Book 7
1.81
The fourth kind employs a disjunctive proposition and one of the two alternatives in the disjunction as premisses, and its conclusion is the contradictory of the other alternative; e.g. Either A or B; but A is, therefore B is not. The fifth kind is that in which the argument as a whole is constructed of a disjunctive proposition and the contradictory of one of the alternatives in the disjunction, its conclusion being the other alternative; e.g. Either it is day or it is night; but it is not night, therefore it is day.
1.81
From a truth a truth follows, according to the Stoics, as e.g. It is light from It is day; and from a falsehood a falsehood, as It is dark from It is night, if this latter be untrue. Also a truth may follow from a falsehood; e.g. from The earth flies will follow The earth exists; whereas from a truth no falsehood will follow, for from the existence of the earth it does not follow that the earth flies aloft.
