What is the epistemological status of Walras' perfectly competitive market and of the process of tâtonnement? The debate is ancient (Bertrand 1883, Edgeworth 1889). Two interpretations prevail. According to the first one (Patinkin, Morishima, Walker), Walras offers a description of the functiunning of a real capitalist economy. According to the second one (Jaffé, Bridel, Rebeyrol), Walras proposes a 'realistic utopia'. This opposition is epistemological. The realistic-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 hypothesis. The market perfectly organized is a rationalist fiction. It is neither an empirical concept, nor (or not only) a moral ideal. It is a thought experience built on the model of Galilei's "machine without friction" or Descartes' s hypothesis about the world's formation.