Lives of Eminent Philosophers 7.80 — Diogenes Laërtius on the Stoics

Diogenes Laërtius, Lives of Eminent Philosophers 7.80

Reported speech · Diogenes Laërtius on the Stoics

The first kind of indemonstrable statement is that in which the whole argument is constructed of a hypothetical proposition and the clause with which the hypothetical proposition begins, while the final clause is the conclusion; as e.g. “If the first, then the second; but the first is, therefore the second is.” The second is that which employs a hypothetical proposition and the contradictory of the consequent, while the conclusion is the contradictory of the antecedent; e.g. “If it is day, it is light; but it is night, therefore it is not day.” Here the minor premiss is the contradictory of the consequent; the conclusion the contradictory of the antecedent. The third kind of indemonstrable employs a conjunction of negative propositions for major premiss and one of the conjoined propositions for minor premiss, concluding thence the contradictory of the remaining proposition; e.g. “It is not the case that Plato is both dead and alive; but he is dead, therefore Plato is not alive.”