In this paper, the authors challenge the common interpretation of Rawls' Theory of Justice as Fairness by showing that this theory, as outlined in the restatement (Rawls, Justice as Fairness: a Restatement, 2001), goes well beyond the definition of a distributive value judgment, in such a way as to embrace efficiency issues as well. A simple model is discussed to support our interpretation of the difference principle, by which inequalities are shown to be permitted as far as they stimulate a greater effort in education in the population, and so economic growth. To their knowledge, this is the only possibility for the inequality to be "bought" by both the most-, and above all, the least-advantaged individual as suggested by the difference principle. Finally, by recalling the old tradition of "universal ex-post efficiency", the authors show that a unique optimal social contract does not exist behind the veil of ignorance; more precisely, the sole set of potentially Rawls-optimal social contracts can be identified a priori, and partial justice orderings derived accordingly.