This is a compilation of six biographies of influential men during the 19th century. From the preface: "Each age has its own prophets, men who bring to it distinctive messages and present them in such effective form as to sway the currents of contemporary thought. No age perhaps has had more diverse theories of life and the meaning of things presented to it than our own, and certainly none has ever given such an opportunity for the original thinker to reach quickly a world-wide audience as he can now through the medium of cheap books and free schools. This volume originated in my own desire to find out what was being said by certain persons who, I had reason to believe, were worth attention. But unless one is abnormally selfish, he always wants to introduce others to an interesting acquaintance. It is then simply as introductions that I would wish the following chapters to be taken. In one way or another such men are influencing the thought of all of us, but since we mostly get their philosophy at second hand-or at third, fourth, or nth hand-we fail to recognize its origin and are apt to misconceive its intent. Ideas that reach us in fragmentary form, and often after multiple translation through minds sometimes alien or hostile, are not very useful. It is always safer to drink at the source. I have endeavored to give some idea of the scope and character of each man's work, so that the reader may judge for himself whether it is profitable for him to follow up the acquaintance. If he does, he will find at the end of the chapter directions how to proceed further. We imagine we can understand a man better if we can see his face, even his photograph. This may be a superstition, but, if so, it is a superstition worth deferring to by one who aspires to be an interpreter. So in the summer of 1910 I went to see the six men included in this first volume in their homes, not with the hope of getting any new and unpublished opinions, not with the expectation of gaining a personal acquaintance that would give me any deeper insight into their mental processes, but merely to convince myself that they are flesh and blood, instead of paper and ink. If I can convince the reader of this, my purpose will be accomplished. In the choice of names to be included in the list, I was guided primarily by the idea that I should be most likely to interest others in the men who have most interested me. Since the object of the book is to serve as an introduction to the works of the authors, not as a substitute for them, the choice was limited to those who have given expression to their philosophical views in a sufficiently popular form to be attractive to the general reader. It was necessary to select representatives of diverse types of thought, and it was not possible to confine the choice to the philosophical profession, for in our day philosophy has escaped from its classroom and often displays more activity outside than in it. So I have included men of science and letters as well as philosophers of the chair. The group comprised in this volume includes: Maurice Maeterlinck, dramatist and essayist, interpreter of the animate and inanimate world; Henri Bergson, of the Collège de France, whose intuitive philosophy has been introduced into America by the late William James; Henri Poincaré, of the French Academy, mathematician and astronomer; Élie Metchnikoff, director of the Pasteur Institute in Paris, author of studies in optimistic philosophy; Wilhelm Ostwald, of Leipzig University, recipient of the Nobel prize for chemistry in 1909, founder of the Annals of Natural Philosophy, and Ernst Haeckel, of Jena University, veteran zoölogist, champion of Darwinism and Monism, author of the "Riddle of the Universe."