For some additional context on today's post, see this post (and its comment thread) at Delta's D&D Hotspot regarding the spell Contact Other Plane.
The Kybalion (by "The Three Initiates," 1912) seems to be the other "go to" theosophic book on the planes. The second chapter, ("The Principle of Correspondence") embodies the idea that there is always a correspondence between the laws of phenomena of the various "planes" of being and life (p.28). This text lists the "Plane of Ethereal Substance" as the 4th sub-plane of the 7 minor physical planes (essentially the same place that Leadbeater puts it). But the Kybalion's ethereal plane itself consists of 7 sub-planes... "This Ethereal Substance forms a connecting link between Matter (so-called) and Energy, and partakes of the nature of each. The Hermetic Teachings, however, instruct that this plane has seven sub-divisions (as have all of the Minor Planes), and that in fact there are seven ethers, instead of but one."
So where am I going with this?
Well, as I stated at the beginning of this post, I'm trying to lay out some ground rules (but, honestly, little more than that, for the sake of fearing being too restrictive and cumbersome) for a "planar travel" appendix to the mystic class/psionics ruleset I'm developing for BX/LL (which is coming along swimmingly, BTW, thanks to some great insight I graciously received from Steve Marsh... the nearly-uncredited genius behind a lot of the planar concepts of D&D1). I've been trying to decide what rules are "givens," and which things are better left to DMs to develop themselves. Based on the above, I think I'm going to stick with the basic concept that one must access the ethereal plane before accessing the astral, even though both the 1e PHB version of the diagram of the planes (and it's predecessor from Dragon Magazine #8, pictured below) suggest that the Astral plane (area 9, in light blue) can be accessed directly from the Prime Material plane (area 1, in purple). This physical-to-ethereal-to-astral model seems to support the "higher consciousness" aspect of the mystic class I'm developing. The ethereal plane will still access all the other material planes (e.g., the elemental planes, and things like the positive and negative material planes, should I keep those "as is"). It's the astral plane and beyond that I'm still contemplating.
But, then again, I might decide to abandon this structure altogether.
There are some other ideas I'm toying with, so I'm sure there will be more posts to come.
1 "When ADVANCED DUNGEONS & DRAGONS was still in its earliest conceptual stage, Steve Marsh and I exchanged considerable correspondence pertaining to the planes." Gary Gygax, "The Sorcerer's Scroll," Dragon Magazine 38 (Vol. IV, No. 6), December 1979.