The present book originated in a series of lectures delivered by the author during the winter and spring 1916—1917 at the University of Groningen, The matter of these was afterwards augmented by the contents of some addresses held about ten years ago at the University of Amsterdam, and by that of some others delivered in various places in this country and in America. In response to a desire, repeatedly expressed by some of his friends, the author has finally resolved to publish these lectures in bookform Only in occasional passages, however, does it betray this devious course of development.