Internal alchemy.

What nèidān is

Internal alchemy — neidan or nei dan 內丹, and in older books the Golden Elixir 金丹.

Internal alchemy takes the language and structure of the older external alchemy — the furnace, the vessel, the firing times — and turns it inward, so that the body becomes the crucible in which the three treasures are refined. It draws on correlative cosmology, the emblems of the Yijing, medical theory, and the meditative and dǎoyǐn practices already covered elsewhere in the curriculum.

It is not a separate subject to graduate onto. It is what the earlier work becomes available for.

Dual cultivation 性命雙修

The approach we follow works both together: innate nature 性 and life-destiny 命. Working one without the other is the classic failure mode in both directions — a settled mind in a body that cannot support it, or a well-conditioned body with nothing steady behind it.

Why it is approached slowly

The alchemical literature is deliberately encoded, and it is easy to read a technique out of it that the text was not describing. The tradition's own texts are unusually candid about what goes wrong when methods are taken up out of sequence, or without the preparatory work — and they treat that as a clinical matter rather than a moral one.

So the sequence is not gatekeeping. Standing, sitting, breath and yǎngshēng are the conditions under which any of this makes sense. Where a practice properly belongs inside a sustained teaching relationship, we say so.

The Longmen material

Much of the alchemical literature we read comes through the Dragon Gate 龍門 stream, and particularly through the anthology gathered by Mǐn Yīdé 閔一得 — which is also the subject of the school's long-running translation work. Reading it in the original is one of the better correctives to reading it too enthusiastically.