At a glance
The pair the texts treat as the inner treasury: essence is what is stored, spirit is what is alert. ‘Keeping them within’ (內守) is this chapter’s plainest statement of what health consists in — and the modern sense of the word as ‘mind’ is a narrowing to only one half.
Read the compound with the modern sense and the line goes flat: “keep the mind inside” says little. Read it as two nouns — what is stored and what is watchful — and the line gives the chapter’s mechanism: the treasury stays shut, so nothing gets in.
The pairing is not incidental. Elsewhere in the corpus 精 and 神 are listed separately; here they are bound into one compound and given one verb, keep within (內守).
Passages where the term does real work, with the original, pinyin and English on the same line.
Every rendering below is a decision about which reader is being addressed. There is no neutral option.