Quotes about Door of all essence

Tao can be talked about, but not the Eternal Tao.
Names can be named, but not the Eternal Name.
As the origin of heaven-and-earth, it is nameless:
As "the Mother" of all things, it is nameable.
So, as ever hidden, we should look at its inner aspects.
As always manifest, we should look at its outer aspects.
These two flow from the same source,
though differently named;
And both are called mysteries.
The Mystery of mysteries is the Door of all essence.
-Tao Teh Ching, Chapter 1, trans. by John C. H. Wu, Shambhala, Boston & London c. 1990