In his earliest works, Steiner already spoke of the "natural and spiritual worlds" as a unity. From 1900 on, he began lecturing about concrete details of the spiritual world(s), culminating in the publication in 1904 of the first of several systematic presentations, his "Theosophy: An Introduction to the Spiritual Processes in Human Life and in the Cosmos". As a starting point for the book Steiner took a quotation from Goethe, describing the method of natural scientific observation, while in the Preface he made clear that the line of thought taken in this book led to the same goal as that in his earlier work, "The Philosophy of Freedom".