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