A huge thank you to those who have helped me, with their advice, their remarks, or their encouragements: to Valérie Lévy-Soussan, fi rst and foremost, for lending me her ear, her advice, and her support every day, but also to Marie-Françoise Plissart, Sandrine Willems, Marc Avelot, Jan Baetens, Jean-Christophe Cambier, Luc Dellisse, Archibald and Vladimir Peeters, Hadrien and Gabriel Pelissier. Thanks also to Sophie Dufour, who transcribed several quotations, with both care and enthusiasm. And particular thanks to Christian Rullier: he knows why. To Sophie Berlin, director of the Human Sciences department at Flammarion, I am immensely indebted. Without her, I would never have had the idea of embarking on this project, nor the energy to bring it to completion.