flaw
In logic and philosophy, a formal fallacy, deductive fallacy, logical fallacy or non sequitur[1] (/ˌnɒnˈsɛkwɪtər/; Latin for 'it does not follow') is a pattern of reasoning rendered invalid by a flaw in its logical structure that can neatly be expressed in a standard logic system, for example propositional logic.[2] It is defined as a deductive argument that is invalid. The argument itself could have true premises, but still have a false conclusion.[3] Thus, a formal fallacy is a fallacy where deduction goes wrong, and is no longer a logical process. This may not affect the truth of the conclusion, since validity and truth are separate in formal logic.
While a logical argument is a non sequitur if, and only if, it 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.
A special case is a mathematical fallacy, an intentionally invalid mathematical proof, often with the error subtle and somehow concealed. Mathematical fallacies are typically crafted and exhibited for educational purposes, usually taking the form of spurious proofs of obvious contradictions.
A formal fallacy is contrasted with an informal fallacy which may have a valid logical form and yet be unsound because one or more premises are false. A formal fallacy; however, may have a true premise, but a false conclusion.
^Barker, Stephen F. (2003) [1965]. "Chapter 6: Fallacies". The Elements of Logic (6th ed.). New York, NY: McGraw-Hill. pp. 160–169. ISBN 0-07-283235-5.
^Harry J. Gensler, The A to Z of Logic (2010) p. 74. Rowman & Littlefield, ISBN 9780810875968
^Labossiere, Michael (1995). "Description of Fallacies". The Nizkor Project. Retrieved 2008-09-09.
In logic and philosophy, a formalfallacy, deductive fallacy, logical fallacy or non sequitur (/ˌnɒn ˈsɛkwɪtər/; Latin for 'it does not follow') is a pattern...
the context in which they are made. Fallacies are commonly divided into "formal" and "informal." A formalfallacy is a flaw in the structure of a deductive...
contain fallacies. Because of their variety, fallacies are challenging to classify. They can be classified by their structure (formalfallacies) or content...
of the argument, as is the case for formalfallacies, but can also be due to their content and context. Fallacies, despite being incorrect, usually appear...
same set, in violation of the laws of probability. It is a type of formalfallacy. I am particularly fond of this example [the Linda problem] because...
Argument from fallacy is the formalfallacy of analyzing an argument and inferring that, since it contains a fallacy, its conclusion must be false. It...
The fallacy of four terms (Latin: quaternio terminorum) is the formalfallacy that occurs when a syllogism has four (or more) terms rather than the requisite...
be confused with formalfallacy, an argument whose conclusion does not follow from its premises; instead, it is that despite its formal consistency it is...
The association fallacy is a formal logical fallacy that asserts that properties of one thing must also be properties of another thing, if both things...
sometimes called converse error, fallacy of the converse, or confusion of necessity and sufficiency, is a formalfallacy of taking a true conditional statement...
The existential fallacy, or existential instantiation, is a formalfallacy. In the existential fallacy, one presupposes that a class has members when one...
An ecological fallacy (also ecological inference fallacy or population fallacy) is a formalfallacy in the interpretation of statistical data that occurs...
Affirming a disjunct – Formalfallacy Essentialism – View that entities have identifying attributes Fallacy of composition – Fallacy of inferring on the...
The fallacy of the undistributed middle (Latin: non distributio medii) is a formalfallacy that is committed when the middle term in a categorical syllogism...
caused B. When B is undesirable, this pattern is often combined with the formalfallacy of denying the antecedent, assuming the logical inverse holds: believing...
called fallacies. For formalfallacies, like affirming the consequent, the error lies in the logical form of the argument. For informal fallacies, like...
usually divided into formal and informal fallacies. Formalfallacies are unsound because of their structure, while informal fallacies are unsound because...
the antecedent, sometimes also called inverse error or fallacy of the inverse, is a formalfallacy of inferring the inverse from an original statement....
deductive arguments, which do not follow a rule of inference, are called formalfallacies. Rules of inference are definitory rules and contrast with strategic...
question or assuming the conclusion (Latin: petītiō principiī) is an informal fallacy that occurs when an argument's premises assume the truth of the conclusion...
contentious Formalfallacy, reasoning of invalid structure Informal fallacy, the complement Informal mathematics, also called naïve mathematics Formal cause...
informal fallacies, critical thinking, and argumentation theory. It examines arguments expressed in natural language while formal logic uses formal language...
The formalfallacy of affirming a disjunct also known as the fallacy of the alternative disjunct or a false exclusionary disjunct occurs when a deductive...
The base rate fallacy, also called base rate neglect or base rate bias, is a type of fallacy in which people tend to ignore the base rate (e.g., general...
Affirmative conclusion from a negative premise (illicit negative) is a formalfallacy that is committed when a categorical syllogism has a positive conclusion...
conclusion. They can be categorized as formal or informal fallacies depending on whether they belong to formal or informal logic. Logic has mostly been...
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