What is the epistemological status of the « theory of determination of prices under a hypothetical regime of perfect free competition » (Walras 1954 : 40)? The debate concerning the interpretation of tâtonnement in ancient (Bertrand 1883, Edgeworth 1889). Two interpretations prevail. According to the first one – followed by Patinkin, Morishima or Walker – Walras offers a description of the functioning of a real capitalist economy. The tâtonnement describes a dynamic and realist process which takes place on real markets. According to the other one – supported by  Jaffé, Bridel or Rebeyrol – Walras proposes a “realist utopia”. Some commentators suggest that Walras may be inconsistent (Ingrao et Israel) or is condemned to give up any “pretence of realism” (Bridel and Huck).

This opposition is an epistemological one. The descriptive-objective interpretation is based on empirism and assumes that theorization is an abstraction from experience. The normative-utopian is based on a rationalist epistemology. We propose a new epistemological hypothesis : the walrasian market is a rationalist fiction, as are the concepts of classical science built by Galileo and Descartes. It is neither an empirical concept – i.e. a concept abstracted from experience through induction – nor  (or not only) a moral ideal: it offers an explanation of the functioning of real markets, as does the “frictionless machine” imagined by Galileo or the Cartesian hypothesis about the world’s formation. If it is well known that Walras has found inspiration in classical physics, it is less known that this physic is built upon fictions seen as the way to the knowledge. These fictions are neither simple abstractions nor utopias but thought experiences which allow to build an imaginary world necessary to  understand the real world.