Marxist political economy makes an analytical distinction between two aspects of economic activity. On the one hand are the ‘social relations’ of production, which relate to the maintenance of social domination, the extraction of an economic surplus, and the exploitation of labour. On the other hand, there are the ‘forces of production’, those elements and relations which are necessary, whatever the social structure, if materials, objects, and forces, drawn from nature, are to be modified into a form suitable to meet some human purpose (‘use value’). There is no agreement about the exact scope of the term ‘forces of production’, but at various times Marx and Engels included the following: ‘raw materials’, the bodies or substances to be worked upon in the labour-process, and always considered by Marx and Engels to be the products of prior expenditures of human labour; ‘instruments of production’, the tools or machinery employed in modifying raw materials (including in some versions, human organs themselves); the human capacity for work (‘labour-power’), a function of bodily organization, fitness, skill, knowledge, and such like; and, finally, the forms of social division and co-ordination of labour required by the particular characteristics of a given labour-process (sometimes called ‘technical relations’ of production). A further category of requirements for production—land, air, water, and other broadly environmental or contextual conditions—was recognized by Marx and Engels, but often mistakenly included among the instruments of production. Marx and Engels postulated a long-run historical trend in human societies, dramatically accelerated by capitalism, for the forces of production (combined human productive powers) to develop. This developmental process would enhance humanity's capacity to control and regulate nature, and so meet universal human needs with a minimum of expenditure of unrewarding effort. This state of developed productive forces was to be a precondition for the future communist realm of freedom beyond scarcity and the necessity for labour.