"Why do you delay," says he, "Why are you idle? Unless you seize the day, it flees." Even though you seize it, it still will flee; therefore you must vie with time's swiftness in the speed of using it, and, as from a torrent that rushes by and will not always flow, you must drink quickly. And, too, the utterance of the bard is most admirably worded to cast censure upon infinite delay, in that he says, not "the fairest age," but "the fairest day." Why, to whatever length your greed inclines, do you stretch before yourself months and years in long array, unconcerned and slow though time flies so fast? The poet speaks to you about the day, and about this very day that is flying. Is there, then, any doubt that for hapless mortals, that is, for men who are engrossed, the fairest day is ever the first to flee? Old age surprises them while their minds are still childish, and they come to it unprepared and unarmed, for they have made no provision for it; they have stumbled upon it suddenly and unexpectedly, they did not notice that it was drawing nearer day by day. Even as conversation or reading or deep meditation on some subject beguiles the traveller, and he finds that he has reached the end of his journey before he was aware that he was approaching it, just so with this unceasing and most swift journey of life, which we make at the same pace whether waking or sleeping; those who are engrossed become aware of it only at the end.
On the Shortness of Life 9.2