How man gets experience, and retains and organizes this experience must be our problem in the chapters on sensations, ideas, memory, and thinking. Individual differences in human capacity make necessary a treatment of the different types and grades of intelligence, and the compilation of tests for determining these differences. We must also treat of the application of psychology to those fields where a knowledge of human nature is necessary. =Applied Psychology.= At the beginning of a subject it is legitimate to inquire concerning the possibility of applying the principles studied to practical uses, and it is very proper to make this inquiry concerning psychology. Psychology, being the science of human nature, ought to be of use in all fields where one needs to know the causes of human action. And psychology is applicable in these fields to the extent that the psychologist is able to work out the laws and principles of human action. In education, for example, we wish to influence children, and we must go to psychology to learn about the nature of children and to find out how we can influence them. Psychology is therefore the basis of the science of education. Since different kinds of work demand, in some cases, different kinds of ability, the psychology of individual differences can be of service in selecting people for special kinds of work. That is to say, we must have sometime, if we do not now, a psychology of professions and vocations. Psychological investigations of the reliability of human evidence make the science of service in the court room. The study of the laws of attention and interest give us the psychology of advertising. The study of suggestion and abnormal states make psychology of use in medicine. It may be said, therefore, that psychology, once abstract and unrelated to any practical interests, will become the most useful of all sciences, as it works out its problems and finds the laws of human behavior. At present, the greatest service of psychology is to education. So true is this that a department has grown up called "educational psychology," which constitutes at the present time the most important subdivision of psychology. While in this book we treat briefly of the various applications of psychology, we shall have in mind chiefly its application to education. =The Science of Education.= Owing to the importance which psychology has in the science of education, it will be well for us to make some inquiry into the nature of education. If the growth, development, and learning of children are all controlled and determined by definite causal factors, then a systematic statement of all these factors would constitute the science of education. In order to see clearly whether there is such a science, or whether there can be, let us inquire more definitely as to the kind of problems a science of education would be expected to solve.