Rudolf Steiner 
Theosophy (Barnes & Noble Digital Library) 
An Introduction to the Supersensible Knowledge of the World and the Destination of Man

Austrian philosopher and author Rudolph Steiner presents an in-depth analysis on the relationship between humans and the spiritual world in
Theosophy, published in 1910. In the book, Steiner introduces and articulates to the reader this central idea: there are three sides to man’s nature, and they are body, soul, and spirit. Touching upon themes of reincarnation and the division of the physical from the spiritual world, Steiner provides an overview of theosophical study in this early twentieth-century text.