In ancient Greece, there was a famous founder of materialist dialectics. His name was Heraclitus (540- 470 BC). He is famous for his philosophical view that "everything flows and everything does not stay". He has two famous sayings: "One cannot step into the same river twice" and "The sun is new every day". The first famous saying is that when you step into the same river for the second time, the old water has already flowed away, and you encounter brand-new water. This is undoubtedly a simple expression of materialist dialectics that "everything is in eternal movement and change".

Two people can't step into the same river at once;

Heraclitus had a student named Kratylos, who was the earliest representative of sophistry in ancient Greece. He pushed the teacher's point of view to the extreme, saying that not only can you not step into the same river twice, but you can't "even once". Kratylos believes that everything is changing and fleeting. So, you can't judge anything, and you can't tell what it is.

Kratylos's view is the sophistry of relativism. Relativism does not recognize the relative stillness of things, denies the stability of things and does not understand the dialectical relationship between motion and stillness. Materialist dialectics holds that motion and stillness are the unity of opposites. On the one hand, it recognizes that movement is eternal, unconditional and absolute, while stillness is temporary, conditional and relative. On the one hand, it is pointed out that motion and stillness are interdependent, interrelated and interpenetrating. Without stillness, motion cannot find its own scale, and we don't know what motion is, which will inevitably lead to agnosticism. Everything that moves absolutely has a relatively static side. Modern science shows that some elementary particles can only exist for ten to twenty seconds, but in this very short time, this elementary particle is this elementary particle. If it doesn't become anything else, we can know it, study it, admit its existence and admit its stillness. Otherwise, things just keep moving, so that we can't know what is moving (what you say is moving, it is changing). Why should we engage in scientific research? We have to "resign ourselves to fate".