=Nature of Emotion.=--An emotion differs from sensuous feeling, not in its content, but in its higher intensity, its greater complexity, and its more elaborate motor response. It may be defined as a succession of interconnected feelings with a more complex physical expression than a simple feeling. On reading an account of a battle, one may feel sad and express this sadness only in a gloomy appearance of the face. But if one finds that in this battle a friend has been killed, the feeling is much intensified and may become an emotion of grief, expressing itself in some complex way, perhaps in tears, in sobbing, in wringing the hands. Similarly, a feeling of slight irritation expressed in a frowning face, if intensified, becomes the emotion of anger, expressed in tense muscles, rapidly beating heart, laboured breathing, perhaps a torrent of words or a hasty blow. =Emotion and Instinct.=--Feeling and instinct are closely related. Every instinct has its affective phase, that is, its satisfaction always involves an element of pleasure or pain. The satisfaction of the instincts of curiosity or physical activity illustrates this fact. On the other hand, every emotion has its characteristic instinctive response. Fear expresses itself in all persons alike in certain characteristic ways inherited from a remote ancestry; anger expresses itself in other instinctive reactions; grief in still others. CONDITIONS OF EMOTION An analysis of a typical emotion will serve to show the conditions under which it makes its appearance. Let us take first the emotion of fear. Suppose a person is walking alone on a dark night along a deserted street. His nervous currents are discharging themselves uninterruptedly over their wonted channels, his current of thought is unimpeded. Suddenly there appears a strange and frightful object in his pathway. His train of thought is violently checked. His nervous currents, which a moment ago were passing out smoothly and without undue resistance into muscles of legs, arms, body, and face, are now suddenly obstructed, or in other words encounter violent resistance. He stands still. His heart momentarily stops beating. A temporary paralysis seizes him. As the nervous currents thus encounter resistance, the feeling tone known as fear is experienced. At the same time the currents burst their barriers and overflow into new channels that are easy of access, the motor centres being especially of this character. Some of the currents, therefore, run to the involuntary muscles, and in consequence the heart beats faster, the breathing becomes heavier, the face grows pale, a cold sweat breaks forth, the hair "stands on end." Other currents, through hereditary influences, pass to the voluntary muscles, and the person shrieks, and turns and flees.