XIII. Now let us examine the arguments which are commonly advanced by this school in opposition to these principles. But, first of all, you have it in your power to become acquainted with what I may call the foundations of their system. They then, first of all, compound a sort of art of those things which we call perceptions, and define their power and kinds; and at the same time they explain what the character of that thing which can be perceived and comprehended is, in the very same words as the Stoics. In the next place, they explain those two principles, which contain, as it were, the whole of this question; and which appear in such a manner that even others may appear in the same, nor is there any difference between them, so that it is impossible that some of them should be perceived, and that others should not be perceived; but that it makes no difference, not only if they are in every part of the same character, but even if they cannot be distinguished. And when these principles are laid down, then these men comprehend the whole cause in the conclusion of one argument. But this conclusion, thus compounded, runs in this way: “Of the things which are seen, some are true and some are false; and what is false cannot be perceived, but that which appears to be true is all of such a character that a thing of the same sort may seem to be also false. And as to those things which are perceived being of such a sort that there is no difference between them, it cannot possibly happen that some of them can be perceived, and that others cannot; there is, then, nothing seen which can really be perceived.” But of the axioms which they assume, in order to draw the conclusions which they desire, they think that two ought to be granted to them; for no one objects to them. They are these: “That those perceptions which are false, cannot really be perceived;” and the second is—“Of those perceptions between which there is no difference, it is impossible that some should be of such a character that they can be perceived, and others of such a character that they cannot.”