Competitive markets and “as if” methodology

Two results showing the limitations of the “as if” methodology are proved under relatively mild assumptions. In an interpretation of the results, a competitive market cannot simulate the outcome of a market M in which the single price assumption does not hold. In a second interpretation, the market M resulting from the aggregation of a finite number of competitive markets is not competitive. In both cases there is no ground to sustain the fiction that M operates as if it were competitive.