There is one idea underneath every mystery tradition on earth, and once you have it, the whole library opens. It is the law of correspondence: as above, so below; as within, so without. The pattern that runs the biggest things shows up in the smallest ones. The order of the cosmos is echoed in the order of the body. The heavens are written into the human being. This is why the old teachers could look at the stars and talk about the soul, and mean the same thing both times.
Before we go further, be clear about what kind of key this is. Correspondence is a way of seeing — a language of comparison that lets you notice the same pattern at different sizes. It is wonderful for insight, quiet reflection, and knowing yourself. It is not a physics equation, and it does not need to be. When a symbol links the spine to the axis of the world, it is not saying the two are literally the same thing. It is training you to feel how they rhyme. Hold the key as a lens to look through, not a lever to force things with, and it will serve you for life.