The now well-known point is that the market is a fantastic information exchange. Changing prices are signals of shortage and surplus. Furthermore, the capitalist market gives people an incentive to respond to these signals in the search to maximize profits (private vices, public virtues again). Take away the market and the profit motive and you remove both signal and incentive. However skilled the planner, it is impossible to gain the quantity of fine-grained information about consumer demand and changing market conditions that even a small market automatically produces. But even if we did have this information, responding efficiently also relies on a level of good will and power we are unlikely to see. Despite its apparent attractions a planned economy just cannot do what it is designed for.