It has been held by several writers that it is Marx's conception of human nature which explains the 'development thesis' (Cohen, 1978) concerning the expansion of the productive forces, which according to Marx, is itself the fundamental driving force of history. If true, this would make his account of human nature perhaps the most fundamental aspect of his work. Geras writes, (1983, pp. 107–108, italics in original) historical materialism itself, this whole distinctive approach to society that originates with Marx, rests squarely upon the idea of a human nature.