a further
development of the world of the plain man. The real things with which
science concerns itself are, after all, only minute touch things,
conceived just as are the things with which the plain man is familiar.
They exist in space and move about in space, as the things about us are
perceived to exist in space and move about in space. They have size
and position, and are separated by distances. We do not _perceive_
them, it is true; but we _conceive_ them after the analogy of the
things that we do perceive, and it is not inconceivable that, if our
senses were vastly more acute, we might perceive them directly.

Now, when we conclude that the things directly perceptible to the sense
of touch are to be regarded as appearances, as signs of the presence of
these minuter things, do we draw such a conclusion arbitrarily? By no
means. The distinction between appearance and reality is drawn here
just as it is drawn in the world of our common everyday experiences.
The great majority of the touch things about us we are not actually
touching at any given moment. We only _see_ the things, _i.e._ we have
certain _signs_ of their presence. None the less we believe that the
things exist all the time. And in the same way the man of science does
not doubt the existence of the real things of which he speaks; he
perceives their _signs_. That certain experiences are to be taken as
signs of such realities he has established by innumerable observations
and careful deductions from those observations. To see the full force
of his reasonings one must read some work setting forth the history of
the atomic theory.

If, then, we ask the question: What is the real external world? it is
clear that we cannot answer it satisfactorily without taking into
consideration the somewhat shifting senses of the word "real." What is
the real external world to the plain man? It is the world of touch
things, of objects upon which he can lay his hands. What is the real
external world to the man of science? It is the world of atoms and
molecules, of minuter touch things that he cannot actually touch, but
which he conceives as though he could touch them.

It should be observed that the man of science has no right to deny the
real world which is revealed in the experience of the plain man. In
all his dealings with the things which interest him in common life, he
refers to this world just as the plain man does. He sees a tree and
walks towards