This is how I conceived the contents of this book when I first wrote it twenty-five years ago. To-day, once again, I have to set down similar sentences if I am to characterise the leading thoughts of my book. At the original writing I contented myself with saying no more than was in the strictest sense connected with the fundamental problems which I have outlined. If anyone should be astonished at not finding in this book as yet any reference to that region of the world of spiritual experience of which I have given an account in my later writings, I would ask him to bear in mind that it was not my purpose at that time to set down the results of spiritual research, but first to lay the foundations on which such results can rest. The Philosophy of Spiritual Activity contains no special results of this spiritual sort, as little as it contains special results of the natural sciences. But what it does contain is, in my judgment, indispensable for anyone who desires a secure foundation for such knowledge. What I have said in this book may be acceptable even to some who, for reasons of their own, refuse to have anything to do with the results of my researches into the Spiritual Realm. But anyone who finds something to attract him in my inquiries into the Spiritual Realm may well appreciate the importance of what I was here trying to do. It is this: to show that open-minded consideration simply of the two problems which I have indicated and which are fundamental for all knowledge, leads to the view that man lives in the midst of a genuine Spiritual World. The aim of this book is to demonstrate, prior to our entry upon spiritual experience, that knowledge of the Spiritual World is a fact. This demonstration is so conducted that it is never necessary, in order to accept the present arguments, to cast furtive glances at the experiences on which I have dwelt in my later writings. All that is necessary is that the reader should be willing and able to adapt himself to the manner of the present discussions. Thus it seems to me that in one sense this book occupies a position completely independent of my writings on strictly spiritual matters. Yet in another sense it seems to be most intimately connected with them. These considerations have moved me now, after a lapse of twenty-five years, to re-publish the contents of this book in the main without essential alterations. I have only made additions of some length to a number of chapters. The misunderstandings of my argument with which I have met seemed to make these more detailed elaborations necessary. Actual changes of text have been made by me only where it seemed to me now that I had said clumsily what I meant to say a quarter of a century ago. (Only malice could find in these changes occasion to suggest that I have changed my fundamental conviction.)