If a man in the fullness of his days, at the end of his life, can pass on the wisdom of his experience to those who grow up after him; if what he has learned in his youth, added to but not discarded in his maturity, still serves him in his old age and is still worth teaching the then young — then his was not an age of revolution, not counting, of course, abortive revolutions. The world into which his children enter is still his world, not because it is entirely unchanged, but because the changes that did occur were gradual and limited enough for him to absorb them into his initial stock and keep abreast of them. If, however, a man in his advancing years has to turn to his children, or grandchildren, to have them tell him what the present is about; if his own acquired knowledge and understanding no longer avail him; if at the end of his days he finds himself to be obsolete rather than wise — then we may term the rate and scope of change that thus overtook him, “revolutionary.”

— Hans Jonas, from Philosophical Essays (1974), p. 46