like manner it will be possible when the syllogisms are ostensive, to deduce to the impossible in the assumed terms when the proposition is taken contradictory to the conclusion. For the same syllogisms arise as those through conversion, so that we have forthwith figures through which each (problem) will be (concluded). It is clear then that every problem is demonstrated by both modes, (viz.) by the impossible and ostensively, and we cannot possibly separate the one from the other.
Chapter 15
In what figure then we may, and in what we may not syllogize from opposite propositions will be manifest thus, and I say that opposite propositions are according to diction four, as for instance (to be present) with every (is opposed) to (to be present) with none; and (to be present) with every to (to be present) not with every; and (to be present) with a certain one to (to be present with) no one; and (to be present with) a certain one to (to be present) not with a certain one; in truth however they are three, for (to be present) with a certain one
