Observing nature, studying history, we often pay attention to the fact that many events occurring at different times and in different places are very similar to each other, as if repeating each other. Is this a coincidence? And if not, then what place does repetition occupy in the development of phenomena? The book by Academician of the USSR Academy of Sciences B. M. Kedrov (1903-1985) is dedicated to this important issue of materialist dialectics and the associated law of negation of negation. It clarifies not only the ideological meaning of the problem under consideration, but also shows its practical significance for scientific knowledge and pedagogical activity. The work uses extensive and interesting factual material. The book is intended for teachers and students of philosophical faculties of universities, as well as everyone who is interested in Marxist-Leninist philosophy.