1. What is a "soul?"
2. What is a soul made from?
3. What or who or how did the stuff that makes souls come into being?
4. What exactly is meant by "spiritual?" Define that word/concept precisely.
5. Where is this spiritual realm?
6. Is this spiritual realm finite or infinite?
7. What is this spiritual realm made out of. It's fundamental building blocks. Like we can say that protons, electrons, and neutrons are the building blocks of the physical realm.
8. How did this spiritual realm come into being?
7. Whether it's spiritual or material. It's just different densities of the same fundamental concept. Material has its' varying densities of solid, liquid, and gas, defined by the atomic mass of their varying constituents. The atoms are mostly composed of empty space, densest at the core which is bound of the next least dense fundamental, that of energy, or, force.
Energy and force I believe are about as close as you can get to the nature of spirit, or, consciousness - similar in density, but different functions. The physical vessel is driven by material electrons, but the animator that commands it is more akin to force; more subtle and less dense.
The forces vary, from the binding force of atoms, and the eventual release of this energy back to the cosmos in the form of radiation, to the invisible electromagnetic that traverses the terrestrial, as well as the aethereal sphere, and of course gravity(attractive) and dark energy(repellant).
So it would be posited here that among all of these, the most fundamental would be spirit, or, consciousness, or said another way(C), via a most renown Hermetic axiom - all is mind.