 Karl Marx (1818–1883) adopted Hegel's single-factor model of historical development but “turned [Hegel] right side up” (1873, 302) by replacing his idealism with materialism. According to Marx, the fundamental fact about a society at a given moment is not its ideological orientation but rather its “productive forces” (1845, 150), by which Marx means its material and technological resources.  Over the long run, the productive forces determine other aspects of the society, starting with the relations of production, the informal and formal rules that define and regulate property (1845, 151).  Marx builds on these assumptions to define capitalism and communism and to predict the former's eventual transformation into the latter. 