INTRODUCTION

The weighing of evidence may be viewed as a mental experiment in which the human mind is used to assess probability much as a pan balance is used to measure weight. As in the measurement of physical quantities, the design of the experiment affects the quality of the result.

Often one design for a mental experiment is superior to another because the questions it asks can be answered with greater confidence and precision. Suppose we want to estimate, on the basis of evidence readily at hand, the number of eggs produced daily in the US. One design might ask us to guess the number of chickens in the US and the average number of eggs laid by each chicken each day. Another design might ask us to guess the number of people in the US, the average number of eggs eaten by each person, and some inflation factor to cover waste and export. For most of us, the second design is manifestly superior, for we can make a reasonable effort to answer the questions it asks.

As this example illustrates, the confidence and precision with which we can answer a question posed in a mental experiment depends on how our knowledge is organized and stored, first in our mind and secondarily in other sources of information available to us.