Excerpt from the book:

The Tao that can be trodden is not the enduring and unchanging Tao. The name that can be named is not the enduring and unchanging name.

(Conceived of as) having no name, it is the Originator of heaven and earth; (conceived of as) having a name, it is the Mother of all things.

Always without desire we must be found,

  If its deep mystery we would sound;

  But if desire always within us be,

  Its outer fringe is all that we shall see.

Under these two aspects, it is really the same; but as development takes place, it receives the different names. Together we call them the Mystery. Where the Mystery is the deepest is the gate of all that is subtle and wonderful.

All in the world know the beauty of the beautiful, and in doing this they have (the idea of) what ugliness is; they all know the skill of the skilful, and in doing this they have (the idea of) what the want of skill is.