Affirming a Disjunct
Affirming a Disjunct is a non-validating form of argument when "or" is inclusive (see below), as it is standardly interpreted in propositional logic. As with other propositional fallacies, an argument which affirms a disjunct is most likely to seem valid when we take into consideration some further information not explicitly mentioned in the argument. In the case of Affirming a Disjunct, this is:
If we have some reason to believe that the two disjuncts are contraries, then the argument may be a valid enthymeme. In contrast, if we cannot rule out the truth of both disjuncts, then the argument is fallacious.
Most logic texts claim that "or" has two meanings:
As a form of argument, Affirming One Disjunct is perfectly valid for the exclusive sense of "or". It is only for the inclusive sense that it is a non-validating form. For this reason, if the textbook account is correct, there is a problem of ambiguity in the above two argument forms, which faces the application of Affirming One Disjunct as a fallacy. In order to accuse an argument of committing this fallacy, we must determine in which sense the "or" in the first premiss is used.