Formal fallacies

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Formal fallacies are fallacies which occur in arguments in formal logic, as well as in ordinary language. A formal fallacy occurs when it produces a contradiction within the system of premises. When a system of premises produces a contradiction, the entire system must be rejected, on the basis that a system containing a contradiction will prove the truth of any proposition, including the negation of the propositions granted by the system.


List of Formal Fallacies

Propositional Logic

Affirming a disjunct is a form of logical fallacy where an alternative possibility is rejected because the first is accepted, even if both are accurate.

Affirming the consequent is a type of logical fallacy where a premise is asserted as true simply because a conclusion implied by the premise is true.

Argument from fallacy is an argument that has one or more fundamentally wrong statements or points.

False dilemma occurs when one provides only two (or a few) answers to a question, giving the illusion that these choices exhaust all possibilities, when in fact they don't.

Quantificational Logic

Existential fallacy is a type of logical fallacy when the existence of a thing is implied, when it otherwise shouldn't from the premises.

Illicit conversion is caused by the inversion of the subject and predicate in a proposition.

Proof by example is caused when an instantiation is claimed as evidence for a universal claim.

Quantifier shift occurs when the quantifiers of a statement are improperly transposed.

Syllogistic Logic

Affirmative conclusion from a negative premise is when a categorical syllogism derives a positive conclusion from a negative premise.

Exclusive premises occurs when both of the premises in a syllogism are negative.

Four-term Fallacy occurs when a categorical syllogism has more than three terms. Categorical syllogisms should only have three terms.

Illicit Major occurs when the major term is not distributed in the premises.

Illicit Minor occurs when the minor term is not distributed in the premises.

Undistributed middle occurs when the middle term is not distributed in the premises.

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