How is this explained, according to Marx? By means of competition, the different rates of profit are levelled to a general rate of profit, which is the average of all the various rates of profit. Thus the capitalists do not realise the surplus value as it is created in any particular factory, but in the form of average rate of profit as it is produced by the operations of the total capital of society. The average rate of profit may be lower or higher than the individual rate of profit, for the "various capitalists," as Marx explains, "so far as profits are concerned, are so many stockholders in a stock company in which the shares of profits are uniformly divided for every 100 shares of capital, so that profits differ in the case of the individual capitalists only according to the amount of capital invested by each of them in the social enterprise, according to his investment in social production as a whole, according to his shares." While thus the individual rates of profit do not proportionately coincide with the rates of surplus value, i.e., while the degree of exploitation of the worker in the individual factory, and the volume of surplus value thus individually created, do not directly determine the individual rate of profit, it is the total mass of social surplus value which is the source of the average rate of profit. If the mass of the surplus value be large, the average rate of profit will also be great. Marx says: "It is here just the same as with average rate of interest which a usurer makes who lends out various portions of his capital at different rates of interest. The level of his average rate depends entirely on how much of his capital he has lent at each of the different rates of interest." The higher the various individual rates of interest, the higher will be the average rate of interest at which his capital has been put out. The individual price of production signifies, therefore, cost price plus the average rate of profit, and not plus surplus value: it does not necessarily correspond with the total amount of the constant and variable portions of capital employed in an individual enterprise, plus the mass of the surplus value: the prices and magnitudes of value of commodities are not manifestly equal, as Marx has often pointed out. Of course, the total profits of the capitalist class coincide with the total surplus value extracted from the working class, provided, of course, that the supply of commodities corresponds with the social needs. Thus the law of surplus value, in spite of all deviations and refractions, holds good in the last resort. "In theory," observes Marx, "it is assumed that the laws of the capitalist mode of production develop freely. In reality, there is always only an approximation."