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.
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.