"One cannot walk into the same river twice" is a famous saying of Heraclitus, an ancient Greek materialist philosopher. This is based on the philosophical truth that "things are constantly moving and developing".

People can't walk into the same river twice, because things are constantly changing, moving and developing, and rivers are constantly flowing. The river they stepped into for the first time was different from the river they stepped into for the second time. Because the river is flowing, the whole river is changing and developing, and the two rivers before and after are not the same. The "same river" here means "exactly the same river".

And "people can't even step into a river once", the moment people step into this river, at that time, people really step into this river. His students only think that rivers are always changing from the philosophical point of view that things are constantly moving and developing, so they can't step into them. However, it ignores another principle in philosophy, "whole and part". Even though the river is always changing, it is made up of countless time points. At that time, the movement is relatively static, and people can step into a river at this time.