St. Paul reveals the existence of our dual nature when he exclaims with passionate fervor, "The good that I would I do not, but the evil which I would not, that do I. I delight in the law of God after the inner man, but I see another law in my members warring against the law of my mind." Xenophon gives, in the Cyropedia, a remarkable speech, expressing almost precisely the same idea. Araspes, a young nobleman of Media, is overwhelmed with mortification on being detected by Cyrus in an indiscretion in regard to a captive princess. Chided by Cyrus, "Alas," said he, "now I am come to a knowledge of myself, and find most plainly that I have two souls: one that inclines me to good, another that incites me to evil ..."—the animal versus the spiritual nature, referred to by St. Paul.
In another place St. Paul, speaking of the "Word of God," says it is "quick and powerful, and sharper than any two-edged sword, piercing even to the dividing asunder of soul and spirit, and of the joints and marrow...." Heb. iv. 12. Hence we may term the two elements of our duality soul and spirit, they being two separate and distinct entities.
The learned Doctor Whedon, in commenting on the forty-fourth verse of the fifteenth chapter of Paul's Epistle to the Corinthians, where the great apostle speaks of the resurrection, says the expression natural body, as distinct from spiritual body, fails utterly to convey to the mind of the English reader the apostle's true idea. "If," he says, "we assume a difference between soul and spirit, and coin the word soulical as the antithesis of spiritual, we present his exact idea. The Greek word psyche, soul or life, when used as antithetical to pneuma, spirit, signifies that animating, formative, and thinking soul or anima which belongs to the animal, and which man, as animal, shares as his lower nature with other animals. Its range is within the limits of the five senses, within which limits it is able to think and to reason. Such is the power of the highest animals. Overlying this is the spirit which man shares with higher natures, by which thought transcends the range of the senses, and man thinks of immensity, eternity, infinity, immortality, the beautiful, the holy, and God—it is certain that man's mind possesses both these classes or sets of thought." Now in regard to the higher of these elements, there are very many well authenticated cases where the extreme susceptibility of the mind (the seat of these elements) to outward impressions, and the reaction of the mental sensation on the nervous system, has led to the most singular and, in some instances, even fatal results. So marvellously delicate is this portion of our organization, that we are not always conscious of this reaction, and as the reaction is conveyed from the nerve centres to the muscular tissue, we actually find ourselves uttering words or making motions unconsciously. So sensitive is the brain through the influence of this higher nature, so subtle its functions, that it is often impressed by means indiscernible to the bodily eye or to the ordinary senses—by means just as mysterious as the action of magnetic attraction or the course of the electric wave.
Byron alludes to this exquisite susceptibility with no less of truth than beauty:
And slight withal may be the things which bring
Back on the heart the weight which it would fling
Aside for ever; it may be a sound,
A tone of music, summer's eve or spring,
A flower, the wind, the ocean, which shall wound,
Striking the electric chain wherewith we are darkly bound.
And how or why we know not, nor can trace
Home to its cloud this lightning of the wind ...
Having referred to the reaction of a mental sensation on the nervous system, let us now examine the course by which the reaction proceeds.
We are told by physiologists that stimuli applied to the nerves in certain cases induce contraction or motion in the muscles by direct conduction of a stimulus along a nerve, or by the conduction of a stimulus to a nervous centre, whence it is reflected along another nerve to the muscles. Not only mechanical and electrical, but psychical stimuli "excite the nerves, whether these are ideational, emotional, or volitional. They proceed from the brain, being themselves sometimes induced by external causes, and sometimes originating primarily in the great nervous centres from the operations of the instinct, the memory, the reason, or the will."
When a stimulus of any kind, whether mechanical, chemical, electrical, or vital, acts upon the living nervous substance, it produces an impression on that nerve substance and excites within it some particular change, and the property by which this takes place in the nerve substance has been called its excitability or neurility. But the nerve substance not only receives such an impression from a stimulus and is excited to such a change, but it possesses the property of conducting that impression in certain definite directions, and this property might be spoken of as conductility.
When such an impression is thus conducted simply along a nerve fibre, and thence to a muscle, it induces or excites, as we have seen, the contraction of that muscle, and so exercises what is called a motor function.