Aristotle

“ That Affirmative is superior to Negative Demonstration Affirmative demonstration is superior to negative. It requires fewer propositions, is more persuasive and comprehensible, and also more immediate, for the negative is only proved through the medium of the affirmative. That affirmative demonstration is superior to negative is plain from the following considerations. We may suppose that, other circumstances being similar, the demonstration which proceeds from fewer postulates, hypotheses, or premises is superior. ”

“ The same method applies to other syllogisms. In affirmative syllogisms the middle term is always used affirmatively when joined with one of the other two terms, but in negative syllogisms the middle term must be negative in one premise. ”