The Discourses of Epictetus, tr. by P.E Matheson, , at sacred-texts.com
Every art and faculty has certain principal things of which it is to take cognizance. When therefore the faculty itself is of like kind with the objects of which it takes cognizance, it must of necessity have power to take cognizance of itself: when it is of unlike kind, it cannot take cognizance of itself. For instance, the shoemaker's art is concerned with hides, but itself is absolutely different from the material of hides: for this reason it does not take cognizance of itself. Grammar again is concerned with written speech: is it then written speech itself? Certainly not: therefore it cannot take cognizance of itself.
For what purpose then have we received reason from nature? That we may deal with impressions aright.
What then is reason itself?
A system framed from impressions of a certain kind. Thus it naturally has the power to take cognizance of itself.
Again, sagacity has been given us. To take cognizance of what? Things good and bad and indifferent.
What is it then itself?
And what is folly?
Bad. Do you see then that of necessity sagacity has the power of taking cognizance of itself and its opposite? Therefore the primary and highest task of the philosopher is to test impressions and distinguish them and to make use of none which is untested. Consider how we have invented an art to test the currency, in which we are admitted to have some interest. Look how many means the assayer uses to test the coin —sight, touch, smell, finally hearing: he breaks the penny and attends to the sound, and is not content with hearing its note once, but by much attention gets an ear for music.
Thus, where we think it makes a serious difference to us whether we are right or wrong, we take great pains to distinguish the possible sources of error, and yet when we have to do with our Governing Principle itself, poor thing, we gape and sleep and are ready to accept any impression that comes: for we do not notice our loss.
When you wish, therefore, to realize how little concerned you are about good and evil, and how eager about things indifferent, consider how you regard physical blindness on the one hand, and mental delusion on the other, and you will recognize that you are far from having a proper feeling in regard to things good and evil.
'Yes, but it needs much preparation and much toil and study.'
What of that? Do you expect that a brief study will enable you to acquire the greatest art? Yet the principal doctrine of philosophers itself is brief enough. If you will learn it, read Zeno's words and you will see. For it is no long matter to say man's end is to follow the gods, and the essence of good is the power of dealing rightly with impressions.
'Tell us then what is "God," and what is "impression," and what is nature in the individual, and what in the universe'.
That is a long story.
Again, if Epicurus should come and say, that the good must be in the flesh, that too means a long discussion; it means we must be taught what is the commanding faculty in us, what constitutes our substantial and true nature. If it is not probable that the good of the snail is in the shell, is it probable that man's good is in his body? Take yourself, Epicurus. What is the more masterful faculty you possess? What is it in you which deliberates, which examines everything, which examines the flesh itself and decides that it is the principal thing? Why do you light a lamp and toil for us, and write such big volumes? Is it that we may not be ignorant of the truth? Who are we? What concern have we with you? So the argument becomes a long one.