Discourses: Book 1 20 : 1

Every art and faculty makes certain things the special object of its contemplation. Now when the art or faculty itself is of like kind with what it contemplates, it becomes inevitably self-contemplative; but when it is of unlike kind, it cannot contemplate itself. For example, the art of leather-working has to do with hides, but the art itself is altogether different from the material of hides, wherefore it is not self-contemplative. Again, the art of grammar has to do with written speech; it is not, therefore, also itself written speech, is it? Not at all. For this reason it cannot contemplate itself. Well then, for what purpose have we received reason from nature? For the proper use of external impressions. What, then, is reason itself? Something composed out of a certain kind of external impressions. Thus it comes naturally to be also self-contemplative. Once more, what are the things that wisdom has been given us to contemplate? Things good, bad, and neither good nor bad. What, then, is wisdom itself? A good. And what is folly? An evil. Do you see, then, that wisdom inevitably comes to contemplate both itself and its opposite? Therefore, the first and greatest task of the philosopher is to test the impressions and discriminate between them, and to apply none that has not been tested. You all see in the matter of coinage, in which it is felt that we have some interest, how we have even invented an art, and how many means the tester employs to test the coinage—sight, touch, smell, finally hearing; he throws the denarius down and then listens to the sound, and is not satisfied with the sound it makes on a single test, but, as a result of his constant attention to the matter, he catches the tune, like a musician. Thus, where we feel that it makes a good deal of difference to us whether we go wrong or do not go wrong, there we apply any amount of attention to discriminating between things that are capable of making us go wrong, but in the case of our governing principle, poor thing, we yawn and sleep and erroneously accept any and every external impression; for here the loss that we suffer does not attract our attention.