yang and yin

The key to the relationship between yang and yin is called hsiang sheng mutual arising or inseparability. As Lao-tzu puts it: 

When everyone knows beauty as beautiful, there is already ugliness;
When everyone knows good as goodness, there is already evil;
"To be" and "not to be" arise mutually;
Difficult and easy are mutually realised;
Long and short are mutually contrasted;
High and low are mutually posited;...
Before and after are in mutual sequence.

They are thus like the different, but inseparable, sides of a coin, the poles of a magnet, or pulse and interval in any vibration. There is never the ultimate possibility that either one will win over the other, for they are more like lovers wrestling than enemies fighting. But it is difficult in our logic to see that being and nonbeing are mutually generative and mutually supportive, for it is the great and imaginary terror of Western man that nothingness will be the permanent end of the universe. We do not easily grasp the point that the void is creative, and that being comes from nonbeing as sound comes from silence and light from space. 

Thirty spokes unit at the wheel's hub;
It is the centre hole [literally, "from their not being"] that makes it useful.
Shape clay into a vessel;
It is the shape within that makes it useful.
Cut out doors and windows for a room;
It is the holes which make it useful.
Therefore profit comes from what is there;
Usefulness from what is not there. 

Tao: The Watercourse Way, Alan Watts