Affirming a disjunct:

Formal Fallacy
Definition Example
When it is claimed that one logical disjunction must be false because the other side of the disjunct is true; A or B; A; therefore not B. My new neighbor looked so happy, I knew he had either a girlfriend or a wife. He told me he has a wife, so I know he does not have a girlfriend.

P or Q
Therefore, not Q

This fallacy is made clear when one follows the same form of the example and says “To have a girlfriend like Tom’s, you have to be either rich or famous. He’s rich, so he can’t be famous.”
Note that this fallacy does not hold for exclusive disjunctions where P and Q cannot both be true.

Case Study One

Some politicians might argue that, to lower the deficit, we’ll need to either cut programs or raise taxes, then suggest that, since they’ve already cut taxes, cutting programs wouldn’t also contribute to alleviating the problem.

Case Study Two

A very common occurance of this fallacy is when someone either explicitly or implicitly suggests that, if I can demonstrate that your religion/ideology is false, my ideology is true. Both religions/ideologies could, of course, be wrong.

Keep in mind that a fallacious argument does not entail an erroneous position.

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