=The Right of Way.= There is a law that comes to the aid of reason in this dilemma and that is the “law of the common path."[7] By this is meant that man is capable of but one intense emotion at a time.  No one can imagine himself strenuously making love while he is shaken by an agony of fear, or ravenously eating while he is in a passion of rage.  The stronger emotion gets the right of way, obtains control of mental and bodily machinery, and leaves no room for opposite states.  If the two emotions are not antagonistic, they may blend together to form a compound emotion, but if in the nature of the case such a blending is impossible, the weaker is for the time being forgotten in the intensity of the stronger.  “The expulsive power of a new affection” is not merely a happy phrase; it is a fact in every day life.  The problem, then, resolves itself into ways of making the desirable emotion the stronger, of learning how to form the habit of giving it the head start and the right of way.  In our chapter on “Choosing the Emotions,” we shall find that much depends on building up the right kind of sentiments, or the permanent organization of instincts around ideas.  However, we must first look more closely at the separate instincts to acquaint ourselves with the purpose and the ways of each, and to discover the nature of the forces with which we have to deal.

[Footnote 7:  Sherrington:  Integrative Action of the Nervous System.]

I THE SELF-PRESERVATIVE INSTINCTS

=Hunger.= Hunger is the most pressing desire of the egoistic or self-preserving impulse.  The yearning for food and the impulse to seek and eat it are aroused organically within the body and are behind much of the activity of every type of life.  As the impulse is so familiar, and its promptings are so little subject to psychic control, it seems unnecessary to do more than mention its importance.

=Flight and Fear.= All through the ages the race has been subject to injury.  Species has been pitted against species, individual against individual.  He who could fight hardest or run fastest has survived and passed his abilities on to his offspring.  Not all could be strongest for fight, and many species have owed their existence to their ability to run and to know when to run.  Thus it is that one of the strongest and most universal tendencies is the instinct for flight, and its emotion, fear.  “Fear is the representation of injury and is born of the innumerable injuries which have been inflicted in the course of evolution."[8] Some babies are frightened if they are held too loosely, even though they have never known a fall.  Some persons have an instinctive fear of cats, a left-over from the time when the race needed to flee from the tiger and others of the cat family.  Almost every one, no matter in what state of culture, fears the unknown because the race before him has had to be afraid of that which was not familiar.