"But those of us who really studied it and arrived at some understanding of its inner significance have a basic appreciation of the truth that no other book seems to supply. HPB said that the next interpretation of the Ageless Wisdom would be a psychological approach, and A Treatise on Cosmic Fire, which I published in 1925, is the psychological key to The Secret Doctrine. None of my books would have been possible had I not at one time made a very close study of The Secret Doctrine."