Theformal fallacy ofaffirming a disjunct also known as thefallacy of the alternative disjunct or afalse exclusionary disjunct occurs when adeductive argument takes the followinglogical form:^[1]

Aor B

A

Therefore, not B

Or inlogical operators:

${\displaystyle p\vee q}$

${\displaystyle p}$

${\displaystyle ⊢}$ ¬${\displaystyle q}$

Where${\displaystyle ⊢}$ denotes alogical assertion.

The fallacy lies in concluding that onedisjunct 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 ofequivocation between the operationsOR andXOR.

Affirming the disjunct should not be confused with the valid argument known as thedisjunctive syllogism.^[2]

The following argument indicates the unsoundness of affirming a disjunct:

Max is a mammal or Max is a cat.

Max is a mammal.

Therefore, Max is not a cat.

Thisinference is unsound becauseall cats, by definition, are mammals.

A second example provides a first proposition that appears realistic and shows how an obviously flawed conclusion still arises under this fallacy.^[3]

To be on the cover ofVogue Magazine, one must be a celebrity or very beautiful.

This month's cover was a celebrity.

Therefore, this celebrity is not very beautiful.

• Exclusive or – True when either but not both inputs are true
• Logical disjunction – Logical connective OR
• Syllogistic fallacy – Failure to parse arguments using Classical Logic
• Black or white thinking – Failure to think in nuancesPages displaying short descriptions of redirect targets

• Fallacy files: affirming a disjunct
• Bennett, Robert "Bo"."Affirming a Disjunct".Logically Fallacious. Retrieved2023-07-29.

• Propositional fallacies
• Syllogistic fallacies

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