Each Mind-Center Must Be Developed Specifically
Recall now that you have a multiplex, not a single brain. That is, your so-called "brain" is made up of innumerable, distinct "brain centers" which function quite independently of one another. No particular unit requires help from any of the others in order to do its especial work with full efficiency. Each center attends only to its specific business in your life. It rests, or relaxes from activity, when it has nothing to do; or when the particular muscles it governs are not in use. And, of course, when a certain brain center rests or is inactive, its associated mind center also rests or is inactive.
As already has been stated, the mind of a man is built up, through the brain instrument, by the sense impressions transmitted to his consciousness. In other words, all he knows with his mind first came into his mental capacity from outside impressions of things and ideas. The fewer the impressions that come into the mind through the brain, the less does a man know. And only the impressions that come into a particular mind center develop that center. (For example, the development of keenest eyesight by many optical impressions would not affect at all a man's ability to discriminate among the tones of music, would not give him "a good ear.")
Weak or Undeveloped Centers
It is evident, therefore, that if a particular brain center temporarily or permanently is deprived of right and sufficient exercise in transmitting sense impressions, its coordinated mind center will be stunted in its growth or starved for lack of mental food. This is why a man is awkward in using his native tongue when he returns to the country of his birth after a long residence among people of a different nation where that language was not spoken. But a little exercise of his brain in transmitting again the sound of his native tongue will quickly stimulate his mind with the renewed supply of this particular mental food to which it formerly was accustomed. In a few weeks he will use the old language naturally; whereas another man, who never had spoken it, would require years to build up such full knowledge from a start of complete ignorance of the language.
Evidently, too, a weak, undeveloped brain center would be incapable of receiving strong mental impulses from its coordinated mind center, and of transmitting them in full strength to the particular muscles governed by that mind center. This is why, if a man's brain center of courage is undeveloped, even the most courageous thoughts will not make his body act bravely. His legs may run away against his will to fight. The physical instrument of his mind (his brain), and also certain associated sets of muscles, must be sufficiently exercised in the action of courage to build up within him the physical structure of fearlessness that will be instantly responsive to a mental attitude of bravery.
Right Exercise for Development
If for any reason the brain instrument is weak or undeveloped, it can handle only weakly either in-coming messages to the ego from the senses, or out-going impulses from the mind to the muscles. So, because of this undeveloped brain instrument, the full capability of neither the inner nor the outer man can be built up and put to use. Obviously, therefore, if one is ambitious to succeed, he needs to know and to practice the coordinated mind-brain-muscle exercises that will increase the quantity and better the quality of his man capacity. Since he is a "many-minded, many-bodied" man, general physical and mental exercise will not develop the particular qualities required to assure his success. Each and every mind-brain-muscle set must be built up individually by specific exercises which strengthen that particular unit of the multiplex man. Then, of course, all his units should be taught to work together to make his success certain with his all-around capability fully developed and coordinated.
The Discriminative-Restrictive Method
Luther Burbank worked out "discriminative-restrictive" methods of growth that may be applied as successfully to men as to plants. He could not have built up the ability of a prune tree to produce delicious fruit if he had not fed into the tree structure, or instrument of production, a sufficient quantity and high quality of the particular plant foods of deliciousness. He restricted his experimental prune trees to the development of specific delicious qualities, by giving them no food except that discriminatively selected for his purpose. That is, he made them develop in one way and in one way only, when he was making a particular test.