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The Theosophic doctrine that the physical world is an externalization of an astral plane or even the higher Platonic doctrine that it crystallizes a world of divine ideation is given to beginners as a help to give them a crude grasp, a first step towards the theory that the world is an idea, until they are mentally developed. When their mind is mature they are then told to discard the astral plane theory and told the pure truth that all existence is idea.
-- Notebooks Category 21: Mentalism > Chapter 3 : The Individual and World Mind > # 65