Underneath the marvels, this universe runs on a coherent set of rules for how one kind of being becomes another. Later fantasy readers will find the system oddly familiar — Chinese web-novels made it famous — but its foundations are two thousand years old, and the classical stories all assume it.
1. Essence accumulates. Everything is made of qi, and the refined essences — of sun and moon, of mountains and rivers, of one’s own body — can be gathered and kept. Whatever gathers enough, gains power and eventually consciousness. This is why old things are dangerous: a thousand-year fox, a thousand-year tree, a thousand-year sword have simply been collecting longer than you.
2. Humans cultivate toward immortality. The human path is called xiulian (修炼): meditation and breath-work to refine the body’s essence; diet and alchemy (the elixirs and cinnabar pills of the Taoist masters); the guarding of one’s vital energies; and above all the loosening of attachment to the world — the lesson of the Handan pillow. Practised long and purely enough, the body is transmuted: the mortal husk is shed, and the cultivator becomes a xian, an immortal, light enough to ride the clouds.
3. Animals cultivate toward humanity. The animal path runs toward human form. A fox gains a tail per century; at some threshold of age and essence it can take human shape, speak, and join human society. This is not a trick of disguise but an actual promotion in kind — which is why fox-wives in the stories can truly love, and truly suffer, as humans do.
4. Advancement is policed by heaven. Great promotions provoke tribulations (劫, jie): ordeals sent by the cosmic order to test the candidate — thunderbolts, temptations, ordeals of character. The snake-spirit who has cultivated a thousand years must survive heaven’s lightning to take full human form. Fail, and centuries of work are burned away. The universe promotes on merit, but it examines.
5. Karma outranks everything. Beneath all mechanics lies the ledger: cause and effect across lives. Cultivation can change what you are; karma decides what happens to you. The tyrant king and the lovers’ graves, the serpent fed on tribute-girls, the filial son visited by heaven — the stories insist, with monotonous moral seriousness, that no amount of power exempts anyone from the accounting. Even gods are audited.
That is the whole system: gather essence, pass the trials, and mind the ledger. Every marvel in this library is a special case of it.