In this paper we study the construction of Keynes' theory, conceived as a structure composed by three general and basic models, which are deduced ones from the others. We show that Say's law as criticized by Keynes can be formalized by means of a recursive supply model. Keynes' criticism leads to a different model, a non-walrasian and general interdepedence one, which does not allow to criticize the adjustement to a full employment equilibrium. It is used by Keynes as a basic model from which, in a third stage, he obtains the Keynesian model, defined as the model which admits at least one involuntary unemployment equilibrium with flexibility of wages and prices. We conclude that the construction of Keynes' theory is logically coherent and we show the limitations of his theoretical heresy.