Affirming a Disjunct

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Affirming a Disjunct

This is also known as False exclusionary disjunct.

Two propositions are given and they are independent. When one of them turns out to be true, the arguer claims the other is false.

  • Proposition A is independent of Proposition B therefore both can be true or neither or just one.
  • Proposition A is found to be true.
  • Therefore the arguer claims that Proposition B is false. ]]


The baby has red hair, or the baby has web feet. The baby does not have web feet therefore it has red hair.

The voter may be a Republican or the voter may be a Christian. The voter is a Christian therefore we say they are not a Republican.

The truck is blockish looking or the truck is blue or the truck is not running. I know the truck is not running, therefore it's also not blue and not blockish.


In this fallacy the arguer is making a mistake of assuming that two propositions are connected when they are not. You should point this out and emphasize that these two things are independent.

The arguer is assuming that an