discharge at first. As higher centers in the brain are
developed, the ingoing currents, aroused by all sense stimuli, find
other connections, and ideas, images, trains of thoughts, are aroused,
and so the energy is consumed; but at first all that these currents can
do is to arouse physical activity. The strength of this instinct is but
little diminished by the time the child comes to school. His natural
inclination is to do things requiring movement of all the growing
muscles. Inhibition, "sitting still," "being quiet," takes real effort
on his part, and is extremely fatiguing. This instinct is extremely
valuable in several ways: it gives the exercise necessary to a growing
body, provides the experience of muscle movements necessary for control,
and stimulates mental growth through the increase and variety of
experiences it gives.

The tendency to enjoy mental activity, to be satisfied with it for its
own sake, is peculiarly a human trait. This capacity shows itself in two
important ways--in the interest in sensory stimuli, usually discussed
under the head of curiosity, and in the delight in "being a cause" or
mental control. The interest in tastes, sounds, sights, touches, etc.,
merely for their own sake, is very evident in a baby. He spends most of
his waking time in just that enjoyment. Though more complex, it is still
strong when the child enters school, and for years any object of sense
which attracts his attention is material which arouses this instinct.
The second form in which the instinct for mental ability shows itself is
later in development and involves the secondary brain connections. It is
the satisfaction aroused by results of which the individual is the
cause. For example, the enjoyment of a child in seeing a ball swing or
hearing a whistle blown would be a manifestation of curiosity, while the
added interest which is always present when the child not only sees the
ball swing but swings it, not only hears the whistle but blows it
himself, is a result of the second tendency, that of joy in being a
cause. As the child grows older the same tendency shows itself on a
higher level when the materials dealt with, instead of being sensations
or percepts, are images or ideas. The interest in following out a train
of ideas to a logical conclusion, of building "castles in the air," of
making plans and getting results, all find their taproot in this
instinctive tendency towards mental activity.

In close connection