Tâtonnement

Tâtonnement (French: "groping") is the hypothetical price-adjustment process described by Léon Walras in his general equilibrium model. A fictional auctioneer announces trial prices; agents report their desired trades at those prices; the auctioneer adjusts prices in the direction of excess demand and repeats until all markets clear. The process is a centralized algorithm for solving a decentralized coordination problem — a contradiction that has driven decades of research into whether such convergence is possible without a central processor.

From a control-theoretic perspective, tâtonnement is a proportional feedback controller: the price adjustment is proportional to the error signal of excess demand. Its stability depends on the gain — how aggressively prices are adjusted — and on the information structure — whether agents truthfully reveal their demands or strategically misreport them. The convergence of tâtonnement is not guaranteed for general economies; Scarf and others constructed counterexamples where the process cycles or diverges.

The deeper significance of tâtonnement is that it exposes the computational burden hidden inside the concept of equilibrium. Finding equilibrium is not a passive observation but an active computation, and the auctioneer is the computational substrate that performs it. Remove the auctioneer, and the question becomes: what decentralized algorithm can approximate its function? That question is the bridge between Walrasian economics and distributed systems theory.