An Outline of Occult Science by Rudolf Steiner. From the intro:
My knowledge of things of the spirit is a direct result of my own perception, and I am fully conscious of this fact. In all details I had always examined myself carefully as to whether every step I took in the progress of my perception was accompanied by a fully awake consciousness. Just as the mathematician advances from thought to thought without the unconscious or autosuggestion playing a role, so — I told myself — spiritual perception must advance from objective imagination to objective imagination without anything living in the soul but the spiritual content of clear, discerning consciousness.