THE "UTOPIAN" SOCIALISTS of the first half of the nineteenth century were mostly humanitarian idealists who preached social justice on religious or moral grounds. Karl Marx and Friedrich Engels, on the other hand, grounded their philosophy of socialism in the science of political economy. Their revolutionary and alarming Communist Manifesto, which appeared in 1848, demanded the immediate abdication of the ruling class. Not ideal justice, not the brotherhood of man were their slogans; boldly and bitterly they indicted the prevailing system of capitalism. Their program of action aimed at the abolition of human exploitation and "the forcible overthrow of the whole extant social order." Confident that the hour of revolution was about to strike, they declared open war against the bourgeoisie. Their battle-cry ended with the rallying challenge: "Let the ruling class tremble at the prospect of a communist revolution. Proletarians have nothing to lose but their chains. They have a world to win. Proletarians of all lands, unite!"
In this aggressive proclamation the two youthful radicals sketched the basic theories which subsequently made up the foundation stones of Marxian socialism. Their later vital and voluminous writings were devoted to the development of their revolutionary philosophy, in which the materialistic conception of history was of primary significance. As Engels defined it: