It has been shown on good grounds that the very sense of external reality has become intensified in the individual by his capacity of living in a social aggregate. The social aggregate strengthens each individual member of the group. The individual is not for the group, but the group is for self-preservation of the individual. Should the individual lose his self-protection, or even have his impulse of self-preservation lowered, the whole aggregate faces ultimate destruction. That is why when in a social aggregate the impulse of self-preservation becomes limited, inhibited, and lowered by tyranny of social commandments, society is sure to decline, degenerate, and finally dissolve; or fall a victim to an external invader,--the fate of tribes, communities, and nations in the past and the present. _The moving power of life is self-preservation of the individual._ A close study of life in general and of animal life in particular brings one to the inevitable conclusion that life in all its forms has self-preservation as its fundamental principle. _Self-preservation is the main impulse, the prime mover of life._ The prime mover of life is not the impulse of species preservation, or of sex, but the impulse of self-preservation of the individual organism. Self-preservation has two aspects, the positive and the negative. In the positive form the primitive impulse is to keep the individual alive, to keep the functions and structure in normal condition, to conduce to the full development and harmonious activity of the individual. The negative form of the preservative impulse is the preservation from injury, degeneration, destruction, and death. This negative aspect of the impulse of self-preservation expresses itself in the higher animals in the form of fear. The fear instinct is an essential constituent of the impulse of life. We may call the fear instinct the guardian of all sentient being. Wherever there is the least scent or even the least suggestion of danger, there the instinct of fear is aroused. Fear is the companion following close on the heels of the impulse of self-preservation. Since every animal is always surrounded by enemies, and since every strange thing or strange occurrence is a menace or possible signal of danger, the fear instinct is aroused on all strange occasions. The fear instinct requires the slightest stimulus to start into function. While most of the instincts require special conditions, and are usually _periodic_, the fear instinct is _ever present_ and can be awakened on all occasions, and under any circumstances. Anything unfamiliar,--darkness, a state of exhaustion, weakness, fatigue, arouses the fear instinct, startling the animal into running, hiding, crouching, and preparation for attack or defense.