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.
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