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It is impossible to step twice in the same river are the words of Heracleitus, Cf. Diels, Frag. der Vorsokratiker, i. p. 96, Heracleitus, no. 91. Plutarch refers to this dictum also in Moralia, 559 c. nor is it possible to lay hold twice of any mortal substance in a permanent state; by the suddenness and swiftness of the change in it there comes dispersion and, at another time, a gathering together; or, rather, not at another time nor later, but at the same instant it both settles into its place and forsakes its place; it is coming and going.