This book studies a large economy. It deals with a static microeconomic model of an exchange market with pure competition. Instead of the sigma-additive theory, the finitely additive theory, the general Jordan content and the general Riemann integration are used respectively. By a specialized probability model, the author obtains a precise interpretation strictly based on microeconomic methods of measurement. In particular, the meaning of an agent and of a coalition is explained and the Core-Walras equivalence is deduced. The author elaborates an elementary representation by broken continuous functions and the classical Riemann integral. A conjecture concerning the reduction of the dynamical case onto generalized differential equations is added.