Those who are familiar with the development of the sciences within recent years have had occasion to remark the fact that psychology has been coming more and more to take its place as an independent science.  Formerly it was regarded as part of the duty of the philosopher to treat of the mind and its knowledge; but the psychologist who pretends to be no more than a psychologist is a product of recent times.  This tendency toward specialization is a natural thing, and is quite in line with what has taken place in other fields of investigation.

When any science becomes an independent discipline, it is recognized that it is a more or less limited field in which work of a certain kind is done in a certain way.  Other fields and other kinds of work are to some extent ignored.  But it is quite to be expected that there should be some dispute, especially at first, as to what does or does not properly fall within the limits of a given science.  Where these limits shall be placed is, after all, a matter of convenience; and sometimes it is not well to be too strict in marking off one field from another.  It is well to watch the actual development of a science, and to note the direction instinctively taken by investigators in that particular field.

If we compare the psychology of a generation or so ago with that of the present day, we cannot but be struck with the fact that there is an increasing tendency to treat psychology as a natural science.  By this is not meant, of course, that there is no difference between psychology and the sciences that concern themselves with the world of material things—­psychology has to do primarily with minds and not with bodies.  But it is meant that, as the other sciences improve upon the knowledge of the plain man without wholly recasting it, as they accept the world in which he finds himself and merely attempt to give us a better account of it, so the psychologist may accept the world of matter and of minds recognized by common thought, and may devote himself to the study of minds, without attempting to solve a class of problems discussed by the metaphysician.  For example, he may refuse to discuss the question whether the mind can really know that there is an external world with which it stands in relation, and from which it receives messages along the avenues of the senses.  He may claim that it is no more his business to treat of this than it is the business of the mathematician to treat of the ultimate nature of space.

Thus the psychologist assumes without question the existence of an external real world, a world of matter and motion.  He finds in this world certain organized bodies that present phenomena which he regards as indicative of the presence of minds.  He accepts it as a fact that each mind knows its own states directly, and knows everything else by inference from those states, receiving messages from the outer world along one set of nerves and reacting along another set.  He conceives of minds as wholly dependent upon messages thus conveyed to them from without.  He tells us how a mind, by the aid of such messages, gradually builds up for itself the notion of the external world and of the other minds which are connected with bodies to be found in that world.