Societies which produce according to capitalist methods present a different picture. We observe that in certain periods all the ingredients of reproduction may be available, both labour and means of production, and yet some vital needs of society for consumer goods may be left unfulfilled. We find that in spite of these resources reproduction may in part be completely suspended and in part curtailed. Here it is no despotic interference with the economic plan that is responsible for the difficulties in the process of production. Quite apart from all technical conditions, reproduction here depends on purely social considerations: only those goods are produced which can with certainty be expected to sell, and not merely to sell, but to sell at the customary profit. Thus profit becomes an end in itself, the decisive factor which determines not only production but also reproduction. Not only does it decide in each case what work is to be undertaken, how it is to be carried out, and how the products are to be distributed; what is more, profit decides, also, at the end of every working period, whether the labour process is to be resumed, and, if so, to what extent and in what direction it should be made to operate.[58] In capitalist society, therefore, the process of reproduction as a whole, constitutes a peculiar and most complicated problem, in consequence of these purely historical and social factors. There is, as we shall see, an external characteristic which shows clearly this specific historical peculiarity of the capitalist process of reproduction. Comprising not only production but also circulation (the process of exchange), it unites these two elements. Capitalist production is primarily production by innumerable private producers without any planned regulation. The only social link between these producers is the act of exchange. In taking account of social requirements reproduction has no clue to go on other than the experiences of the preceding labour period. These experiences, however, remain the private experiences of individual producers and are not integrated into a comprehensive and social form. Moreover, they do not always refer positively and directly to the needs of society. They are often rather indirect and negative, for it is only on the basis of price fluctuations that they indicate whether the aggregate of produced commodities falls short of the effective demand or exceeds it. Yet the individual private producers make recurrent use of these experiences of the preceding labour period when they re-engage in reproduction, so that glut or shortage are bound to occur again in the following period. Individual branches of production may develop independently, so that there may be a surplus in one branch and a deficiency in another. But as nearly all individual branches of production are interdependent technically, glut or shortage in some of the larger branches of production lead to the same phenomenon in most of the others.