The three yang take heaven as their work · san yang tian wei ye

In brief

The clause stands at the head of the chapter’s doctrine and is best read as a definition by office. 三陽天爲業 — the three yang have heaven as their work — and what follows is the consequence: 上下無常，合而病至，偏害陰陽. Because their work is heaven’s, their qi has no fixed place above or below; because it has no fixed place, when the currents gather the illness arrives, and it harms yin and yang one-sidedly. 王冰 reads the clause through the old correspondence of father and heaven: the three yang are the greater yang, the greater yang is the father, the father is heaven, and so heaven is its work. The point of the phrase in the chapter is not the correspondence but the mobility — a qi whose office is the whole sky cannot be expected to keep to one region of the body, and the whole doctrine of 獨至 and 并至 follows from exactly that.