[111] Here we can leave out of account instances of products capable in part of entering the process of production without any exchange, such as coal in the mines. Within capitalist production as a whole such cases are rare (cf. Marx, _Theorien_ ..., vol. ii, part 2, pp. 255 ff.). [112] _Capital_, vol. ii, p. 503. _CHAPTER VIII_ MARX'S ATTEMPT TO RESOLVE THE DIFFICULTY Complete abstraction from the circulation of money, though making the process of accumulation so smooth and simple in the diagram of enlarged reproduction, has great disadvantages of its own, we see. There was much to be said for this method in the analysis of simple reproduction, where consumption is the be-all and end-all of production. Money there had an ephemeral part, mediating the distribution of the social product among the various groups of consumers--the agent for the renewal of capital. In the process of accumulation, however, the money form has an essential function: it no longer serves as a mere agent in the circulation of commodities--here it has come to be a feature of capital itself, an element in the circulation of capital. Even if the transformation of the surplus value is not essential to real reproduction, it is the economic _sine qua non_ of capitalist accumulation. In the transition from production to reproduction, the surplus product is thus subjected to two metamorphoses: first it casts off its use-form and then it assumes a natural form which is fit for the purpose of accumulation. The point here is not that the different cycles of production are counted off in units of years. It would be just as well to take the month; for that matter, the successive transformation of individual portions of the surplus value in Departments I and II may even intersect in time. Series of years here do not mean units of time but really intend the sequence of economic transformations. What matters is that this sequence must be observed if accumulation is to keep its capitalist character, whether it extends over a longer or a shorter period of time. This brings us back to the old question: How, and by whom, is the accumulated surplus value to be realised? Marx was well aware that his seemingly water-tight scheme of accumulation did not cover this point adequately, and he himself kept reviewing the problem from various angles. What he says is this: