There are three layers of souls.
The soul, the source of power for humans and monsters. Cloaking themselves in their souls, the most brittle existence, the astral bodies Establishing themselves in power, spiritual bodies. Those who have a direct connection to this world, material bodies.
The soul is the entity itself, there is no “being” outside of it. Any kind of consideration requires the presence of an astral body. Of course, should it be based on the astral body alone, the being will dissipate into thin air. So a medium to record the memories is needed, the spiritual body. However, the spiritual body preserves kind of like virtual memory, therefore cannot serve as a permanent medium. Hence the flesh.
So for those with tempered souls, should they even sustain brain damage, recovery of certain memories becomes possible. And, there are monsters who live with only a soul. That kind of being, however, is inferior to the rest. But, even if it is but a soul, should it obtain some level of intelligence, it will be recognized as a monster.
These are what the world knows as the four dragons, the highest form of being. But, even this special race requires a soul.