Earlier I explained the fallacies of Undistributed Middle and Illicit Major/Minor. But what about the fallacies regarding the quality of the statements? One such rule of validity states,

A valid syllogism cannot have two affirmative premise and a negative conclusion.

Why is this the case? What prevents two affirmative statements from implying a negative one? The easiest way to show this is to consider counterexamples for syllogisms with two affirmative premises and a negative conclusion, in which the premises are necessarily true, and the conclusion necessarily false. We will do this with a trick. Continue reading Another rule of validity →