Like other ancient branches of learning, psychology has undergone in the last hundred years a change and development amounting to a revolution. Not only has there been rapid growth in knowledge and in the number of persons devoting their time and ingenuity to the
increase of knowledge in this field, but there has occurred a remarkable change in attitude, method, and standards. The change can be characterized, in a word, by saying that psychology has become an empirical science. It has ceased to be a chapter in general philosophy, and become one of the 'special sciences*. Leaving
the parental roof, it has followed its older brothers, physics,chemistry, and biology, out into the world, and set up business for itself. The transformation of psychology is a phase of the general scientific movement properly to be called the great outstanding fact in the
history of the nineteenth century. As the social movement of the past century was a result of the industrial development, and this in turn dependent on the progress of science, the latter may rightly be named the real fundamental movement of the century. It was the extension of scientific interest and method from the inorganic world to the realm of living creatures, and from life in general to the special forms of living activity which we call mental, that fructified the mental philosophy of the older day, and gave us the psychology of the present.
At the opening of the nineteenth century, psychology, as we call it today, though the name was then little used, could already boast of a long history. It could scarcely have been true that the philosophic minds of early days should have omitted from their view the mental performances of mankind. Socrates, in fact, taught that to 'know thyself was the prime factor in wisdom ; and Aristotle, among the numerous writings in which he reduced to order the thought of the ancient Greeks, composed a treatise on psychology, the 'science of the soul', destined to remain for many centuries' without a serious rival. In the early modern period, while 'natural philosophy',developing a technique of its own, split off from the parent stem and became the
science of physics, 'mental philosophy' remained bound up with general philosophy to such a degree that now it is almost impossible, in reading the philosophers, to disentangle their psychology fromtheir teachings on logic, ethics, and the criticism of knowledge.
Locke, the founder of the British empirical school in philosophy, wrote an Essay Concerning Human Understanding, a title appropriate, one would suppose, for a chapter in psychology.
