This is based on the philosophical principle that "things are constantly moving and developing".

People cannot step into the same river twice because things are constantly changing, moving and developing. The river is constantly flowing. The river you step into for the first time is different from the river you step into the second time. . Because the water of the river is flowing and the entire river is changing and developing, the two rivers before and after are no longer the same river. The "same river" here means "the exact same river."

And "people cannot step into a river even once." The moment a person steps into the river, at that point in time, the person has indeed stepped into the river. His students only believed that the river was always changing from the philosophical perspective of "things are constantly moving and developing", so they could not step into it. However, another principle in philosophy, "whole and part", is ignored. Even if the river is always changing, it is also composed of countless points in time. At that point in time, the movement is relatively static. At this time, people can step into a river. river.