什么是知识？自从有阶级的社会存在以来，世界上的知识只有两门，一门叫做生产斗争知识，一门叫做阶级斗争知识。自然科学、社会科学，就是这两门知识的结晶，哲学则是关于自然知识和社会知识的概括和总结。 - Mao Zedong

All reactionaries are paper tigers. In appearance, the reactionaries are terrifying, but in reality they are not so powerful. From a long-term point of view, it is not the reactionaries but the people who are really powerful.

It is absolutely necessary for educated young people to go to the countryside to be re-educated by the poor and lower-middle peasants. Cadres and other city people should be persuaded to send their sons and daughters who have finished junior or senior middle school, college, or university to the countryside. Let us mobilize. Comrades throughout the countryside should welcome them.

Man's social practice is not confined to activity in production, but takes many other forms--class struggle, political life, scientific and artistic pursuits; in short, as a social being, man participates in all spheres of the practical life of society. Thus man, in varying degrees, comes to know the different relations between man and man, not only through his material life but also through his political and cultural life (both of which are intimately bound up with material life). Of these other types of social practice, class struggle in particular, in all its various forms, exerts a profound influence on the development of man's knowledge. In class society everyone lives as a member of a particular class, and every kind of thinking, without exception, is stamped with the brand of a class.