Repeatedly dwell on the swiftness of the passage and departure of things that are and of things that come to be. For substance is like a river in perpetual flux, its activities are in continuous changes, and its causes in myriad varieties, and there is scarce anything which stands still, even what is near at hand; dwell, too, on the infinite gulf of the past and the future, in which all things vanish away. Then how is he not a fool who in all this is puffed up or distracted or takes it hardly, as if he were in some lasting scene, which has troubled him for long?