Many historians of eighteenth-century Spain have addressed, in one way or another, the introduction of physiocracy and its influence in Spain (Sarrailh 1957, 547, 549; Herr 1958, 45). In general, these references are based on a rather vague definition of the term, one which stresses a kind of agrarianism, holding agriculture to be the most important (but not the only) productive sector. Occasionally there are references to the idea of a single tax (although not necessarily in relation to agricultural production), but not much else. In actuality, physiocracy was defined by a precise conceptual model, created in order to engage in the controversies on economic policies of the period (Francois Quesnay, 1957; Vaggi 1991). Physiocrats defined themselves more by the almost sectarian defense of this theoretical and conceptual model, and the language that expressed it, than by their proposals on policy questions. This theoretical model, in its core, included the following ideas: that agriculture was the only productive sector, the concept of produit net and its circulation through the Tableau oeconomique including, accordingly, the protection of a single tax and of free trade.