Tao Te Ching 52

The Tao which originated all under the sky is to be considered as the mother of them all.

When the mother is found, we know what her children should be. When one knows that he is his mother's child, and proceeds to guard the qualities of the mother that belong to him, to the end of his life he will be free from all peril.

Let him keep his mouth closed, and shut up the portals of his nostrils, and all his life he will be exempt from laborious exertion. 

Let him keep his mouth open, and spend his breath in the promotion of his affairs, and all his life there will be no safety for him.

The perception of what is small is the secret of clear-sightedness; the guarding of what is soft and tender is the secret of strength.

Who uses well his light,
Reverting to its source so bright,
Will from his body ward all blight,
And hides the unchanging from men's sight.