Liberatore’s treatise (1889) is noticeable first, as a source of the doctrines expounded in the Encyclical Rerum Novarum (1891), its author being the main drafter of the document; and second, as a manifestation of the Roman Catholic Church authorities’ attitude toward political economy. Liberatore’s purpose was to emend political economy and bring it into line with Catholic principles, but his two main objections to it, ethical and epistemological, imply that the task is impossible. Thus he does not press them and instead engages in a series of skirmishes with political economy on its own ground, which is the fascinating and unique aspect of the book. In the discussion he also works out some principles of social philosophy. Capital is underplayed: the only original all-important production factor is land. The importance given to the right of private property in land and, through it, paradoxically, the legitimacy afforded to capital and interest, are the main themes of the book, and recur in Rerum Novarum. Some of the anomalies of the Encyclical—for example, the fact that while its nominal subject is the industrial society, it seems to deal with a preindustrial world—can thus find an explanation. The permanence and evolution of Liberatore’s hostility toward political economy among the church’s present authorities is also analyzed.