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.
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.
This inference is unsound because all cats, by definition, are mammals.
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