On this day, 75 years ago, on December 29, 1950 (November 21, 1950, the lunar calendar), Mao Zedong's "On Practice" was published. On December 29, 1950, Mao Zedong's "On Practice" was published. This article was written in July 1937. The article summarizes the party's historical experience and lessons from the epistemological perspective, exposes the "Left" and Right Righteousness mistakes, and focuses on criticizing the idealism and transcendentalism of "Left" adventurists such as Wang Ming. It points out that all opportunism is characterized by the separation of subjectivity and objectivity, and the separation of understanding and practice. Mao Zedong pointed out that Marxist philosophical dialectical materialism has two distinctive characteristics: one is its class nature, which serves the proletariat; the other is its practical nature, which emphasizes that theory is for practice and in turn serves practice. It comprehensively clarifies the relationship between understanding and practice, namely knowledge and action, analyzes in detail the two leaps in the process of understanding, and points out that the struggle of the proletariat and the revolutionary people to transform the world includes both tasks of transforming the objective world and transforming the subjective world. "On Practice" inherits, defends and develops Marxist epistemology and is Mao Zedong's important contribution to the treasure house of Marxism-Leninism philosophy.