It is in the transitory identity of vacant urban areas where lies its potential to serve as a laboratory for developing new forms of urban life and as a political instrument for implementing progress. The Petite Ceinture, a former 19th-century railway line, is one of the last vacant spaces in Paris. It is at the crossroads of two different periods and projects of a Paris capital of progress, the Petite Ceinture and the Parc des Buttes-Chaumont, that a new type of political architecture is sketched. If Paris is a political stage, let us have a theater to build this scenario together.