Certainly much has been written about motivation. To this extent Drucker's observation is correct. The Reader's Guide Index lists scores of books on motivation written in the last decade alone, and these do not include the hundreds of research articles and technical reports churned out each year. What is less clear, however – and this is Drucker's concern – is the nature of our understanding. Actually, we do know a good deal about motivation, but on closer inspection our knowledge is quite uneven. We know how to arouse people to greater effort, especially for short periods of time: how, for example, to arrange incentives for factory workers so that production improves and absenteeism falls; and even how to rearrange the social organization of schools so that students are more willing to pursue learning for its own sake. But knowing how to motivate is not the same as knowing what is motivation. Here Drucker makes his point. Whatever is being aroused by the clever use of rewards and incentives, namely, motivation itself, remains mysterious and elusive. Motivation, like the concept of gravity, is easier to describe (in terms of its outward, observable effects) than it is to define. Of course, this has not stopped researchers from trying. The history of various attempts to grasp the essence of motivation is the first main theme of this book.