Notan: the Dark-Light Principle

We put thirty spokes together and call it a wheel;
But it is on the space where there is nothing that the utility of the wheel depends.
We turn clay to make a vessel;
But it is on the space where there is nothing that the utility of the vessel depends.
We pierce doors and windows to make a house; 
and it is on these spaces where there is nothing that the utility of the house depends.
Therefore, just as we take advantage of what is,
we should recognize the utility of what is not.

- Lao Tse, c. 2000 years ago (translated by Arthur Waley in The Way and its Power, Houghton Mufflin Co., Boston, 1935)