There is a close analogy between the exercise of the brain-cells and the exercise of the muscles of the body. Both respond to reasonable exercise; both are injured by overwork; both degenerate by disuse. As Brooks says: "The mind grows by its own inherent energies. Mental exercise is thus the law of mental development. As a muscle grows strong by use, so any faculty of the mind is developed by its proper use and exercise. An inactive mind, like an unused muscle, becomes weak and unskilful. Hang the arm in a sling and the muscle becomes flabby and loses its vigor and skill; let the mind remain inactive and it acquires a mental flabbiness that unfits it for any severe or prolonged activity. An idle mind loses its tone and strength like an unused muscle; the mental powers go to rust through idleness and inaction. To develop the faculties of the mind and secure their highest activity and efficiency, there must be a constant and judicious exercise of these faculties. The object of culture is to stimulate and direct the activity of the mind." Experiments conducted by scientists upon dogs have shown that in the case of dogs specially trained to unusual mental activity, there has been a corresponding increase of the number of active brain-cells in the particular parts of the brain concerned with those mental activities. Microscopic examination of the brain tissues showed the greatest difference between the brain structure of the trained dogs and untrained ones of the same brood. So carefully were the experiments conducted that it was possible to distinguish between the dogs trained in one set of activities from those trained in another. Biologists have demonstrated the correctness of the brain-cell development theory beyond reasonable doubt, and ordinary human experience also adds its testimony in its favor. In view of the above, it will be seen that by intelligent exercise and use any and all faculties of the mind may be developed and cultivated, just as may any special muscle of the body. And this exercise can come only from actual use of the faculties themselves. Development must come from within and not from without. No system of outward stimulation will develop the faculties of the mind--they may be cultivated only by an exercise in their own particular field of work. The only way to exercise any particular faculty of thought is to _think_ through that faculty.