Let us suppose now that the process of production has a normal course, that is to say, that it comes out as the capitalist wishes. He has begun with a sum, A, and ends by possessing A + a. We must now explain this surplus a, which, in the terminology of capitalistic production, is called surplus-value. The surplus obtained from the labor of slaves is easily explained. The owner leaves the slaves a part of the product of their own labor to live on. The rest is his. His surplus springs from the labor of others. The relation of the serf and his lord is, if possible, even clearer. The serf works part of the week for himself and on the remaining days for his master. The explanation of the surplus produced by capital employed at usury or in primitive commerce (the most ancient forms under which capital was employed) no longer offer any great difficulties. The usurer appropriated to himself the possessions of the borrower little by little and so ruined him completely. The primitive merchant made himself a surplus by selling dear something that he had bought at a trivial price, a transaction which involved no increase in value. Now it is just this increase in value that is to be explained upon the basis of the law that things of equal value are exchanged, and not, as in the cases cited above, upon the exceptions to the law. If we represent the transaction of one who buys, not to make a profit, but to exchange something which has no use-value for him for something which has such a value (the simple circulation of commodities), by the formula C—M—C, in which C stands for commodities and M for money, we can represent the transaction of the capitalist by M—C—(M + m). In this formula m stands for the surplus-value accruing to the capitalist at the end of a successful operation. The latter formula is composed of the factors M—C, the purchase of the commodity, and C—(M + m), the sale. According to the law of the circulation of commodities the value of M ought to be equal to C, but C in turn must be equal to M + m, a thing which is possible only if C is a commodity which, while it is being consumed, produces a value greater than what it has. However, there is no value without labor; consequently the formula cited can harmonize with reality only if labor is itself a commodity. And, as we have seen above, it is such from the moment that the economic development has reached a certain point.