Capital vol. 1, Chapter 2, footnote 2.

Proudhon begins by taking his ideal of Justice, of “justice éternelle,” from the juridical relations that correspond to the production of commodities: thereby, it may be noted, he proves, to the consolation of all good citizens, that the production of commodities is a form of production as everlasting as justice. Then he turns round and seeks to reform the actual production of commodities, and the actual legal system corresponding thereto, in accordance with this ideal. What opinion should we have of a chemist, who, instead of studying the actual laws of the molecular changes in the composition and decomposition of matter, and on that foundation solving definite problems, claimed to regulate the composition and decomposition of matter by means of the “eternal ideas,” of “naturalité” and “affinité”? Do we really know any more about “usury,” when we say it contradicts “justice éternelle,” équité éternelle “mutualité éternelle,” and other vérités éternelles than the fathers of the church did when they said it was incompatible with “grâce éternelle,” “foi éternelle,” and “la volonté éternelle de Dieu”?