This book addresses a growing concern as to why Psychology, now more than a hundred years after becoming an independent research area, does not yet meet the basic requirements of a scientific discipline on a par with other sciences such as physics and biology. These requirements include: agree­ ment on definition and delimitation of the range of features and properties of the phenomena or subject matter to be investigated; secondly, the development of concepts and methods which unambiguously specify the phenomena and systematic investigation of their features and properties. A third equally important requirement, implicit in the first two, is exclusion from enquiry of all other mattes with which the discipline is not concerned. To these requirements must then be added the development of basic assumptions about the nature of what is under investigation, and of principles to account for its properties and to serve as a guide as to what are relevant questions to ask and theories to develop about them.