The Emerald Tablet's compact formula, that what is below corresponds to what is above and what is above to what is below, in order to accomplish the miracle of the one thing, functions in the Hermetic and alchemical traditions as something close to a master key: it licenses the practice of reading planetary influence in metals, bodily organs in zodiacal signs, and cosmic process in the alchemical vessel, on the working assumption that these are not arbitrary symbolic associations invented for convenience but genuine structural mirrors linking every level of reality to every other.
Robert Fludd took the principle to its most exhaustive Renaissance elaboration, producing detailed diagrams mapping the human being, the microcosm, point for point against the macrocosm of the wider universe, proportion for proportion, organ for celestial sphere, arguing that the human body is a literal, structural miniature of the cosmos rather than merely a poetic comparison to it. The doctrine's epistemological promise is considerable and is exactly what makes it so useful to divinatory and alchemical practice: if the lower genuinely mirrors the higher, then careful, systematic study of accessible, lower-order patterns, an astrological chart, an alchemical process, the human body itself, becomes a legitimate way of gaining real knowledge about otherwise inaccessible higher-order, spiritual, or cosmic truths, rather than remaining pure guesswork or metaphor.