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E see from the General Law of mental
growth that mental activity is two-fold.
First, new forms of activity are excited
through the senses; secondly, these are

unified by the mind in consequence of its own constitutional tendency to unification. Physiologists find a corresponding two-fold growth in the nervous system. This system, in outline, consists of nerve cells and nerve filaments. A nerve filament may connect an organ of sensation to a nerve cell, one nerve cell to another, or a nerve cell to a muscular fiber. By comparing nerve systems in different stages of development, it has been found that as mental growth progresses, the cells increase in number and develop in strength of walls; and the filaments are multiplied, connecting a larger number of cells into close relationship, and affording shorter lines of communication between different parts of the organism. The most important collection of nerve cells is the brain, and the most important of the filaments are those that cover the organs of sensation and lead to the brain.

2. The correspondence between the development of the mind and the nervous system leads us to infer some relation of dependence between them, but it does not destroy their separate unity. The Science of Physiology and the Science of Mind remain distinct from each other because the chain of cause and effect in either, however similar to that in the other, is distinct from it. The

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activity of one may be a condition of the activity of the other, but even if we could have a conception of the two, such that we might think a transference of energy possible, there would still remain a separate system of laws for each.

3. The arrangement of cells and filaments in the nervous system allows of three distinct courses of nervous activity. First, nervous energy may pass by the shortest way from a nerve of sensation or from a nerve cell to the muscles; secondly, it may pass from the organs of sensation to the nerve cells, and there stop; or, thirdly, it may have its beginning and end in the nerve cells.

4. There is a form of nerve activity that produces muscular action without consciousness. This is seen in the action of the heart and lungs and other movements of the body that we can not trace by direct consciousness. The energy producing these movements passes directly from the exciting cause to the muscles without arousing the general activity of the nervous system so as to produce consciousness. If consciousness is produced, it is not the cause of the muscular activity, for, as in the case of an injury to the eye, it is found that the muscular activity, like the closing of the lid, precedes the consciousness. Physiologists call this reflex action. Such activities as walking, habitual and skillful exercise, and all those actions to which we have become so accustomed that we perform them almost without consciousness, seem to be similar in nature, although they originate in an exercise of the will. If we were to suppose that the degree of consciousness depends upon the extent to which the activity of the nervous system is involved, and that habitual action produces shorter and shorter lines of nervous communication, the low degree of consciousness would be explained.

5. Nervous energy tends to disseminate itself throughout the system, and without arousing conscious discriminations produces a general feeling of happiness or unrest.

6. In reflex action all consciousness is wanting; in the general diffusion of nervous energy, discrimination is wanting But there are localized centers of nervous activity where the ends of nerve filaments are multiplied and spread over a limited surface, through which the mind is excited to discriminating conscious activity. These centers are the organs of sense. We classify them physiologically, and arrange them in the order of their importance as sources of knowledge. This order is also the order of their location, and may be the order of their development in the animal kingdom. We begin with the lowest, and name them touch, taste, smell, hearing, sight. The sense of touch is spread more or less over the entire surface of the body, but as a discriminating sense it is mainly located at the ends of the fingers.

7. There are some well-established Laws setting forth the relations between the physiological organism and mental development which deserve careful attention.

LAW 1.—ALL DISTINCT FORMS OF CONSCIOUS ACTIVITY ARE TO BE TRACED TO THE SENSES FOR THEIR FIRST IM

PULSE.

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First Proof.The first distinct manifestation of consciousness in a child is given in response to the influence of some object, as a lighted candle, upon the senses.

Second Proof.-In the process of abstract thinking, we are obliged continually to bring concrete illustrations taken from the perceptions of our senses to the aid of reason, and we have no conceptions that can not be traced in more or less perfect forms to experience.

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Third Proof.The terms we use in speaking even of the most abstract conceptions, in their primary use, represent impressions of the senses.

Fourth Proof.We may see how abstract ideas have grown from the very process which we call abstraction. We first consider the concrete, which is complex in thought, and then withdraw the attention from one part or quality and another until there is left only the conception which we wish to consider. The abstract conception is the final result of the process, -- not a conception previously formed, which we determine to hold apart from every thing else.

Fifth Proof.--To develop abstract notions in children, we must begin with the concrete. A child can not attain to the abstract conception of number till it has first learned to count things.

OBSERVATIONS.

The inferences from this law are manifold :

(I.) Each of the senses should be developed to make discriminations as exact as possible. The extent and accuracy of practical knowledge, and the reliability of memory, imagination, and reasoning, depend upon the extent and exactness with which we are accustomed to use the senses. Merchants must be able to judge by the sense of feeling of the grades of flour, cloth, leather, and other goods in which they deal. The senses of taste and smell are generally so dull that the ordinary grocer must depend upon a specialist to determine the quality of tea and other articles that please the taste or smell, but the fact that specialists exist is evidence of the cultivation that is possible. The ear and eye are continually exercised, and experience corrects many of their errors, but grave.

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