Think, often, of how swiftly all things pass away and are no more--the
works of Nature and the works of man. The substance of the
Universe--matter--is like unto a river that flows on forever. All things
are not only in a constant state of change, but they are the cause of
constant and infinite change in other things. Upon a narrow ledge thou
standest! Behind thee, the bottomless abyss of the Past! In front of
thee, the Future that will swallow up all things that now are! Over what
things, then, in this present life, wilt thou, O foolish man, be
disquieted or exalted--making thyself wretched; seeing that they can vex
thee only for a time--a brief, brief time! (Book v., §23.)