The theory with which this volume is concerned is that of science. In very general terms it may be defined as a system of hypotheses, one following from another, which permits the reader to derive from known facts and predictions of other facts. If the knowledge of the fact which we predict is important for us, in the sense that it permits us to achieve better the objectives we pursue, the theory may have a practical value. This book is therefore not only a contribution to the analysis of expectations but also an introduction to a number of selected topics in economic theory.