The distinction between nonatomicity and thick markets as the source of perfect competition is examined. The authors construct a model of an imperfectly competitive economy with a nonatomic continuum of traders and a continuum of differentiated commodities for which Walrasian equilibria exist....

The first theorem of welfare economics rests on the assumption that individuals have neither price-making nor market-making capacities. The authors offer a revision in which individuals have such capacities. The revision emphasizes two keys for market efficiency: the need to align private...