Of all of his works, The Philosophy of Spiritual Activity is the one that Steiner himself believed would have the longest life and the greatest spiritual and cultural consequences. It was written as a phenomenological account of the results of observing the human soul according to the methods of natural science. This seminal work asserts that free spiritual activity – understood as the human ability to think and act independently of physical nature – is the suitable path for human beings today to gain true knowledge of themselves and of the universe.