The title of this book names what is simplest in the universe, and what is, for us, most complex in it. A very large part of the book will be occupied with the mind; but I shall endeavour to exhibit minds in the order of realities which begins with mere events in space and time and ends with God. No explanation is needed for leaving the notion of deity to the end. However immediately we may be aware of God in the religious sentiment, in philosophy there is no short road to deity. But I propose in this introductory chapter to explain the reasons why I begin with Space and Time and not with mind; and by a preliminary and provisional description of the relation of mind to its objects, to show how an inquiry into this secondary topic leads on to the more fundamental one.