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.