In this volume I made an attempt to formulate the fundamental assumptions and main principles that un derlie normal and abnormal psychology. Every science, mathematical, physical or biological, has its postulates as the foundation of its structure. Psychology as a science has also its own assumptions which have to be clearly formulated. The object of the first part of this volume is the unravelling of the principal concepts and hypotheses which form the basis of the study of men tal phenomena. All through the domain of the sciences there is a vast movement for the search of fundamental concepts and for the close investigation of such concepts. Even such an exact science as mathematics has felt this spirit of examination of its fundamental assumptions, axioms, and postulates. Men like Lobatchevsky, Bolyai, Rie man and others have given the start and a number of mathematicians have recently followed in their foot steps, with the result of getting a wider horizon and of opening unknown regions. The same we find in the case of physical sciences, such as physics, mechanics and chemistry. Mach, Poincaré, Ostwald, Pearson and others have contributed to this spirit of investigation in the domain of physical sciences. This spirit of inquiry has become of late specially intensified by the revolu tionary discoveries of radio-active bodies.