For unless the nature of things is thoroughly known, we shall have no means by which we can defend the judgments formed by our senses. Moreover, whatever we discern by our intellect, all arises from the senses. And if our senses are all correct, as the theory of Epicurus affirms, then something may be discerned and understood accurately; but as to those men who deny the power of the senses, and say that nothing can be known by them, those very men, if the senses are discarded, will be unable to explain that very point which they are arguing about. Besides, if all knowledge and science is put out of the question, then there is an end also of all settled principles of living and of doing anything.
Thus, by means of natural philosophy, courage is desired to withstand the fear of death, and constancy to put aside the claims engendered by superstition; and by removing ignorance of all secret things, tranquillity of mind is produced; and by explaining the nature of desires and their different kinds, we get moderation: and (as I just now explained) by means of this rule of knowledge, and of the judgment which is established and corrected by it, the power of distinguishing truth from falsehood is put into man's hands.
XX. There remains a topic necessary above all others to this discussion, that of friendship, namely: which you, if pleasure is the chief good, affirm to have no existence at all. Concerning which Epicurus speaks thus: "That of all the things which wisdom has collected to enable man to live happily, nothing is more important, more influential, or more delightful than friendship." Nor did he prove this assertion by words only, but still more by his life, and conduct, and actions. And how important a thing it is, the fables of the ancients abundantly intimate, in which, many and varied as they are, and traced back to the remotest antiquity, scarcely three pairs of friends are found, even if you begin as far back as Theseus, and come down to Orestes. But in one single house, and that a small one, what great crowds of friends did Epicurus collect, and how strong was the bond of affection that held them together! And this is the case even now among the Epicureans. However, let us return to our subject: it is not necessary for us to be discussing men.