For Marx the unity of theory and practice meant the resolution of theoretical problems by practical activity. ‘The development of the materialist theory of history’ examines how Marx came upon his theory of world history in which practical human activity, rather than thought, plays the crucial role. At this point in his life, Marx starts to make more use of historical data and less of abstract philosophical reasoning about the way the world should be, but his interest in alienation persists. The materialist conception of history tells us that humans are completely subject to forces they do not comprehend and are unable to control. These forces are the productive powers of human beings themselves.