Samiel
One Thousand Club
If I were to stick with my "souls = essence filters" theory, I guess I'd justify that by saying that the body is the essence accumulator, the lower soul is what manages essence to animate the body and keep you alive, and the higher soul is the seat of reasoning and how one spends essence consciously or through direct effort (i.e., Prayer).
However, you can live (with no small discomfort) without a lower soul, but it takes away your vigour, so we might assume that the higher soul can perform the autonomic functions as well, when there's no lower soul to do so. Just not as well.
Without the body to accumulate essence, the Po seeks essence from other living bodies, killing them by instinct and drinking their essence through their blood or flesh. When in the underworld, it does the same to higher souls to harvest essence for itself. However, it cannot consciously use this essence, simply animating itself thereby. This fits the role of a "disposable" soul designed only to animate, whereas the higher soul is permanent by design and can consciously apply the essence outside itself.
However, you can live (with no small discomfort) without a lower soul, but it takes away your vigour, so we might assume that the higher soul can perform the autonomic functions as well, when there's no lower soul to do so. Just not as well.
Without the body to accumulate essence, the Po seeks essence from other living bodies, killing them by instinct and drinking their essence through their blood or flesh. When in the underworld, it does the same to higher souls to harvest essence for itself. However, it cannot consciously use this essence, simply animating itself thereby. This fits the role of a "disposable" soul designed only to animate, whereas the higher soul is permanent by design and can consciously apply the essence outside itself.