From The Essays of Montaigne:
We are never present with, but always beyond ourselves: fear, desire, hope, still push us on towards the future, depriving us, in the meantime, of the sense and consideration of that which is to amuse us with the thought of what shall be, even when we shall be no more.—[Rousseau, Emile, livre ii.]
"Calamitosus est animus futuri auxius."
["The mind anxious about the future is unhappy."
—Seneca, Epist., 98.]
We find this great precept often repeated in Plato, "Do thine own work, and know thyself." Of which two parts, both the one and the other generally, comprehend our whole duty, and do each of them in like manner involve the other; for who will do his own work aright will find that his first lesson is to know what he is, and that which is proper to himself; and who rightly understands himself will never mistake another man's work for his own, but will love and improve himself above all other things, will refuse superfluous employments, and reject all unprofitable thoughts and propositions. As folly, on the one side, though it should enjoy all it desire, would notwithstanding never be content, so, on the other, wisdom, acquiescing in the present, is never dissatisfied with itself. —[Cicero, Tusc. Quae., 57, v. 18.]—Epicurus dispenses his sages from all foresight and care of the future.