“Changes in society are due chiefly to the development of the internal contradictions in society, that is, the contradiction between the productive forces and the relations of production, the contradiction between classes and the contradiction between the old and the new; it is the development of these contradictions that pushes society forward and gives the impetus for the suppression of the old society by the new.”
On Contradiction (1937)
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Mao Zedong 181
Chairman of the Central Committee of the Communist Party of… 1893–1976Related quotes

The Green Book (1975)
The Green Book (1975)
Context: Any class which becomes heir to a society, inherits, at the same time, its characteristics. That is to say that if the working class crushes all other classes, for instance, it becomes heir of the society, that is, it becomes the material and social base of the society. The heir bears the traits of the one he inherits from, though they may not be evident at once. As time passes, attributes of other eliminated classes emerge in the very ranks of the working class. And the possessors of those characteristics take the attitudes and points of view appropriate to their characteristics. Thus the working class turns out to be a separate society, showing the same contradictions as the old society.

Imperialism and World Economy (1917), Ch. 15 http://www.marxists.org/archive/bukharin/works/1917/imperial/index.htm
“The roots of struggle lie in the necessary truth that contradictions propel society forward.”
"Philosophy and History" (2018)

Pedagogia do oprimido (Pedagogy of the Oppressed) (1968, English trans. 1970)

On Contradiction (1937)
Original: (zh-CN) 共产党内正确思想和错误思想的矛盾,如前所说,在阶级存在的时候,这是阶级矛盾对于党内的反映。这种矛盾,在开始的时候,或在个别的问题上,并不一定马上表现为对抗性的。但随着阶级斗争的发展,这种矛盾也就可能发展为对抗性的。苏联共产党的历史告诉我们:列宁、斯大林的正确思想和托洛茨基、布哈林等人的错误思想的矛盾,在开始的时候还没有表现为对抗的形式,但随后就发展为对抗的了。中国共产党的历史也有过这样的情形。我们党内许多同志的正确思想和陈独秀、张国焘等人的错误思想的矛盾,在开始的时候也没有表现为对抗的形式,但随后就发展为对抗的了。目前我们党内的正确思想和错误思想的矛盾,没有表现为对抗的形式,如果犯错误的同志能够改正自己的错误,那就不会发展为对抗性的东西。因此,党一方面必须对于错误思想进行严肃的斗争,另方面又必须充分地给犯错误的同志留有自己觉悟的机会。在这样的情况下,过火的斗争,显然是不适当的。但如果犯错误的人坚持错误,并扩大下去,这种矛盾也就存在着发展为对抗性的东西的可能性

On the Correct Handling of Contradictions Among the People
Original: (zh-CN) 敌我之间的矛盾是对抗性的矛盾。人民内部的矛盾,在劳动人民之间说来,是非对抗性的;在被剥削阶级和剥削阶级之间说来,除了对抗性的一面以外,还有非对抗性的一面。