Does dialectics oppose the identity of things or concepts? It does not. Dialectics recognizes the relative identity of things or concepts. Why then does dialectics oppose the law of identity of formal logic? It is because the law of identity of formal logic is an absolute law which rejects contradictions. Dialectics acknowledges the identity of things or concepts, but asserts that they simultaneously contain contradictions and are interconnected; this kind of identity indicates the interconnection of contradictions, it is relative and temporary. Since the law of identity of formal logic is an absolute law which rejects contradictions, it cannot but advance the law of excluded middle which opposes one concept changing into another concept, or one thing changing into another thing. Dialectics on the other hand regards the identity of a thing or concept as temporary, relative and conditional; because the struggle of contradictions guides the regularities
This problem has two aspects, the first part of which is that contradiction exists in all processes; the second is that in every process the movement of contradiction exists from start to finish. This is called the universality or absoluteness of contradictions.[4-409]
Engels said “motion itself is a contradiction”. Lenin defined the law of the unity of opposites
Why do we say that contradiction is motion? Haven’t there been those who have disputed Engels’ assertion? This is because the theory of contradiction discussed by Marx, Engels, and Lenin has become the most important theoretical base of the proletarian revolution. This has led to all-out attacks by bourgeois theorists who constantly hope to overturn Engel’s law that “motion is contradiction”. Raising aloft their obstreperous refutations they have moreover produced the following reasons: the motion of things in the real world is in different instances of time, and through different points in space; when a thing is positioned at a certain point, it occupies that point, and when it moves to another point it occupies that other point. In this way, the motion of things in time and space is divided into many sections; there are no contradictions, for if there were contradictions there could be no movement.
Lenin has pointed out the absurdity of this reasoning, pointed out that this reasoning in fact, by observing continual motion as many sections in time and space and as many static conditions, results in denying motion. They do not know that when a thing occupies a new position, it is because the thing has moved in space from one point to another; namely, as a result of motion. Without the contradiction in so-called motion in which a thing occupies a point and at the same time does not occupy a point, and without this continuous and interrupted unity, the unity of motion and rest, inaction and action, motion would be fundamentally impossible. To deny contradiction is to deny motion. All motion in nature, society, and thought is this motion of the unity of contradictions.
Ceaseless contradiction is the basis of the simple forms of motion (for instance, the mechanical motion discussed above) and is moreover [p. 248] the basis of all complex forms of motion.