Chapter six is six lines and among the most quoted in the book. The spirit of the valley does not die, and is called the dark female; the gate of the dark female is called the root of heaven and earth; barely there, it goes on, and using it never wears it out.
The valley is the book's standing figure for what is low, empty and receptive, and it is always a compliment. A valley is defined by the hills that are not there. It collects what runs down and gives rise to what grows, without doing anything at all, and rivers and seas rule the hundred valleys in chapter sixty-six by the same logic, because they lie below them.
The gate is a generative image and the text does not soften it. What is being described is a source that produces continuously, that is not depleted by producing, and that is characterised by yielding rather than by force. The last line repeats the promise made about the vessel in chapter four and the bellows in chapter five, that use does not exhaust it, and the three chapters together establish a single idea in three images.
Later Daoist practice took this chapter in directions the text does not obviously authorise, reading the valley, the gate and the unfailing breath as internal anatomy in traditions of physiological cultivation. That reading is a real part of the chapter's history and it belongs to those traditions rather than to the text.