Here are ten quotes I have found repeatedly profitable, from authors or books I have enjoyed reading. I might post more such periodically. They appear in chronological order of the author's year of birth:
"Life, says Erasmus's Folly, is theater: we each have lines to say and a part to play. One kind of actor, recognizing that he is in a play, will go on playing nevertheless; another kind of actor, shocked to find he is participating in an illusion, will try to step off the stage and out of the play. The second actor is mistaken. For there is nothing outside the theater, no alternative life one can join instead. The show is, so to speak, the only show in town. All one can do is to go on playing one's part, though perhaps with a new awareness, a comic awareness."
"Literature is not like music; it isn't for the young; there are no prodigies in writing. The knowledge or experience a writer seeks to transmit is social or sentimental; it takes time, it can take much of a man's life, to process that experience, to understand what he has been through; and it takes great care and tact, then, for the nature of the experience not to be lost, not to be diluted by the wrong forms. The other man's forms served the other man's thoughts."
"The essence of being human [rather than a saint] is that one does not seek perfection, that one is sometimes willing to commit sins for the sake of loyalty, that one does not push asceticism to the point where it makes friendly intercourse impossible, and that one is prepared in the end to be defeated and broken up by life, which is the inevitable price of fastening one's love upon other human individuals."
"Life's splendor forever lies in wait about each one of us in all its fullness, but veiled from view, deep down, invisible ... But it is there, not hostile, not reluctant, not deaf. If you summon it by the right word, by its right name, it will come."
"I am neither a pessimist, nor an optimist; I watch, I examine, without any preconceived notion, without any prepared idealism, and I am in no hurry to reach a verdict ... I prize every fleeting pleasure, every minute of joy, for there are fewer and fewer of them ... I should not say that my present point of view is a particularly consoling one, but I have grown calmer; I have stopped being angry with life because it does not give what it cannot give."
"Every moment think steadily as a Roman and a man to do what thou hast in hand with perfect and simple dignity, and feeling of affection, and freedom, and justice; and to give thyself relief from all other thoughts. And thou wilt give thyself relief, if thou doest every act of thy life as if it were the last, laying aside all carelessness and passionate aversion from the commands of reason, and all hypocrisy, and self-love, and discontent with the portion which has been given to thee. Thou seest how few the things are, which if a man lays hold of, he is able to live a life which flows in quiet, and is like the existence of the gods; for the gods on their part will require nothing more from him who observes these things."
"When we say that pleasure is the goal, we mean ... being neither pained in the body nor troubled in the soul ... it is not possible to live pleasurably without living sensibly and nobly and justly. A just man is least troubled but an unjust man is loaded with troubles ... the pleasant life is produced not by a string of drinking-bouts and revelries, nor by the enjoyment of boys and women, nor by fish and other items on an expensive menu, but by sober reasoning."
"A nation that draws too broad a difference between its scholars and its warriors will have its thinking done by cowards and its fighting by fools."
"This, however, I know – that if every nation were to bring all its evil deeds to a given place in order to make an exchange with some other nation, when they had all looked carefully at their neighbors’ faults, they would truly be glad to carry their own back again."
Anonymous (from the Song of Creation, The Rig Veda, c. 1000 BCE):
"Who really knows? Who will here proclaim it? Whence was it produced? Whence is this creation? The gods came afterwards, with the creation of this universe. Who then knows whence it has arisen?
Whence this creation has arisen—perhaps it has formed itself, or perhaps it did not—the one who looks down on it, in the highest heaven, only he knows—or perhaps he does not know."