The Treaty of Rome, which established the European Economic Community (EEC) in 1957, is not ideologically neutral. On the contrary, it takes root in a particular conception of political economy: neo-liberalism. To understand their relationship, and how this particular view was institutionalized in a common European market, this article explores the visions of the economy carried by the promoters of European integration since the 1940s up to the period of treaty negotiations. It explains that the neo-liberal character comes from the pre-existence of a group located in the “border spaces”, between the administrative and academic worlds, between the national and international level, which played a key role in the genesis of the Treaty. It describes a series of “ratchet effects” that induced a progressive reduction of the space of the thinkable about the prospect of a customs union based on a free market economy, and outlines the particular context in which political leaders have been able to conduct the project of founding a new European social order.