The relationship between the growing social diffusion of the claims related to the commons and their legal definition is far from being clarified. This essay focuses on this conundrum, which is considered to be paradigmatic of that "refractional" relation between the multiplicity of social practices and the autonomy of legal concepts. After a reading of the still scanty legal literature on the commons, this will appear to be pivoting around a dichotomy: the one separating function from entitlement. Concepts such as "Community" or the "Common" are usually called into question as potential responses to this problem. This legal impasse, which seems more a second-best than a solution, is also a chance to propose a "Roman" genealogy of the commons. Through a close reading of two crucial essays by Yan Thomas on the civil institution of the city and on the res religiosae the essay will show that procedure, and especially collective legal actions such as class actions, might be the actual domain which allows for a stronger politics of the commons.