Tao Te Ching: Chapter 10
When the intelligent and animal souls are held together in one embrace, they can be kept from separating. When one gives undivided attention to the (vital) breath, and brings it to the utmost degree of pliancy, he can become as a (tender) babe. When he has cleansed away the most mysterious sights (of his imagination), he can become without a flaw.
In loving the people and ruling the state, cannot he proceed without any (purpose of) action? In the opening and shutting of his gates of heaven, cannot he do so as a female bird? While his intelligence reaches in every direction, cannot he (appear to) be without knowledge?
(The Tao) produces (all things) and nourishes them; it produces them and does not claim them as its own; it does all, and yet does not boast of it; it presides over all, and yet does not control them. This is what is called 'The mysterious Quality' (of the Tao).
The thirty spokes unite in the one nave; but it is on the empty space (for the axle), that the use of the wheel depends. Clay is fashioned into vessels; but it is on their empty hollowness, that their use depends. The door and windows are cut out (from the walls) to form an apartment; but it is on the empty space (within), that its use depends. Therefore, what has a (positive) existence serves for profitable adaptation, and what has not that for (actual) usefulness.