The will-power develops by focusing one's thought on a certain object of one's concentration; and therefore one can develop that particular thing better than any other thing by one's willpower. For instance, those who play brass instruments in a band naturally develop the power of blowing instruments, and they will also be able to play the wood instruments, clarinet or flute; but at the same time if they have practiced the horn, they can play the horn yet better than the flute; because, although there is blowing in both, they are accustomed to that special instrument.