Yesod, the Foundation, the ninth Sephirah, the operation of the sphere of the moon. This is the airy sphere of the fourth dimension, termed in occultism the astral plane. Here we find the subtle electromagnetic substance into which all the higher forces are focused, the ether, and it constitutes the basis or final model upon which the physical world is built. It’s elemental attribution is that of air, ever flowing, shifting and in constant flux - yet because of that flux, in perpetual stability.

- The Hermedic Order of the Golden Dawn, Isreal Regardie