Friedrich Engels: Just as Darwin discovered the law of development or organic nature, so Marx discovered the law of development of human history: the simple fact, hitherto concealed by an overgrowth of ideology, that mankind must first of all eat, drink, have shelter and clothing, before it can pursue politics, science, art, religion, etc.; that therefore the production of the immediate material means, and consequently the degree of economic development attained by a given people or during a given epoch, form the foundation upon which the state institutions, the legal conceptions, art, and even the ideas on religion, of the people concerned have been evolved, and in the light of which they must, therefore, be explained, instead of vice versa, as had hitherto been the case. But that is not all. Marx also discovered the special law of motion governing the present-day capitalist mode of production, and the bourgeois society that this mode of production has created. The discovery of surplus value suddenly threw light on the problem, in trying to solve which all previous investigations, of both bourgeois economists and socialist critics, had been groping in the dark. https://wordsmith.social/protestation/quotes#quote9516