19.

The department of dialectic questions remains to be discussed; for, as you are aware, all questions in the pursuit of wisdom are classified under three heads,--Ethics, Physics, and Dialectics. When, therefore, the Epicureans said that the senses are never deceived, and, though the Stoics admitted that they sometimes are mistaken, both placed in the senses the standard by which truth is to be comprehended, who would listen to the Platonists when both of these sects opposed them? Who would look upon them as entitled to be esteemed men at all, and much less wise men, if, without hesitation or qualification, they affirmed not only that there is something which cannot be discerned by touch, or smell, or taste, or hearing, or sight, and which cannot be conceived of by any image borrowed from the things with which the senses acquaint us, but that this alone truly exists, and is alone capable of being perceived, because it is alone unchangeable and eternal, but is perceived only by reason, the faculty whereby alone truth, in so far as it can be discovered by us, is found?