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.The knowledge that an imagination is not a mere subjective picture, but a representation in picture form of an objective spiritual content is attained by means of healthy inner experience. This is achieved in a psycho-spiritual way, just as in the realm of sense-perception one is able with a healthy organism to distinguish properly between mere imaginings and objective perceptions. Thus the results of my perception stood before me.”
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Rudolf Steiner – An Outline of Occult Science