INTRODUCTION

Probabilistic and statistical ideas have transformed experimental psychology not once but twice in the past fifty years: first at the level of method, and then at the level of theory. The two transformations are in fact intimately connected, and in a way perhaps peculiar to psychology. Because psychologists observe and theorize about their subjects just as their subjects observe and theorize about the world, the methods of the psychologists are prone to become models for the mental processes they study. Once psychologists came to view statistics as an indispensable method, it was not long before they began to conceive of the mind itself as an intuitive statistician.

Around 1960, our understanding of cognitive processes such as perception and thinking was radically transformed by a new metaphor: the mind as statistician. Questions such as how does the mind discriminate between two sounds, recognize an object as a person, attribute a cause to an event, or solve a problem, were approached from this new point of view. Brain functions are today described in terms of calculating probabilities and likelihood ratios, taking random samples, setting a decision criterion and estimating prior probabilities. This metaphor emerged at about the same time as another, similarly powerful one: the mind as computer.