The sixth sense is not something that one can take off and put on at will. Ability to use this great power comes slowly, through application of the other principles outlined in this book. Seldom does any individual come into workable knowledge of the sixth sense before the age of forty. More often the knowledge is not available until one is well past fifty, and this, for the reason that the spiritual forces, with which the sixth sense is so closely related, do not mature and become usable except through years of meditation, self-examination, and serious thought.
~ Napoleon Hill, Think & Grow Rich (1937)
