Chapter eighteen can easily be summarized as a perspective on where Marx has gone thus far in terms of looking at production capital from the standpoint of individual capital. A simultaneous movement is made in this last section: one from production towards accounting for circulation; and hence one from individual to total social capital.
He begins by stating that the "determining motive" of the production process is the production and valorization of surplus-value. "On top" of production, one must add circulation, and their combination forms the "overall circuit" that is "constantly repeated afresh at definite intervals" (427).
Circulation for Marx can be broken down into "1) The circuit of capital proper and 2) the circuit of the commodities which enter into individual consumption, consequently of the commodities for which the labourer expends his wages and the capitalist his surplus-value (or a part of it)." However, "the expenditure of this surplus-value and wages for commodities does not form a link in the circulation of capital, although at least the expenditure of wages is essential for this circulation" (428).
This is a tricky quote to unravel. My understanding is that Marx wants to ensure we understand that spending wages is a movement of M - C, not C - M, and so no value can be realized. Circulation can only entail the movement of C - M or C - P. That is, it includes the process of turning constant capital into a commodity and selling it, and it includes tracking the value attached to capital such as labor-power and its subsequent valorization. That is, circulation only counts for the things that remain in the capitalists hands. Once in the hands of the workers, capital becomes money, that is, nothing that can be self-valorized in the exchange for commodities.
Marx then summarizes the prior work up until now:
"In Book I the process of capitalist production was analysed as an individual act as well as a process of reproduction: the production of surplus-value and the production of capital itself...
"In the first part of this Book II, the various forms were considered which capital assumes its circular movement, and the various forms of this movement itself. The circulation time must now be added to the working times discussed in Book I....
"But in both the first and the second Parts it was always only a question of some individual capital, of the movement of some individualised part of social capital. However the circuits of the individual capitals intertwine, presuppose and necessitate one another, and form, precisely in this interlacing, the movement of the total social capital.
"We have now to study the process of circulation (which in its entirety is a form of the process of reproduction) of the individual capitals as components of the aggregate social capital, that is to say, the process of circulation of this aggregate social capital."
All the components must now be synthesized.