Sextus Empiricus Pyrrhoniae Hypotyposes 2.13.153

[153] For he who declares that any particular argument is inconclusive through inconsistency will, if he is merely uttering a statement, find himself opposed by a statement which contradicts his own; while if he tries to prove it by argument, he will be told that this argument of his must itself be conclusive before he can prove that the premisses of the argument said to be inconsistent are devoid of consistency. But we shall not know whether it is probative, since we have no agreed test of the syllogism whereby to judge whether the conclusion follows the logical connexion formed by the premisses. And thus, also, we shall be unable to distinguish the argument that is faulty through inconsistency from those that are conclusive.