....................Do you ask what is the foundation of a sound mind? It is, not to find joy in useless things. I said that it was the foundation; it is really the pinnacle. We have reached the heights if we know what it is that we find joy in and if we have not placed our happiness in the control of externals. The man who is goaded ahead by hope of anything, though it be within reach, though it be easy of access, and though his ambitions have never played him false, is troubled and unsure of himself. Above all, my dear Lucilius, make this your business: learn how to feel joy.
Related audiobook: The Moral Epistles: 124 Letters to Lucilius
Related quotes, which I've also added to the Stoicism quotes, thoughts, and readings page:
"People look for retreats for themselves, in the country, by the coast, or in the hills; and you too are especially inclined to feel this desire. But this is altogether unphilosophical, when it is possible for you to retreat into yourself at any time you want. There is nowhere that a person can find a more peaceful and trouble-free retreat than in his own mind, especially if he has within himself the kind of thoughts that let him dip into them and so at once gain complete ease of mind; and by ease of mind, I mean nothing but having one’s own mind in good order. So constantly give yourself this retreat and renew yourself. You should have to hand concise and fundamental principles, which will be enough, as soon as you encounter them, to cleanse you from all distress and send you back without resentment at the activities to which you return." —Marcus Aurelius
"Practice, then, from the very beginning to say to every rough impression, ‘You’re an impression and not at all what you appear to be.’ Then examine it and test it by the standards that you have, and first and foremost by this one, whether the impression relates to those things which are within our power or those which aren’t up to us; and if it relates to those things which aren’t within our power, be ready to reply, ‘That’s nothing to me’." —Epictetus