The political constitution of markets is promoted by Montchrestien whereas, on the contrary, the absolute autonomy of the sphere of exchange is favoured by Cantillon. Yet, this article seeks to demonstrate that both authors also participate in the emergence of a modern way of thinking about the economy. In both cases, we find the idea that the sphere of exchange acquires an autonomy which requires the application of a specific science. In the case of Cantillon, this autonomy is absolute and anticipates the foundations of contemporary economics. On the other hand, Montchrestien's work forms part of a ``political economy''. This article also aims to show that the mercantilism attributed to Montchrestien in no way implies that, in principle, the economy when theorised as such is a simple transposition of a war-like model. Similarly, it will seek to show that Cantillon, a supposed mercantilist, does not suggest that state intervention is futile, even if he adheres to the fundamental principles of economic liberalism. This modernity has two sides, represented by these texts which present economics as a science for the first time. It will be suggested that it allows us to call into question the postmodern theses which imagine a contemporary economic world in which the classical notion of sovereignty is totally absent.