The _geist_ of social and political evolution is economic, according to the Socialist philosophy. This view of the importance of man's economic relations involves some very radical changes in the methods and terminology of political economy. The philosophical view of social and political evolution as a world-process, through revolutions formed in the matrices of economic conditions, at once limits and expands the scope of political economy. It destroys on the one hand the idea of the eternality of economic laws and limits them to particular epochs. On the other hand, it enhances the importance of the science of political economy as a study of the motive force of social evolution. With Marx and his followers, political economy is more than an analysis of the production and distribution of wealth; it is a study of the principal determinant factor in the social and political progress of society, consciously recognized as such. The sociological viewpoint appears throughout the whole of Marxian economic thought. It appears, for instance, in the definition of a commodity as the unit of wealth _in those societies in which the capitalist mode of production prevails_. Likewise wealth and capital connote special social relations or categories. Wealth, which in certain simpler forms of social organization consists in the ownership of use-values, under the capitalist system consists in the ownership of exchange-values. Capital is not a thing, but a social relation between persons established through the medium of things. Robinson Crusoe's spade, the Indian's bow and arrow, and all similar illustrations given by the "orthodox" economists, do not constitute capital any more than an infant's spoon is capital. They do not serve as the medium of the social relation between wage-worker and capitalist which characterizes the capitalist system of production. The essential feature of capitalist society is the production of wealth in the commodity form; that is to say, in the form of objects that, instead of being consumed by the producer, are intended to be exchanged or sold at a profit. Capital, therefore, is wealth set aside for the production of other wealth with a view to its exchange at a profit. A house may consist of certain definite quantities of bricks, timber, lime, iron, and other substances, but similar quantities of these substances piled up without plan will not constitute a house. Bricks, timber, lime, and iron become a house only in certain circumstances, when they bear a given ordered relation to each other. "A negro is a negro; it is only under certain conditions that he becomes a slave. A certain machine, for example, is a machine for spinning cotton; it is only under certain defined conditions that it becomes capital. Apart from these conditions, it is no more capital than gold _per se_ is money; capital is a social relation of production."[163]