The publication of the present work on _The Wonders of Life_ has been occasioned by the success of _The Riddle of the Universe_, which I wrote five years ago. Within a few months of the issue of this study of the monistic philosophy, in the autumn of 1899, ten thousand copies were sold. Moreover, the publisher having been solicited on many sides to issue a popular edition of the work, more than a hundred thousand copies of this were sold within a year.[1] This extraordinary and--as far as I was concerned--unexpected success of a philosophical work which was by no means light reading, and which had no particular charm of presentation, affords ample proof of the intense interest taken by even the general reader in the object of the work--the construction of a rational and solid philosophy of life. Naturally, the clear opposition of my monistic philosophy, based as it was on the most advanced and sound scientific knowledge, to the conventional ideas and to an outworn "revelation," led to the publication of a vast number of criticisms and attacks. During the first twelve months more than a hundred reviews and a dozen large pamphlets appeared, full of the most contradictory strictures and the most curious observations. One of the ablest of my pupils, Heinrich Schmidt, gave a summary and criticism of them in his _Der Kampf um die Welträthsel_, in the autumn of 1900. However, the literary struggle went on to assume gigantic proportions when twelve different translations of the _Riddle_ appeared, and led to an ever-increasing agitation in every educated country of the Old and the New World. I gave a brief reply to the chief of these attacks in April, 1903, in the appendix to the popular edition of the _Riddle_. It would be useless to go further into this controversy and meet the many attacks that have since been made. It is a question here of that profound and irreconcilable opposition between knowledge and faith, between a real knowledge of nature and an alleged "revelation," which has occupied the thoughtful and inquiring mind for thousands of years. I base my monistic philosophy exclusively on the convictions which I have gained during fifty years' close and indefatigable study of nature and its harmonious working. My dualistic opponents grant only a restricted value to these experiences; they would subordinate them to the fantastic ideas which they have reached by faith in a supernatural world of spirits. An honest and impartial consideration of this palpable contradiction discovers it to be irreconcilable--_either_ science and experience, or faith and revelation!