Anger is the motion of the spirit within the body toward the outside, together with the blood and the humors. This is found in animals also. Fear is the entrance of the soul within, leaving the surface of the body, and causing the extremities to become cold. Shame is a motion inward, and forthwith again outward. Sorrow is caused in the same way as fear, except that fear is sudden, while sorrow is gradual. This is why fear sometimes kills when the body is weak. Joy is motion outward. Joy may kill too, when it is very great, and the person is weak and without control. Joy is of the nature of pleasure, except that pleasure is gradual, while joy is sudden.
Pain is that feeling we have when we are taken out of our natural state and put into an unnatural. Pleasure is felt when we are restored to the natural. Take, for example, the heat of the sun. When a person is exposed to it, the sun takes him out of his natural state. Heat is then painful, and pleasure is produced by the thing which restores him to his natural state; in this case a cold spring and a drink of cold water. Similarly a person walking in the snow and cold air feels pain by reason of the cold taking him out of his natural state. Heat then gives him pleasure by restoring him. The same thing applies to hunger and thirst, sleeping and waking, and other things which give us pleasure and pain. Without pain there is no pleasure, and the pleasure varies in accordance with the antecedent pain.
Life is the effect of the animal soul. The disappearance of the effect does not necessarily involve the disappearance of the cause, as the disappearance of the smoke does not require the cessation of the fire. Death means simply the separation of the soul, not the destruction thereof. It does not follow because the human soul remains after the death of the body, that the soul of the ox and the ass continues likewise, for the two souls are different. Animals were created for the sake of man, whereas man exists for his own sake. Moreover, man's life is ultimately derived from his rational soul. For if the animal soul of man were the ultimate source of life, the rational soul too would be dependent for its life upon the former, and hence would be inferior to it, which is absurd. It remains then that the _rational soul gives existence to the animal soul in man_.