the French poodle, the greyhound and dachshund. A con-
cept such as this is typical of the general run of concepts
which require no unusual penetration to disclose the funda-
mental elements of similarity in spite of the wide range of
differences.

3. There is, however, a third class of concepts which
require more than ordinary insight, it may be the insight of
genius, in order to discover the unity which lies hidden be-
neath an obscuring mass of manifold differences. It required
the analytic mind of a Newton to grasp under one concept
such diverse phenomena as the fall of a body to the earth
and the moon's revolution about its orbit. In the one case
there is motion in a straight line, in the other the motion
is in the path of an ellipse; in the one the body actually
falls to the earth, in the other it is forever falling but never
falls. Nevertheless, the two are similar. The course of
the moon may be resolved into two distinct motions; the one
centripetal, which is a direct falling toward the earth, the
other the centrifugal, which holds the former in check and
modifies the direct fall toward the earth so that the result
is the present elliptical orbit of the moon. Therefore it may
be truly said that the moon is always falling toward the
earth in a manner precisely similar to that of the ordinary
falling body upon the earth's surface. The only difference
is the counter force which is operative in the one case and
not in the other. Such a difference however so obscures
the general features of resemblance in the resulting phe-
nomena that a surface observation devoid of any deeper
reflection may pronounce them so different as to possess no
point in common. It is characteristic of the trained mind
that it is able to penetrate beneath the surface and discover
points of similarity which escape the notice of unreflecting
observation. Fortunately for the generality of intelligence,
the phenomena of human experience for the most part fall
together into natural groups whose underlying bond of unity
is perfectly obvious. Nature is so prodigal of her creations

-14-