All content copyright © 2010-2019 Frank Revelo
As above, so below, and as below, so above, is the principle of correspondence. "Above" refers to the spiritual or inner world, while "below" refers to the material or outer world. Relationships between sets of purely material world phenomena (such as material world earth and sky) are not Hermetic correspondences, but rather fall into the domain of material world science, and hence are governed by the standard material world scientific requirements for testable hypotheses and reproducible experiments.
Material phenomena as metaphors for spiritual phenomena compensates for lack of vocabulary for describing the latter. Also, ideas concerning the spiritual world are easily lost or garbled, since those to whom such ideas appeal and who are capable of grasping and communicating such ideas without introduction of error are few and far between, and there is no guarantee these chosen few will be able to find one another. It is thus better to encode the message in a form which is resistant to change and which appeals to the masses. This ensures that the message is widely disseminated through space and time, with the esoteric interpretation always available for decoding by those with "eyes to see and ears to hear". This rationale for encoding wisdom was especially relevant in olden times, when books were rare (and the internet not yet invented) and wisdom had to be communicated orally.
Encoding spiritual wisdom also protects against suppression by the authorities. First, encoding limits those who can receive the message, and thus limits the threat to society and thus the motivation to suppress. Second, if suppression does occur, it typically opens the authorities to ridicule, given the absurdity of the literal interpretation, and also awakens more interest in the encoded wisdom, so that the suppression backfires. Unless the authorities are truly foolish, they will anticipate both the risk of making fools of themselves and the "lure of the forbidden" factor, and so will refrain from suppressing encoded spiritual wisdom.
Astrology and alchemy developed out of the material world sciences of astronomy and chemistry. A tendency of material world scientists to wander in their investigations towards the study of the spiritual world is natural, since understanding and controlling the spiritual world is typically far more useful, especially in the short run, than understanding and controlling the material world.