preface A Contribution to the Critique of Political Economy (1859). All constituent features of a society (social classes, political pyramid and ideologies) are assumed to stem from economic activity, forming what is considered the base and superstructure. The base and superstructure metaphor describes the totality of social relations by which humans produce and re-produce their social existence. According to Marx, the "sum total of the forces of production accessible to men determines the condition of society" and forms a society's economic base. The base includes the material forces of production such as the labour, means of production and relations of production, i.e. the social and political arrangements that regulate production and distribution. From this base rises a superstructure of legal and political "forms of social consciousness" that derive from the economic base that conditions both the superstructure and the dominant ideology of a society. Conflicts between the development of material productive forces and the relations of production provoke social revolutions, whereby changes to the economic base lead to the superstructure's social transformation. This relationship is reflexive in that the base initially gives rise to the superstructure and remains the foundation of a form of social organization. Those newly formed social organizations can then act again upon both parts of the base and superstructure so that rather than being static, the relationship is dialectic, expressed and driven by conflicts and contradictions. Engels clarified: "The history of all hitherto existing society is the history of class struggles. Freeman and slave, patrician and plebeian, lord and serf, guild-master and journeyman, in a word, oppressor and oppressed, stood in constant opposition to one another, carried on uninterrupted, now hidden, now open fight, a fight that each time ended, either in a revolutionary reconstitution of society at large, or in the common ruin of the contending classes." Marx considered recurring class conflicts as the driving force of human history as such conflicts have manifested as distinct transitional stages of development in Western Europe. Accordingly, Marx designated human history as encompassing four stages of development in relations of production: Primitive communism: cooperative tribal societies. Slave society: development of tribal to city-state in which aristocracy is born. Feudalism: aristocrats are the ruling class, while merchants evolve into the bourgeoisie. Capitalism: capitalists are the ruling class who create and employ the proletariat. While historical materialism has been referred to as a materialist theory of history, Marx did not