Affirming a disjunct

Affirming a disjunct is when you take "A or B" and "A is true" to mean that B must be false. That's a mistake, because "or" in everyday reasoning usually allows both to be true. The argument assumes that only one of the options can hold, so confirming one seems to rule out the other. In logic, unless we explicitly say "A or B but not both," we can't conclude that B is false just because A is true.

Examples

• The light is on or the battery is dead. The light is on. So the battery isn't dead.

• She's either at the office or at the gym. She's at the office. So she's not at the gym.

• The error is in the software or in the hardware. We found an error in the software. So the hardware is fine.

• He passed by studying hard or by getting help. He studied hard. So he didn't get help.

• Revenue went up because of price increases or because of more sales. It was the price increases. So sales didn't contribute.