Jiang Shigong quote: “The roots of struggle lie in the necessary truth that contradictions propel society forward.”

“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.”

Mao Zedong book On Contradiction

On Contradiction (1937)

“Opposition and struggle between ideas of different kinds constantly occur within the Party; this is a reflection within the Party of contradictions between classes and between the new and the old in society. If there were no contradictions in the Party and no ideological struggles to resolve them, the Party's life would come to an end.”

Mao Zedong book On Contradiction

On Contradiction (1937)
Original: (zh-CN) 党内不同思想的对立和斗争是经常发生的，这是社会的阶级矛盾和新旧事物的矛盾在党内的反映。党内如果没有矛盾和解决矛盾的思想斗争，党的生命也就停止了。

“Membership in the social democracy means a struggle, a political struggle with grievous persecutions, and a private struggle for existence, a struggle that for the majority is far more difficult and heavy than the political struggle. And it is necessary, because the courage for this twofold struggle is created only by the consciousness that the injustice of society by which the great majority of mankind are to-day oppressed, corrupted and crippled, can only be abolished through a revolutionary movement, that is, a movement that shall completely exterminate capitalism with every fiber of its roots.”

Wilhelm Liebknecht (1826–1900) German socialist politician

No Compromise – No Political Trading (1899)

“Contradiction and struggle are universal and absolute”

Mao Zedong book On Contradiction

On Contradiction (1937)
Context: Contradiction and struggle are universal and absolute, but the methods of resolving contradictions, that is, the forms of struggle, differ according to the differences in the nature of the contradictions. Some contradictions are characterized by open antagonism and others are not. In accordance with the concrete development of things, some contradictions, which were originally non-antagonistic, develop into antagonistic ones, while others which were originally antagonistic develop into non-antagonistic ones.

“The first and most necessary topic in philosophy is that of the use of moral theorems, such as, "We ought not to lie;" the second is that of demonstrations, such as, "What is the origin of our obligation not to lie;" the third gives strength and articulation to the other two, such as, "What is the origin of this is a demonstration." For what is demonstration? What is consequence? What contradiction? What truth? What falsehood? The third topic, then, is necessary on the account of the second, and the second on the account of the first. But the most necessary, and that whereon we ought to rest, is the first. But we act just on the contrary. For we spend all our time on the third topic, and employ all our diligence about that, and entirely neglect the first.”

Epictetus (50–138) philosopher from Ancient Greece

51
The Enchiridion (c. 135)

“We are in presence of the contradiction of a style of living which cultivates sincerity and is at the same time a fraud. There is truth only in an existence which feels its acts as irrevocably necessary.”

José Ortega Y Gasset book The Revolt of the Masses

Source: The Revolt of the Masses (1929), Chapter XIV: Who Rules The World?
Context: No one knows toward what center human things are going to gravitate in the near future, and hence the life of the world has become scandalously provisional. Everything that today is done in public and in private — even in one's inner conscience — is provisional, the only exception being certain portions of certain sciences. He will be a wise man who puts no trust in all that is proclaimed, upheld, essayed, and lauded at the present day. All that will disappear as quickly as it came. All of it, from the mania for physical sports (the mania, not the sports themselves) to political violence; from "new art" to sun-baths at idiotic fashionable watering-places. Nothing of all that has any roots; it is all pure invention, in the bad sense of the word, which makes it equivalent to fickle caprice. It is not a creation based on the solid substratum of life; it is not a genuine impulse or need. In a word, from the point of view of life it is false.
We are in presence of the contradiction of a style of living which cultivates sincerity and is at the same time a fraud. There is truth only in an existence which feels its acts as irrevocably necessary. There exists today no politician who feels the inevitableness of his policy, and the more extreme his attitudes, the more frivolous, the less inspired by destiny they are. The only life with its roots fixed in earth, the only autochthonous life, is that which is made of inevitable acts. All the rest, all that it is in our power to take or to leave or to exchange for something else, is mere falsification of life. Life today is the fruit of an interregnum, of an empty space between two organizations of historical rule — that which was, that which is to be. For this reason it is essentially provisional. Men do not know what institutions to serve in truth; women do not know what type of men they in truth prefer.
The European cannot live unless embarked upon some great unifying enterprise. When this is lacking, he becomes degraded, grows slack, his soul is paralyzed. We have a commencement of this before our eyes today. The groups which up to today have been known as nations arrived about a century ago at their highest point of expansion. Nothing more can be done with them except lead them to a higher evolution. They are now mere past accumulating all around Europe, weighing it down, imprisoning it. With more vital freedom than ever, we feel that we cannot breathe the air within our nations, because it is confined air. What was before a nation open to all the winds of heaven, has turned into something provincial, an enclosing space.