The plasticity of neurone groups seems to vary in two respects—­as to modifiability and as to power to hold modifications.  The neurone groups controlling the reflex and physiological operations are least easily modified, while those controlling the higher mental processes are most easily modified.  The neurone groups controlling the instincts hold a middle place.  So far as permanence goes, connections between sensorimotor neurone groups seem to hold modifications longer than do connections between either associative-motor or associative-association.

It is probably because of this fact that habit in the minds of so many people refers to some physical activity.  Of course this is a misconception.  Wherever the nervous system is employed, habits are formed.  There are intellectual, moral, emotional, temperamental habits, just as truly as physical habits.  In the intellectual field every operation that involves association or memory also involves habits.  Good temper, or the reverse, truthfulness, patriotism, thoughtfulness for others, open-mindedness, are as much matters of learning and of habit as talking or skating or sewing.  Habit is found in all three lines of mental development:  intellect, character, and skill.

Not only does the law of habit operate in all fields of mental activity, but the characteristics which mark its operation are the same.  Two of these are important.  In the first place, habit formation results in a lessening of attention to the process.  Any process that is habitual can be taken care of by a minimum of attention.  In other words, it need no longer be in the focal point, but can be relegated to the fringe.  At the beginning of the modification of the neurone tract focal attention is often necessary, but as it progresses less and less attention is needed until the activity becomes automatic, apparently running by itself.  Not all habits reach this stage of perfection, but this is the general tendency.  This lessening of the need for attention means that less energy is used by the activity, and the individual doing the work is less likely to be fatigued.  In the second place, habit tends to make the process more and more sure in its results.  As the resistance is removed from the synapses, and the one particular series of units come to act more and more as a unit, the current shoots along the path with no sidetracking, and the act is performed or the thought reached unwaveringly with very little chance of error.  If the habit being formed is that of writing, the appropriate movements are made with no hesitation, and the chances that certain ones will be made the first time increase in probability.  This means a saving of time and an increase in confidence as to the results.