Center on Capitalism and Society at Columbia and Brandeis University, none of whom bears This paper is an exploratory analysis of the role that banks play in supporting what Jevons called the mechanism of exchange. It considers a model economy in which exchange activities are facilitated and coordinated by a self-organizing network of entrepreneurial trading firms. Collectively these firms play the part of the Walrasian auctioneer, matching buyers with sellers and helping the economy to approximate equilibrium prices that no individual is able to calculate. Banks affect macroeconomic performance in this economy because their lending activities facilitate entry of trading firms and also influence their exit decisions. Both entry and exit have conflicting effects on performance, and we resort to computational analysis to understand how these conflicting effects are resolved. Our analysis sheds new light on the conflict between micro prudential bank regulation and macroeconomic stability; under some circumstances the economy performs better when bank regulation pays less attention to micro prudence (ie when capital adequacy ratios are lower and allowable loan-to-value ratios are higher). Related to this, the