Affirming a disjunct

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The formal fallacy of affirming a disjunct also known as the fallacy of the alternative disjunct or a false exclusionary disjunct occurs when a deductive argument takes the following logical form:

A or B
A
Therefore, it is not the case that B

Or in logical operators:

 p \vee q
 p
{} \vdash {} ¬ q

Where {} \vdash {} denotes a logical assertion.

Explanation[edit]

The 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.

Example[edit]

The following argument indicates the invalidity of affirming a disjunct:

Max is a cat or Max is a mammal.
Max is a cat.
Therefore, Max is not a mammal.

This inference is invalid. If Max is a cat then Max is also a mammal. (Remember "or" is defined in an inclusive sense not an exclusive sense.)

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

To be on the cover of Vogue Magazine, one must be a celebrity or very beautiful.
This month's cover was a celebrity.
Therefore, this celebrity is not very beautiful.

See also[edit]

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