Formal fallacies

A formal fallacy is a pattern of reasoning rendered invalid by a flaw in its logical structure that can neatly be expressed in a standard logic system.

An argument that is formally fallacious is always considered wrong.

Affirmative conclusion from a negative premise

Affirmative conclusion from a negative premise (illicit negative) is a formal fallacy that is committed when a categorical syllogism has a positive conclusion, but one or two negative premises.

Affirming a disjunct

Affirming a disjunct fallacy lies in concluding that one disjunct must be false because the other disjunct is true; in fact they may both be true because OR is defined inclusively rather than exclusively. It is a fallacy of equivocation between the operations OR and XOR. Affirming the disjunct should not be confused with the valid argument known as the disjunctive syllogism.

Affirming the consequent

Affirming the consequent, sometimes called converse error, fallacy of the converse or confusion of necessity and sufficiency, is a formal fallacy of inferring the converse from the original statement.

Appeal to probability

An appeal to probability (or appeal to possibility) is the logical fallacy of taking something for granted because it would probably be the case (or might possibly be the case)

Argument from fallacy

Argument from fallacy is the formal fallacy of analyzing an argument and inferring that, since it contains a fallacy, its conclusion must be false.

Base rate fallacy

Base rate fallacy, also called base rate neglect or base rate bias, is a formal fallacy. If presented with related base rate information (i.e. generic, general information) and specific information (information only pertaining to a certain case), the mind tends to ignore the former and focus on the latter.

Conjunction fallacy

The conjunction fallacy is a formal fallacy that occurs when it is assumed that specific conditions are more probable than a single general one.

Denying the antecedent

Denying the antecedent, sometimes also called inverse error or fallacy of the inverse, is a formal fallacy of inferring the inverse from the original statement.

Existential fallacy

The existential fallacy, or existential instantiation, is a formal fallacy, where we presuppose that a class has members when we are not supposed to do so; that is, when we should not assume existential import.

Fallacy of exclusive premises

The fallacy of exclusive premises is a syllogistic formal fallacy committed in a categorical syllogism that is invalid because both of its premises are negative.

Fallacy of four terms

The fallacy of four terms (Latin: quaternio terminorum) is the formal fallacy that occurs when a syllogism has four (or more) terms rather than the requisite three. This form of argument is thus invalid.

Fallacy of the undistributed middle

The fallacy of the undistributed middle (Lat. non distributio medii) is a formal fallacy that is committed when the middle term in a categorical syllogism is not distributed in either the minor premise or the major premise.

Illicit major

Illicit major is a formal fallacy committed in a categorical syllogism that is invalid because its major term is undistributed in the major premise but distributed in the conclusion.

Illicit minor

Illicit minor is a formal fallacy committed in a categorical syllogism that is invalid because its minor term is undistributed in the minor premise but distributed in the conclusion.

Masked-man fallacy

The masked-man fallacy (also known as the intensional fallacy and the epistemic fallacy) is committed when one makes an illicit use of Leibniz's law in an argument. Leibniz's law states that, if one object has a certain property, while another object does not have the same property, the two objects cannot be identical.

Negative conclusion from affirmative premises

Negative conclusion from affirmative premises is a syllogistic fallacy committed when a categorical syllogism has a negative conclusion yet both premises are affirmative. The inability of affirmative premises to reach a negative conclusion is usually cited as one of the basic rules of constructing a valid categorical syllogism.