I remember reading some of the Michael teachings years ago but it was not "mathematical" enough for me. Far to many words with nothing much being said. Years ago I repaired and calibrated precision instruments and I grew to appreciate precision. This also applies to 'teachings'. Even Castaneda spoke with precision, imo, even though much of what he said was kinda 'out there'.
I often viewed the soul as a formless dimension-less point of pure Will that comes from the highest level of creation that decends into the material world to begin its journey thru the world of form to develope consciousnes and awareness. In its journey it grows in awareness and knowledge as it ascends 'upwards' towards its point of origin from whence it came but now with consciousness and awareness and the "mastering" of the material world of form. It proceeds upward thru the three dimensions of space(and 4th?).
First it proceeds thru plant-1st dimension . Then animal-2nd dimension and then human-3rd dimension and then possibly the 4th dimension on its return trip to its origin point. So it may be that the four dimensional manifold are really all material, and in actual terms (and not just mathematical abstractions) the higher dimensions can possibly be viewed as really extensions of matter.
Beyond the 4th dimension or 'extension' the soul proceeds thru the non-material formless realms (5th, 6th, (and then 7th?) having now fully cognized and mastered the world of form with greater knowledge, awareness, and experience.
It may be that there are two aspects to the soul. The form and formless aspects. The formless aspect is real but does not exist in actuality and the form aspect exists in potential but is not yet real. Only when they are brought or married together that one might say that the soul is REAL both actually and potentially. So it might be that when we say that the soul "grows" it really means the lower centers, that is, the lower mind organizing itself so as to recieve the higher mind mediated by that point of awareness (the Will) that came from the highest levels of creation.
It may be that the soul can be defined as the mind/information organized, both in its higher and lower aspects by an act of the Will.
Just some thoughts here and I'm not sure if I explained this right.