According to Li Gan, the Spirit Theory explains why People of the Land die while Adventurers and demihumans respawn. What all of them have in common is a spirit which consist of a soul, the mind's power, and an essence, the body's strength. The stronger the soul, the higher the magical power and MP. The stronger the essence, the higher the Ki and HP.
The psyche, personality, ambition, character, emotions, and other information are stored in both soul and essence. When essence is diminished, it will start dispersing. This process is called failing essence. How long it takes depends on how strong the essence is. Low-level essence may disperse in a matter of minutes while high-level essence may take up to a half a day.
When a healer cast a resurrection spell, it will gather the dispersed essence around the area, and reconstruct and send it back to the body. If Ki is insufficient during the reconstruction process, the healer will top off with their own Ki. The information needed for reconstruction is taken from the body and calculated through reverse engineering. If there is any mistake in calculation, it will result in the body losing EXP, which is called data corruption.
If a resurrection spell is not used, after the essence has dispersed completely, the body will start to decompose itself. In this condition, the People of the Land are dead. A resurrection spell works on them by casting immediately after their death; however, it only delays their death since they cannot be revived.
For Adventurers, when the failing essence process has completed, the body and equipment will be disintegrated into light particles and sent to the Cathedral using the soul's power. The body will be reconstructed. The Cathedral will provide Ki necessary to repair the essence. After repairing the body, the soul will merge with the body to complete the resurrection process. Since this process exhausts both soul and essence, it results in data corruption and lost of EXP.
For demihumans, when the failing essence process has completed, the body will decompose itself, just like the People of the Land. The equipment will be left behind. The soul will be immediately reborn in another body. Its psyche might be able to retain memories, but they lose the ability to recall the memories freely and to recognize who they were.
Effect of the Spirit Theory
Shiroe suspects that death for Adventurers is not risk-free and may result in memory loss, since an Adventurer's soul is no longer in front of the monitor but in the game now. As a result, the continuity of memories is no longer a guarantee. This hypothesis has been confirmed by Krusty who has experienced death twice after the Catastrophe such that he can no longer recall his cat's name ("Nobody") or how it looked even though he claims himself to have a very good memory. Nonetheless, the memory loss does not seem to have a major impact on Adventurers' daily lives.