must be depressed when a weight is put in it, so the mind, too, must yield to what is evident; for just as it is impossible for any animal to forbear discerning what is manifestly suited to its nature (the Greeks call that οἰκεῖον), so it is equally impossible for it to withhold its assent to a manifest fact which is brought under its notice.
Although, if those principles which we have been maintaining are true, there is no advantage whatever in discussing assent. For he who perceives anything, assents immediately. But these inferences also follow,—that memory can have no existence without assent, no more can notions of things or arts. And what is most important of all is, that, although some things may be in our power, yet they will not be in the power of that man who assents to nothing. Where, then, is virtue, if nothing depends on ourselves? But it is above all things absurd that vices should be in the power of the agents, and that no one should do wrong except by deliberate consent to do so, and yet that this should not be the case with virtue; all the consistency and firmness of which depends on the things to which it has assented, and which it has approved. And altogether it is necessary that something should be perceived before we act, and before we assent to what is perceived; wherefore, he who denies the existence of perception or assent, puts an end to all action in life.
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.
