The instinct of physical activity is one of the most noticeable ones in babyhood.  The young baby seems to be in constant movement.  Even when asleep, the twitchings and squirmings may continue.  This continued muscular activity is necessary because the motor nerves offer the only possible path of 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.