Although produced in controversy, this book is not a controversial work. The calming effects of the years that have passed since the tumultuous days in Lubeck are enough to guarantee that these pages will accurately trace the coming and going of opinions, the battle for the truth and the recognition of error. In only a few passages, especially in Part Six, will one be able to tell from the tone of the book that it comes out of this struggle. For these I ask the indulgence of my reader, since they contain explanations the extent of which probably does not correspond either to the difficulty of the questions treated or to their influence. But in such passages the extent of treatment could not - as was otherwise the case - be made to depend solely on a judgment as to the value and significance of the investigations presented. There considerations of defense, more than concern for symmetry, had to determine the structure.