Avoid studies of which the result dies with the worker. Men are in error when they lament the flight of time, accusing it of being too swift, and not perceiving that it is sufficient as it passes; but good memory, with which nature has endowed us, causes things long past to seem present. Learning acquired in youth arrests the evil of old age; and if you understand that old age has wisdom for its food, you will so conduct yourself in youth that your old age will not lack for nourishment. The acquisition of any knowledge is always of use to the intellect, because it may thus drive out useless things and retain the good. For nothing can be loved or hated unless it is first known. As a day well spent procures a happy sleep, so a life well employed procures a happy death. The water you touch in a river is the last of that which has passed, and the first of that which is coming. Thus it is with time present. Life if well spent, is long. Just as food eaten without caring for it is turned into loathsome nourishment, so study without a taste for it spoils memory, by retaining nothing which it has taken in. Just as eating against one's will is injurious to health, so study without a liking for it spoils the memory, and it retains nothing it takes in. On Mount Etna the words freeze in your mouth and you may make ice of them.[Footnote 2: There is no clue to explain this strange sentence.]