Tao te Ching: 6

The spirit of the valley never dies.
It is called the subtle and profound female.
The gate of the subtle and profound female
Is the root of Heaven and Earth.
It is continuous, and seems to be always existing.
Use it and you will never wear it out.

I think this probably means more when taken in a historical context. I don't know what the spirit of the valley is, but Lao Tzu does speak of returning to the dusty earth and later will say that a man should dwell in the low places. This may be the spirit of the valley: the sage in the low place, but I don't think so. I think the spirit of the valley is the Tao. It could be that it is both the sage and the Tao, since the sage emulates the Tao.

So then is the gate of the subtle and profound female what separates Heaven and Earth? It is the root of Heaven and Earth and proceeds from the spirit of the valley, which is the Tao. If so, then this gate always exists, and like the Tao it can be used without wearing it out.

This fits with his earlier chapters, but I haven't even started to comprehend what Lao Tzu is telling us here. Perhaps it is just that Heaven and Earth are just as enduring as the eternal Tao, and they too can be used and will never wear out. I look at the environment as we know it and see that this isn't demonstrably true, unless we consider Earth as all of the cosmos: the physical plane of existence. If so, Heaven is that higher plane that we seem to experience from time to time but have absolutely zero rational evidence for. The worldly and the sublime, descending from the eternal unnameable Tao.