Mao also addresses this in On Practice

Marxists hold that man’s social practice alone is the criterion of the truth of his knowledge of the external world. What actually happens is that man’s knowledge is verified only when he achieves the anticipated results in the process of social practice (material production, class struggle or scientific experiment). If a man wants to succeed in his work, that is, to achieve the anticipated results, he must bring his ideas into correspondence with the laws of the objective external world; if they do not correspond, he will fail in his practice. After he fails, he draws his lessons, corrects his ideas to make them correspond to the laws of the external world, and can thus turn failure into success; this is what is meant by “failure is the mother of success” and “a fall into the pit, a gain in your wit”. The dialectical-materialist theory of knowledge places practice in the primary position, holding that human knowledge can in no way be separated from practice and repudiating all the erroneous theories which deny the importance of practice or separate knowledge from practice. Thus Lenin said, “Practice is higher than (theoretical) knowledge, for it has not only the dignity of universality, but also of immediate actuality.” The Marxist philosophy of dialectical materialism has two outstanding characteristics. One is its class nature: it openly avows that dialectical materialism is in the service of the proletariat. The other is its practicality: it emphasizes the dependence of theory on practice, emphasizes that theory is based on practice and in turn serves practice. The truth of any knowledge or theory is determined not by subjective feelings, but by objective results in social practice. Only social practice can be the criterion of truth. The standpoint of practice is the primary and basic standpoint in the dialectical materialist theory of knowledge.