Dialectics is a philosophical theory about unity of opposites, struggle and movement, universal connection and change and development. The core of dialectics is struggle (contradiction theory). In other words, dialectics is a theory of contradiction. Now it is the concept of philosophical evolution in the fields of thinking, nature and history, and it is also used to refer to a world outlook and methodology as opposed to metaphysics. There are only two methodologies in the world, one is dialectics and the other is metaphysics, and dialectics and metaphysics are fundamentally opposite. The basic feature of dialectics is that struggle and unity are linked. This idea is also the basic idea of dialectics as a methodology. The basic feature of metaphysics is that struggle is divorced from unity. It either only sees struggle and does not see unity, or only sees unity and does not see struggle. The "dialectics" and "metaphysics" mentioned here are both generalizations in form, not in content. Philosophers in different periods have different understandings of dialectics. Ancient Greek philosophy holds that dialectics is a method to demonstrate and analyze contradictions in propositions, expose contradictions in conversations, and overcome contradictions. German philosopher G. Hegel believes that the self-contradiction of the object of dialectics is not only a way of thinking, but also a world view. Marxism critically inherited Hegel's thought, holding that dialectics is the inherent law of the objective world itself, dialectics in thinking is the reflection of objective laws in people's minds, and dialectics is the theory of universal connection and development. Dialectics has three basic historical forms: ancient naive dialectics, idealistic dialectics represented by Hegel and Marxist materialist dialectics. In ancient Greek thinkers, dialectics has a wide range of meanings, from a rebuttal skill in debate to a systematic evaluation of definitions, and then to the study and division of the relationship between special concepts and general concepts. Heraclitus is one of the founders of simple materialist dialectics in ancient Greece. He is famous for his philosophical view that "everything flows and everything does not die". He has two famous sayings: "One cannot step into the same river twice" and "The sun is new every day". Socrates, Plato, Aristotle and so on. It is also the representative of ancient dialectics. Most of them are based on idealism and contain many reasonable dialectical cores. For example, they believe that truth is always concrete and relative, and can be transformed into opposites under certain conditions. Many schools in ancient China had simple dialectical thoughts. For example, in Taoism represented by Laozi, Laozi once had famous sayings, such as "whether there is mutual strength, whether it is difficult to complement each other, and whether the length is compared", "The weak use Tao when moving in the opposite direction", "Everything is negative and eager to think of peace", "Happiness lies in misfortune, and disaster lies in it", "Flexibility wins rigidity, teeth are just broken, and the tongue is soft". Another example is the theory of yin-yang and five elements. The Book of Changes also contains the concepts of combining rigidity with softness, mutual change of yin and yang, and endless life. Another example is the Confucian "golden mean", "too much is not enough" and so on. All these explain the relativity of things; Things are moving, changing and developing; Nothing in the world is absolutely unchanged, only change itself. In particular, China's military classic The Art of War is also full of dialectics. The book discusses the opposition and transformation of a series of war-related contradictions, such as enemy and me, subject and object, number of people, strength, attack and defense, advance and retreat, victory and defeat, strangeness, reality, courage, interest, mobility, straightforwardness, pros and cons, life and death. The book's "Know yourself and know yourself, and win every battle" and "defeat the enemy" represent the simple materialistic thought of "subjective should conform to objective" and the dialectical thought of "concrete analysis of specific problems". The book also emphasizes the conditions for the transformation of opposites, the most important of which is people's subjective initiative. Sun Tzu believes that the victory or defeat of a war depends not only on the objective situation, but also on whether the subjective guidance to the war is correct. On the one hand, he said that "victory is knowable, but it cannot be done", and victory is foreseeable, but it can not be achieved by subjective wishes; On the other hand, he also said that "victory can be achieved", because as long as we study the specific situation of the enemy and ourselves, correctly decide on actions, adopt correct strategies and tactics, and foster strengths and avoid weaknesses, we can create conditions for victory. It can be said that the Art of War put forward its war strategy and tactics on the basis of studying various contradictions and their transformation conditions in war. Many people think that ancient dialectics is simple and intuitive, and most of them only describe the general nature of the whole world, and they are not clear about the elements and parts that make up the world, so it is incomplete and has not formed a complete dialectical ideological system. From the Stoic philosopher's era to the end of the Middle Ages, idealist dialectics has been closely related to formal logic. Later, Kant used the term "transcendental dialectics" to express his efforts to reveal hallucinations, which appeared when he tried to apply intellectual categories and principles outside the scope of phenomena and possible experiences. Hegel regards dialectics as: a concept is the result of its own internal contradictions and conflicts between the two sides. Everything has two sides, and it is not absolute, such as surfing the Internet.