“Marxist philosophy holds that the law of the unity of opposites is the fundamental law of the universe. This law operates universally, whether in the natural world, in human society, or in man's thinking. Between the opposites in a contradiction there is at once unity and struggle, and it is this that impels things to move and change. Contradictions exist everywhere, but they differ in accordance with the different nature of different things. In any given phenomenon or thing, the unity of opposites is conditional, temporary and transitory, and hence relative, whereas the struggle of opposites is absolute.”

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。

On the Correct Handling of Contradictions Among the People
Original: (zh-CN) 在一九四二年，我们曾经把解决人民内部矛盾的这种民主的方法，具体化为一个公式，叫做“团结——批评——团结”。讲详细一点，就是从团结的愿望出发，经过批评或者斗争使矛盾得到解决，从而在新的基础上达到新的团结。按照我们的经验，这是解决人民内部矛盾的一个正确的方法。

Source: The Book on the Taboo Against Knowing Who You Are (1966), p. 107-108

Cause, Principle, and Unity (1584)

pg 99, italics are feyerabends
Farewell to Reason (1987)

Les Loix du Mouvement et du Repos, déduites d'un Principe Métaphysique (1746)

Upon the Sovereign Sun (362)
Context: That divine and all-beauteous World, which from the highest vault of Heaven down to the lowest Earth is held together by the immutable providence of God, and which has existed from all eternity, without creation, and shall be eternal for all time to come, and which is not regulated by anything, except approximately by the Fifth Body (of which the principle is the solar light) placed, as it were, on the second step below the world of intelligence; and finally by the means of the "Sovereign of all things, around whom all things stand." This Being, whether properly to be called "That which is above comprehension," or the "Type of things existing," or "The One," (inasmuch as Unity appears to be the most ancient of all things), or "The Good," as Plato regularly designates Him, This, then, is the Single Principle of all things, and which serves to the universe as a model of indescribable beauty, perfection, unity, and power. And after the pattern of the primary substance that dwells within the Principle, He hath sent forth out of Himself, and like in all things unto Himself, the Sun, a mighty god, made up of equal parts of intelligible and creative causes. And this is the sense of the divine Plato, where he writes, "You may say (replied I) that I mean the offspring of the Good, whom the Good has produced, similar to itself; in order that, what the Good is in the region of intelligence, and as regards things only appreciable by the mind, its offspring should be the same in the region that is visible, and in the things that are appreciable by the sight." For this reason I believe that the light of the Sun bears the same relation to things visible as Truth does to things intelligible. But this Whole, inasmuch as it emanates from the Model and "Idea" of the primal and supreme Good, and exists from all eternity around his immutable being, has received sovereignty also over the gods appreciable by the intellect alone, and communicates to them the same good things, (because they belong to the world of intelligence), as are poured down from the Supreme Good upon the other objects of Intelligence. For to these latter, the Supreme Good is the source, as I believe, of beauty, perfection, existence, and union; holding them together and illuminating them by its own virtue which is the "Idea" of the Good.

Source: The Production of Security (1849), p. 25

Ein scheinbarer Widerspruch gegen ein Naturgesetz ist nur die selten vorkommende Betätigung eines andern Naturgesetzes.
Source: Aphorisms (1880/1893), p. 36.