"One cannot cross the same river twice" is a famous saying of Heraclitus, an ancient Greek materialist philosopher, and Lenin called him "one of the founders of dialectics". This famous saying means that the water in the river is constantly flowing. When you step into the river this time, the water flows away. When you step into the river next time, new water flows. River. The water keeps flowing, so you can't step into the same river. Obviously, this famous saying has its specific meaning, not the difference between this river and that one. Heraclitus advocated that "everything moves" and "everything flows", which made him an outstanding representative of the "mobile school" with simple dialectical thought at that time.

Heraclitus's famous saying shows that objective things move forever. Such a truth that is changing and developing. Engels once commented: "This primitive, simple but essentially correct world view is the world view of ancient Greek philosophy, and it was clearly expressed by Heraclitus for the first time: everything exists, but it does not exist, because everything is flowing, constantly changing, constantly producing and disappearing." Heraclitus also believes that everything is transformed into each other. Cold becomes hot, hot becomes cold, wet becomes dry, and dry becomes wet. He also clearly asserted: "We went down without going down the same river. We exist and don't exist. "