THINGS don't always turn out as we expect them to. Many events occur unintentionally. In Adam Ferguson's memorable phrase, “History is the result of human action, not of human design.” His contemporary, Adam Smith, referred to an “invisible hand” that shapes human affairs. Half a century later, Hegel invoked the “cunning of Reason,” and thirty years after him Marx talked about people's “alienation” from their own action. The theme of unintended consequences of action was one of two central concerns of the emerging social sciences in this period, the other being the vision of society as an organic unity. These two images are still with us. On the one hand, there is the idea of individual actions interfering with each other to produce an unintended outcome. On the other hand, there is the idea – more fully discussed in the next chapter – of mutual adjustment among individual plans, allowing all to be carried out without distortion.

Before I consider the unintended consequences that arise from social interaction and interference, I would like to point out some mechanisms that operate within the mind. As explained earlier, an action is the result of desires and opportunities. But action can also shape desires and opportunities, in unintended ways (Fig. X.1).