This was done in the book which appeared at Paris in two volumes in 1846, _The System of Economic Contradictions, or the Philosophy of Misery_, which deserves to be called his masterpiece, both because it contains the philosophic and economic foundations of his theory in a perfectly comprehensive and clear exposition, and because it is impossible to understand Proudhon without a knowledge of these contradictions. In his first work upon property, Proudhon had represented it as something equivalent to theft. But now we have another doctrine proposed: that Property is Liberty. These two propositions were thought by Proudhon to be proved in the same way. "Property considered in the totality of social institutions has, so to speak, two current accounts. One is the thought of the good which it produces, and which flows directly from its nature; the other is the disadvantages which it produces, and the sacrifices which it causes, and which also result directly, just as much as the good, from its nature. In property evil, or the abuse of it, is inseparable from the good, just as in book-keeping by double entry the debtor is inseparable from the creditor side. The one necessarily implies the other. To suppress the abuse of property means to extinguish it, just as much as to strike out an entry on the debtor side means also striking it out on the creditor side of an account." He proceeded in the same way with all "economic categories." Labour, he tells us in the _Contradictions_ more explicitly, is the principle of wealth, the power which creates or abolishes values, or puts them in proportion one to another, and also distributes them. Labour thus in itself, at the same time, is a force that makes for equilibrium and productivity, which one might think should secure mankind against every want. But in order to work, labour must define and determine itself--that is, organise itself. What are, then, the organs of labour, that is, the forms in which human labour produces and fixes values and keeps off want? These forms or categories are: division of labour, machinery, competition, monopoly, the State or centralisation, free exchange, credit, property, and partnership. However much labour in itself is the source of wealth, yet those means which are invented for the purpose of increasing wealth, become, through their antagonism and through that antithetical character, which, according to Proudhon, lies in the very nature of all social forms, just as many causes of want and pauperism. Labour gains by its division a more than natural fertility, but, at the same time, this divided labour, which debases the workman, sinks, owing to the manner in which this division is carried out, with great rapidity below its own level and only creates an insufficient value. After it has increased consumption by the superfluity of products, it leaves them in the lurch owing to the low rate of pay; instead of keeping off want it actually produces it.