To have given clear and unified answers in familiar empirical terms to those theoretical questions which most occupied men's minds at the time, and to have deduced from them clear practical directives without creating obviously artificial links between the two, was the principle achievement of Marx's theory. The sociological treatment of historical and moral problems, which Comte and after him, Spencer and Taine, had discussed and mapped, became a precise and concrete study only when the attack of militant Marxism made its conclusions a burning issue, and so made the search for evidence more zealous and the attention to method more intense.
Isaiah Berlin Karl Marx: His Life and Environment. 3rd edition (1967). Time Inc Book Division, New York. pp. 13-14