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:

Tomorrow it will either rain or the sun will shine.

The weather forecast said it would rain tomorrow.

Therefore, the sun will not shine tomorrow.

This inference is obviously false, because the sun can be shining while it is raining (they are not exclusive events).

See also: syllogistic fallacy.

External links

• Fallacy files: affirming a disjunct