Man’s nerve cell organization may be classified into two main systems:

(I) The inferior, the reflex, the instinctive, the automatic centers.

(II) The superior, the controlling, selective, and inhibitory brain-centers of the cortex.

The double systems of nerve-centers have correspondingly a double mental activity, or double-consciousness as it is sometimes called, the inferior, the organic, the instinctive, the automatic, the reflex consciousness, or briefly termed the subconsciousness, consciousness below the threshold of self-consciousness; and the superior, the choosing, the willing, the critical, the will-consciousness. This controlling will-consciousness may also be characterized as the guardian-consciousness of the individual.

From an evolutionary standpoint, we can well realize the biological function or importance of this guardian-consciousness. The external world bombards the living organism with innumerable stimuli. From all sides thousands of impressions come crowding upon the senses of the individual. Each neuron system with its appropriate receptors has its corresponding system of reactions which, if not modified or counteracted, may end in some harmful or fatal result.

It is not of advantage to an individual of a complex organization to respond with reaction to all impressions coming from the external environment. Hence, that organism will succeed best in the struggle for existence that possesses some selective, critical, inhibitory “choice and will” centers. The more organized and the more sensitive and delicate those centers are, the better will the organism succeed in its life existence.

The guardian-consciousness wards off, so far as it is possible, the harmful blows given by the stimuli of the external environment. In man, this same guardian consciousness keeps on constructing, by a series of elimination and selection, a new environment, individual and social, which leads to an ever higher and more perfect development and realization of the inner powers of individuality and personality.

Under normal conditions man’s superior and inferior centers with their corresponding upper, critical, controlling consciousness together with the inferior automatic, reflex centers and their concomitant subconscious consciousness, keep on functioning in full harmony. The upper and lower consciousness form one organic unity,—one conscious, active personality.

Under certain abnormal conditions, however, the two systems of nerve-centers with their corresponding mental activities may become dissociated. The superior nerve-centers with their critical, controlling consciousness may become inhibited, split off from the rest of the nervous system. The reflex, automatic, instinctive, subconscious centers with their mental functions are laid bare, thus becoming directly accessible to the stimuli of the outside world; they fall a prey to the influences of external surroundings, influences termed suggestions.

The critical, controlling, guardian-consciousness, being cut off and absent, the reduced individuality lacks the rational guidance and orientation given by the upper choice- and will-centers, and becomes the helpless plaything of all sorts of suggestions, sinking into the trance states of the subconscious. It is this subconscious that forms the highway of suggestions. Suggestibility is the essential characteristic of the subconscious.

The subconscious rises to the surface of consciousness, so to say, whenever there is a weakening, paralysis, or inhibition of the upper, controlling will and choice-centers. In other words, whenever there is a disaggregation of the superior from the inferior nerve-centers, there follows an increase of ideo-sensory, ideo-motor, sensori-secretory, reflex excitability; and ideationally, or rationally there is present an abnormal intensity of suggestibility.[6]