It is unfortunate that logic and system demand that the two most difficult chapters, this and the next-but-one, should come early. The reader, however, should not be discouraged by the difficulty of this or any other chapter. For many, if not most, chapters are quite easy, and all are written in such a way as to be understood without recourse to the others. The least central and/or most difficult sections in the chapter are (ix), (xi), (xii), (xiii), (xiv), (xv), (xvi), (xvii) and (xviii), which I have marked with a hash mark (#). The reader is recommended to read the prologue before the book.