Text: "The Philosopher's Stone." This is not a stone in the usual sense of that term, but an allegorical expression, meaning the principle of wisdom upon which the philosopher who has obtained it by practical experience (not the one who is merely speculating about it) may fully rely on, as he would rely on the value of a precious stone, or as he would trust to a solid rock upon which to build the foundation of his (spiritual) house.

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Source: 131