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Prior Analytics - Book II   


proposition AB is converted as in the universal syllogism, i.e 'B

belongs to some of that to some of which A does not belong': otherwise

no syllogism results because the particular premiss is negative.



6



In the second figure it is not possible to prove an affirmative

proposition in this way, but a negative proposition may be proved.

An affirmative proposition is not proved because both premisses of the

new syllogism are not affirmative (for the conclusion is negative) but

an affirmative proposition is (as we saw) proved from premisses

which are both affirmative. The negative is proved as follows. Let A

belong to all B, and to no C: we conclude that B belongs to no C. If

then it is assumed that B belongs to all A, it is necessary that A

should belong to no C: for we get the second figure, with B as middle.

But if the premiss AB was negative, and the other affirmative, we

shall have the first figure. For C belongs to all A and B to no C,

consequently B belongs to no A: neither then does A belong to B.

Through the conclusion, therefore, and one premiss, we get no

syllogism, but if another premiss is assumed in addition, a

syllogism will be possible. But if the syllogism not universal, the

universal premiss cannot be proved, for the same reason as we gave

above, but the particular premiss can be proved whenever the universal

statement is affirmative. Let A belong to all B, and not to all C: the

conclusion is BC. If then it is assumed that B belongs to all A, but

not to all C, A will not belong to some C, B being middle. But if

the universal premiss is negative, the premiss AC will not be

demonstrated by the conversion of AB: for it turns out that either

both or one of the premisses is negative; consequently a syllogism

will not be possible. But the proof will proceed as in the universal

syllogisms, if it is assumed that A belongs to some of that to some of

which B does not belong.

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