Affirming a Disjunct

This fallacy takes the following form:

P1. A or B

P2. A

C. Therefore Not B

Explanation: this fallacy reasons that one disjunct must be false because the other is known to be true. This is fallacious because it is possible that both disjuncts are true. This fallacy may arise due to misunderstanding the logical operator ‘or’ as being equivalent to ‘exclusive or’ (one or the other but not both), when in fact logical-or is inclusive (one or the other or both).

Example: “Someone either works for the money or because they genuinely like what they do. I know that he is desperate to earn a lot of money, so deep down he must really hate his job.”

Further Reading

Affirming a disjunct: overview from LogicallyFallacious

Affirming a disjunct: overview from philosophy-index.com

Affirming a disjunct: overview from FallacyFiles