Though this book has been written with scrupulous care to avoid words or terms of vague meaning—and though it often may seem coldly critical of things metaphysical, it has not been written with indifference to that great, perhaps the greatest, urge of the human heart—the craving for spiritual truth—our yearning for the higher potentialities of that which we call “mind,” “soul” and “spirit”—but it has been written with the deep desire to find the source of these qualities, their scientific significance and a scientific proof of them, so that they may be approached and studied by the best minds of the world without the digressions, and misinterpretations that are caused by the color and the confusion of personal emotions; and if the book be read with care, it will be seen that, though the clarifying definition of the classes of life has been chiefly used in the book for its great carrying power in the _practical_ world, its greatest help will ultimately be in guiding the investigation, the right valuation and especially the control and use of the higher human powers. In writing this book I have been not only introducing new ideas and new methods of analysis, but I have been using a tongue new to me. The original manuscript was very crude and foreign in form, and I am greatly indebted to various friends for their patient kindness in correcting the many errors of my poor English. I am also under great obligations to Walter Polakov, Doctor of Engineering, for his exceedingly helpful suggestions, not only in giving me a thorough criticism from the point of view of the Engineer, but also in devoting his energies to organizing the first “Time-binding Club” where these problems have been discussed and criticized, with great practical results. To all those who have read and criticized the manuscript or helped otherwise—Professors E. H. Moore, C. J. Keyser, J. H. Robinson, Burges Johnson, E. A. Ross, A. Petrunkevitch; and Doctors J. Grove-Korski, Charles P. Steinmetz, J. P. Warbasse; Robert B. Wolf, Vice-President of the American Society of Mechanical Engineers; Champlain L. Riley, Vice-President of the American Society of Heating and Ventilating Engineers; Miss Josephine Osborn; to the authors, L. Brandeis, E. G. Conklin, C. J. Keyser, J. Loeb, E. S. Mead, H. O’Higgins, W. Polakov, J. H. Robinson, R. B. Wolf, for their kind permission to quote them, I wish to express my sincere appreciation. I wish also to acknowledge the deepest gratitude to my wife, formerly Mira Edgerly, who has found in this discovery of the natural law for the human class of life, the solution of her life long search, and who, because of her interest in my work, has given me incomparably inspiring help and valuable criticism. It is not an exaggeration to state that except for her steady and relentless work and _her time, which saved my time_, this book could not have been produced in such a comparatively short time.