rmerly 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 pre