From an Alan Watts lecture, titled "Taoism"
The tao is a certain kind of order, and this kind of order is not quite what we call order when we arrange everything geometrically in boxes, or in rows. That is a very crude kind of order, but when you look at a plant it is perfectly obvious that the plant has order. We recognize at once that is not a mess, but it is not symmetrical and it is not geometrical looking.
The plant looks like a Chinese drawing, because they appreciated this kind of non-symmetrical order so much that it became an integral aspect of their painting. In the Chinese language this is called li, and the character for li means the markings in jade. It also means the grain in wood and the fiber in muscle.
We could say, too, that clouds have li, marble has li, the human body has li. We all recognize it, and the artist copies it whether he is a landscape painter, a portrait painter, an abstract painter, or a non-objective painter. They all are trying to express the essence of li.
The interesting thing is, that although we all know what it is, there is no way of defining it. Because tao is the course, we can also call li the watercourse, and the patterns of li are also the patterns of flowing water. We see those patterns of flow memorialized, as it were, as sculpture in the grain in wood, which is the flow of sap, in marble, in bones, in muscles.
All these things are patterned according to the basic principles of flow. In the patterns of flowing water you will see all kind of motifs from Chinese art, immediately recognizable, including the S-curve in the circle of yang-yin.
So li means then the order of flow, the wonderful dancing pattern of liquid, because Lao-tzu likens tao to water:
The great tao flows everywhere,
to the left and to the right,
It loves and nourishes all things,
but does not lord it over them.