Lives of Eminent Philosophers 7.81 — Diogenes Laërtius on the Stoics

Diogenes Laërtius, Lives of Eminent Philosophers 7.81

Reported speech · Diogenes Laërtius on the Stoics

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.” 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.