The treatise falls into two parts, the first biological-psychological, the second logical-philosophical. It may be objected that I should have done better to make two books, the one treating of purely physical science, the other introspective. It was necessary to be done with biology before turning to psychology. The second part treats of certain psychical problems in a fashion totally different from the method of any contemporary naturalist, and for that reason I think that the removal of the first part of the book would have been at some risk to many readers. Moreover, the first part of the book challenges an attention and criticism from natural science possible in a few places only in the second part, which is chiefly introspective. Because the second part starts from a conception of the universe that is anti-positivistic, many will think it unscientific (although there is given a strong proof against Positivism). For the present I must be content with the conviction that I have rendered its due to Biology, and that I have established an enduring position for non-biological, non-physiological psychology. My investigation may be objected to as in certain points not being supported by enough proof, but I see little force in such an objection. For in these matters what can “proof” mean? I am not dealing with mathematics or with the theory of cognition (except with the latter in two cases); I am dealing with empirical knowledge, and in that one can do no more than point to what exists; in this region proof means no more than the agreement of new experience with old experience, and it is much the same whether the new phenomena have been produced experimentally by men, or have come straight from the creative hand of nature. Of such latter proofs my book contains many. Finally, I should like to say that my book, if I may be allowed to judge it, is for the most part not of a quality to be understood and absorbed at the first glance. I point out this myself, to guide and protect the reader. The less I found myself able in both parts of the book (and especially in the second) to confirm what now passes for knowledge, the more anxious I have been to point out coincidences where I found myself in agreement with what has already been known and said. I have to thank Professor Dr. Laurenz Müllner for the great assistance he has given me, and Professor Dr. Friedrich Jodl for the kindly interest he has taken in my work from the beginning. I am specially indebted to the kind friends who have helped me with correction of the proofs. CONTENTS _Page_ AUTHOR’S PREFACE TO THE FIRST GERMAN EDITION ix FIRST OR PREPARATORY PART SEXUAL COMPLEXITY INTRODUCTION 1