Next, Aristotle compares the Affirmative with the Negative demonstration, and shows that the Affirmative is the better. Of two demonstrations (he lays it down) that one which proceeds upon a smaller number of postulates, assumptions, or propositions, is better than the other; for, to say nothing of other reasons, it conducts you more speedily to knowledge than the other, and that is an advantage. Now, both in the affirmative and in the negative syllogism, you must have three terms and two propositions; but in the affirmative you assume only that something _is_; while in the negative you assume both that something _is_, and that something _is not_. Here is a double assumption instead of a single; therefore the negative is the worse or inferior of the two.[77] Moreover, for the demonstration of a negative conclusion, you require one affirmative premiss (since from two negative premisses nothing whatever can be concluded); while for the demonstration of an affirmative conclusion, you must have two affirmative premisses, and you cannot admit a negative. This, again, shows that the affirmative is logically prior, more trustworthy, and better than the negative.[78] The negative is only intelligible and knowable through the affirmative, just as _Non-Ens_ is knowable only through _Ens_. The affirmative demonstration therefore, as involving better principles, is, on this ground also, better than the negative.[79] _A fortiori_, it is also better than the demonstration by way of _Reductio ad Absurdum_, which was the last case to be considered. This, as concluding only indirectly and from impossibility of the contradictory, is worse even than the negative; much more therefore is it worse than the direct affirmative.[80] [Footnote 77: Analyt. Post. I. xxv. p. 86, a. 31-b. 9.] [Footnote 78: Ibid. b. 10-30.] [Footnote 79: Ibid. b. 30-39.] [Footnote 80: Ibid. I. xxvi. p. 87, a. 2-30. Waitz (II. p. 370), says: "deductio (ad absurdum), quippe quæ per ambages cogat, post ponenda, est demonstrationi rectæ." Philoponus says (Schol. pp. 234-235**, Brand.) that the Commentators all censured Aristotle for the manner in which he here laid out the Syllogism [Greek: di' a)duna/tou]. I do not, however, find any such censure in Themistius. Philoponus defends Aristotle from the censure.]