# Formal fallacies

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.

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## 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.