So far, we have only considered circulation as it takes place between the two large departments of production. Yet 4,000 units of the first Department's produce remain there in the form of means of production to renew its constant capital of 4,000_c_. Moreover 500 of the consumer goods produced in Department II [corresponding to the surplus value II(500_s_)] also remain in this department in the form of consumer goods for the capitalist class. Since in both departments the mode of production is capitalistic, that is unplanned, private production, each department can distribute its own products--means of production in Department I and consumer goods in Department II--amongst its own capitalists only by way of commodity exchange, i.e. by a large number of individual sale transactions between capitalists of the same department. Therefore the capitalists of both departments must have a reserve of money with which to perform these exchange transactions--to renew both the means of production in Department I and the consumer goods for the capitalist class in Department II. This part of circulation does not present any features of specific interest, as it is merely simple commodity circulation. Vendor and purchaser alike belong to the same category of agents of production, and circulation is concerned only with money and commodity changing hands within the same class and department. All the same, the money needed for this circulation must from the outset be in the hands of the capitalist class: it is part of their capital. So far, the circulation of total social capital presents no peculiarities, even if we consider the circulation of money. From the very outset it is self-evident that society must possess a certain quantity of money to make this circulation possible, and this for two reasons: first, the general form of capitalist production is that of commodity production which implies the circulation of money; secondly, the circulation of capital is based upon the continuous alternation of the three forms of capital: money capital, productive capital, and commodity capital. And as it is this very money, finally, which operates as capital--our diagram referring to capitalist production exclusively--the capitalist class must have possession of this money, as it has possession of every other form of capital; it throws it into circulation in order to regain possession as soon as the process of circulation has been completed.