Leaning more on the example of the Soviet Union, which involved the silencing and subjugation of adversarial political forces to help bring about a cultural change, Mao called for his followers to speak openly and critically about revisionist forces that they were observing in society and to expel them, assuring them that their actions would be endorsed by the party and that their efforts would in no way be interfered with. This warrant granted to the public ultimately lead to roughly ten years in which those seen as "Revisionist" forces - largely understood to mean landlords, rich peasants, and the so called "bourgeoise academic" - were publicly criticised and denounced in places of gathering, and in more extreme examples had physical violence inflicted on them, including being beaten, tortured, and/or killed for their perceived crimes. Contradiction Mao drew from the writings of Karl Marx, Friedrich Engels, and Vladimir Lenin in elaborating his theory. Philosophically, his most important reflections emerge on the concept of "contradiction" (maodun). In two major essays, On Contradiction and On the Correct Handling of Contradictions Among the People, he adopts the positivist-empiricist idea (shared by Engels) that contradiction is present in matter itself and thus also in the ideas of the brain. Matter always develops through a dialectical contradiction: "The interdependence of the contradictory aspects present in all things and the struggle between these aspects determine the life of things and push their development forward. There is nothing that does not contain contradiction; without contradiction nothing would exist". Mao held that contradictions were the essential feature of society, and a wide range of contradictions dominates society, this calls for various strategies. Revolution is necessary to resolve antagonistic contradictions between labour and capital fully. Contradictions within the revolutionary movement call for an ideological correction to prevent them from becoming antagonistic. Furthermore, each contradiction (including class struggle, the contradiction holding between relations of production and the concrete development of forces of production) expresses itself in a series of other contradictions, some dominant, others not. "There are many contradictions in the process of development of a complex thing, and one of them is necessarily the principal contradiction whose existence and development determine or influence the existence and development of the other contradictions". The principal contradiction should be tackled with priority when trying to make the fundamental contradiction "solidify". Mao elaborates on this theme in the essay On Practice, "on the