To all who taught me, and to all who will. Over the past fifteen years the notions of efficiency and sustainability have, more than any others, influenced the academic and public discussion concerning the intertemporal allocation of resources, especially as regards the economics of growth and environment. This treatise formally develops and counterposes these notions by means of the construct of a trajectorial objective, which is here developed, along with its implications, as a natural advance upon the classical scalar objective. In the course of this study it becomes clear that efficiency...