The Discourses of Epictetus, tr. by P.E Matheson, , at sacred-texts.com
If you give up any external possession, mind you see what you are to get in exchange for it: and if it is worth more, then never say, 'I have been a loser.' You will not lose if you get a horse for an ass, an ox for a sheep, a noble action for a piece of money, true peace instead of pedantry, self-respect instead of foul language. If you remember this you will everywhere preserve your character as it ought to be: if you do not remember it, I warn you that your time perishes for nought, and you will waste and overthrow all the pains that you now spend upon yourself. It needs but a little to overthrow and destroy everything—just a slight aberration from reason. For the helmsman to wreck his vessel, he does not need the same resources, as he needs to save it: if he turn it but a little too far to the wind, he is lost; yes, and if he do it not deliberately but from mere want of attention, he is lost all the same. It is very much the same in life: if you doze but a little, all that you have amassed up till now leaves you. Keep awake then and watch your impressions: it is no trifle you have in keeping, but self-respect, honour, constancy, a quiet mind, untouched by distress, or fear, or agitation—in a word, freedom.
What are you going to sell all this for? Look and see what your purchase is worth.
'But I am not going to sell my freedom for anything of that kind.'
Well, suppose you waive external gain, consider what the exchange is that you are making. It is yours to say, 'Self-control for me, a tribunate for him: a praetorship for him, self-respect for me. I do not clamour, when to do so is unseemly, I will not jump from my seat, when I ought not, for I am free and God's friend, to obey Him of my own free will. I must not lay claim to anything else—body, property, office, reputation, anything in short, for He does not wish me to lay claim to them: had He wished it, He would have made them good for me, but He has not done so, and therefore I cannot transgress any of His commands.' In everything you do, guard what is your own good: for the rest, be content just to take anything that is given you, so far as you may use it rationally. Otherwise you will be wretched and miserable, hampered and hindered. These are the laws that are sent you from God, these are His ordinances. These you must expound, and these obey, not those of Masurius and Cassius.