The Discourses of Epictetus (1.20)

Every art, and every faculty, contemplates certain things as its principal objects. Whenever, therefore, it is of the same nature with the objects of its contemplation, it necessarily contemplates itself too; but where it is of a different nature, it cannot contemplate itself. The art of shoemaking, for instance, is exercised upon leather, but is itself entirely distinct from the materials it works upon; therefore it does not contemplate itself. Again, grammar is exercised on articulate speech. Is the art of grammar itself, then, articulate speech? By no means. Therefore, it cannot contemplate itself. To what purpose, then, is reason appointed by nature? To a proper use of the phenomena of existence. And what is reason? The art of systematizing these phenomena. Thus, by its nature, it becomes contemplative of itself too.

Again, what subjects of contemplation belong to prudence? Good and evil, and that which is indifferent. What, then, is prudence itself? Good. What imprudence? Evil.

You see, then, that it necessarily contemplates both itself and its contrary. Therefore, the first and greatest work of a philosopher is to try to distinguish the phenomena of existence, and to admit none untried. Even in money, where our interest seems to be concerned, you see what an art we have invented, and how many ways an assayer uses to try its value,—by the sight, the touch, the smell, and, lastly, the hearing. He throws the piece down, and attends to the jingle; and is not contented with its jingling only once, but, by frequent attention to it, trains his ear for sound. So, when we think it of consequence whether we are deceived or not, we use the utmost attention to discern those things which may deceive us. But, yawning and slumbering over our poor neglected reason, we are imposed upon by every appearance, nor know the mischief done. Would you know, then, how very languidly you are affected by good and evil, and how vehemently by things indifferent, consider how you feel with regard to bodily blindness, and how with regard to being deceived; and you will find that you are far from being moved, as you ought, in relation to good and evil.

But trained powers and much labor and learning are here needed.

What then? Do you expect the greatest of arts to be acquired by slight endeavors? And yet the principal doctrine of the philosophers is in itself short. If you have a mind to know it, read Zeno, and you will see. It is not a long story to say, Our end is to serve the gods, and The essence of good consists in the proper use of the phenomena of existence. If you say, what then is God; what are phenomena; what is particular, what universal nature,—here the long story comes in. And so, if Epicurus should come and say that good lies in the body, here, too, it will be a long story; and it will be necessary to hear what is the principal, and substantial, and essential part in us. It is unlikely that the good of a snail should be placed in the shell; and is it likely that the good of a man should? You yourself, Epicurus, have in you something superior to this. What is that in you which deliberates, which examines, which recognizes the body as the principal part? Why light your lamp, and labor for us, and write so many books? That we may not be ignorant of the truth? But what are we? What are we to you? Thus the doctrine becomes a long story.