Written in July, 1937, subtitled "On the Relationship between Cognition and Practice-the Relationship between Knowing and Doing". It was originally a section in the second chapter of Dialectical Materialism (syllabus). The outline of the lecture was published in mimeographed form in September 1937, and was published in a separate edition of the Eighth Route Army Military and Political Magazine in Yan 'an the following year, but the author was not signed. On February 29th, 1950, 1950, People's Daily was officially published, and then it was included in the first volume of Selected Works of Mao Zedong and Mao Zedong (new edition). In our party, some dogmatic comrades have long rejected the experience of the China revolution, denied the truth that Marxism is not a dogma but a guide to action, and ate the words in Marxist books alive to scare people; There are also some empirical comrades who stick to their own piecemeal experience, fail to understand the importance of theory to revolutionary practice, and fail to see the overall situation of the revolution. Although they are also working hard, they are all blind efforts. These two kinds of comrades' mistakes, especially dogmatism, once caused great losses to the China Revolution in 193 1-1934. Dogmatists, on the other hand, put on the cloak of Marxism and confuse the broad masses of comrades. Mao Zedong's book exposes dogmatism and empiricism within the Party, especially the subjectivist errors of dogmatism from the perspective of Marxist epistemology. Because its focus is to expose dogmatic subjectivism that despises practice, it is entitled "On Practice". Mao Zedong once gave a speech at Yan 'an Anti-Japanese Military and Political University with the viewpoint of this article. Focusing on the dialectical relationship between cognition and practice, this paper comprehensively and systematically expounds and develops the basic principles of Marxist epistemology. Firstly, it expounds the basic form of practice, the relationship between practice and cognition, and the position and function of practice in cognition. It is pointed out that Marxism believes that human production activities are the most basic practical activities and social practices of human beings, which are not limited to one form of life activities, but also include many other forms, such as class struggle, political life, scientific and artistic activities and so on. In short, social people participate in all fields of social real life. Therefore, dialectical materialist epistemology puts practice in the first place, holds that human knowledge cannot be divorced from practice at all, and rejects all wrong theories that deny the importance of practice and make knowledge divorced from practice. Moreover, no matter from the source of cognition, the motive force of cognition development, or the standard to test whether cognition is correct or not, it is inseparable from human practical activities. In a word, practical viewpoint is the primary and basic viewpoint of dialectical materialism epistemology. Then, the paper expounds the development process of cognition and points out that the process of cognition has to go through two leaps: from perceptual cognition to rational cognition and from theory to practice. After realizing two leaps, the cognitive movement is both completed and unfinished, because the development process of cognition is infinite, which is the unity of stages and infinity. In addition, the article expounds the relativity and absoluteness of truth. Some emphasize absolute truth, but it exists in relative truth. Without relative truth, there is no absolute truth. The sum of countless relative truths is absolute truth. It is emphasized that Marxism-Leninism has not ended the truth, but has constantly opened up the way to know the truth in practice. The article also puts forward the task of transforming the objective world and the subjective world, pointing out that the struggle of the proletariat and the revolutionary people to transform the world includes the following tasks; Transform the objective world and transform your own subjective world. Finally, the main contents of dialectical materialism epistemology are highly summarized, pointing out that truth is discovered in practice, and truth is confirmed and developed in practice. From perceptual knowledge to rational knowledge, and from rational knowledge to actively guiding revolutionary practice, transforming the subjective world and the objective world, practice, knowledge, re-practice and re-understanding are endless, and the content of each practice and knowledge cycle has reached a higher level. This is the whole epistemology of dialectical materialism, and this is dialectical materialism's view of the unity of knowledge and action. This is an outstanding and creative work of Marxist philosophy, which not only enriches and develops the basic idea of "opposing bookishness" about proceeding from reality, combining investigation and research with theory and practice, but also sublimates the philosophy of "strategic issues in China's revolutionary war". He profoundly expounded Mao Zedong Thought's basic viewpoint of seeking truth from facts with a fresh soul, scientifically solved the problem of the relationship between knowing and doing that has been debated for thousands of years in the history of China's philosophy, and added an extremely glorious page to the history of China's philosophy. Together with "On Contradiction" and other works, it laid the theoretical foundation of our party's scientific ideological line, made theoretical preparations for Yan 'an rectification, and laid the ideological foundation for the successful development of the new-democratic revolution. Its publication marks the maturity of Mao Zedong's philosophy. Mao Zedong himself once said that his first published philosophical work, On Practice, was more important than other philosophical works of his time (quoted from the commentator's article of People's Daily on February 26th 1978: Guiding us to Win the Victory of Modernization), which shows the importance of this book in Mao Zedong Thought's system.