Affirming a disjunct

The logical fallacy of affirming a disjunct occurs in a disjunctive syllogism when an argument takes the form:

Either A or B (this is the disjunct)

A (Affirming the middle term)

Therefore, not B

The fallacy lies in concluding that B must be false because A is true; in fact they may both be true. The second, or "minor premise" must be negative in order for this kind of argument to be valid.

NOTE: if the or is really a xor then this is not a fallacy.

Example:

See also: syllogistic fallacy.