JEAN-PIERRE DUPUY PHILOSOPHICAL FOUNDATIONS OF A NEW CONCEPT OF EQUILIBRIUM IN THE SOCIAL SCIENCES: PROJECTED EQUILIBRIUM (Accepted December 23 1998) 1. INTRODUCTION My broader aim is to succeed where admittedly such authors as David Gauthier, 1 Edward McClennen 2 and others have failed: in grounding a form of Kantian rationalism in a Hobbesian view of the world where Rational Choice Theory (RCT) is held to provide the best account of how agents form mental states such as beliefs, desires and intentions, and of how they reason and act. An intermediate aim is to shake the foundations of our conven- tional decision-making model. The agent faces all the possibles and chooses the one that maximizes a certain index that supposedly represents her self-interest. She could have acted otherwise: herein resides her freedom. I will show that this model does not exhaust what we mean by practical reason. These two objectives seem to contradict each other. If the founda- tions of decision theory are shattered, how could one possibly expect to build something as grandiose as Ethics on RCT? It will be my task to show that RCT has the resources to accommodate a model of decision-making radically different from the orthodox account. 2. THE BACKWARD INDUCTION PARADOX My starting point will be one of the most nagging paradoxes that seem to undermine the foundations of RCT itself: the class of the so-called “Backward Induction Paradoxes” (BIPs). For most decision problems with a finite horizon, whether strategic or with a single agent, working backwards in time, step Philosophical Studies 100: 323–345, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.