“But the old idealist conception of history, which was not yet dislodged, knew nothing of class struggles based upon economic interests, knew nothing of economic interests; production and all economic relations appeared in it only as incidental, subordinate elements in the ‘history of civilization.’
The new facts made imperative a new examination of all past history. Then it was seen that all past history, with the exception of its primitive stages, was the history of class struggles; that these warring classes of society are always the products of the modes of production and of exchange — in a word, of the economic conditions of their time; that the economic structure of society always furnishes the real basis, starting from which we can alone work out the ultimate explanation of the whole superstructure of juridical and political institutions as well as of the religious, philosophical, and other ideas of a given historical period. Hegel has freed history from metaphysics — he made it dialectic; but his conception of history was essentially idealistic. But now idealism was driven from its last refuge, the philosophy of history; now a materialistic treatment of history was propounded, and a method found of explaining man’s ‘knowing’ by his ‘being,’ instead of, as heretofore, his ‘being’ by his ‘knowing.’
From that time forward, Socialism was no longer an accidental discovery of this or that ingenious brain, but the necessary outcome of the struggle between two historically developed classes — the proletariat and the bourgeoisie. Its task was no longer to manufacture a system of society as perfect as possible, but to examine the historico-economic succession of events from which these classes and their antagonism had of necessity sprung, and to discover in the economic conditions thus created the means of ending the conflict. But the Socialism of earlier days was as incompatible with this materialist conception as the conception of Nature of the French materialists was with dialectics and modern natural science. The Socialism of earlier days certainly criticized the existing capitalistic mode of production and its consequences. But it could not explain them, and, therefore, could not get the mastery of them. It could only simply reject them as bad. The more strongly this earlier Socialism denounced the exploitations of the working-class, inevitable under Capitalism, the less able was it clearly to show in what this exploitation consisted and how it arose. but for this it was necessary —
to present the capitalistic mode of production in its historical connection and its inevitableness during a particular historical period, and therefore, also, to present its inevitable downfall; and
to lay bare its essential character, which was still a secret. This was done by the discovery of surplus-value.
It was shown that the appropriation of unpaid labor is the basis of the capitalist mode of production and of the exploitation of the worker that occurs under it; that even if the capitalist buys the labor power of his laborer at its full value as a commodity on the market, he yet extracts more value from it than he paid for; and that in the ultimate analysis, this surplus-value forms those sums of value from which are heaped up constantly increasing masses of capital in the hands of the possessing classes. The genesis of capitalist production and the production of capital were both explained.
These two great discoveries, the materialistic conception of history and the revelation of the secret of capitalistic production through surplus-value, we owe to Marx. With these discoveries, Socialism became a science.”
– Friedrich Engels, “Socialism: Utopian and Scientific”