In this article I compare the approaches to process and order of classical political economy and marginalist economics, taking into account the implicit ontological commitments of each perspective in their explanation of capitalism. I draw on the social ontology developed by Tony Lawson, especially the notion of social positioning. The classical political economists studied the capitalist economy as a process of reproduction and distribution of the economic surplus, where socio-economic order depends on the division of society into social classes. After the marginal revolution, the classical approach is definitely abandoned, in a context where the analysis of human institutions in terms of social positions is progressively replaced by methodological individualism. This leads to a conception where the notion of socio-economic order is interpreted always in terms of market exchange between individuals, and in many cases replaced with a concern with the stability of an equilibrium situation.