The Tao is expressed in the duality of "yin" and "yang" which is represented in this well-known symbol (also known as Tai Chi):

Around the outside there is a circle having no beginning and no end, which represents the origin of all things and contains all things. But things are not all the same as they would be in an undivided circle - there is a differentiation between things. There is a distinction between yin and yang, and from this there follows the difference between all the other many things. Some things are light and some dark, some are solid and some are hollow, some male and some female, some hot and some cold. But as experience shows, the world isn't just made of opposites, life isn't just "black and white". So rather than one side of the circle being black and the other white, it is as if black and white are circling around, showing that yin and yang are related to each other and not just separated by a straight boundary. There would be no "day" if there was not a "night" as well. And although day and night may seem separate, they are actually the result of one thing - of earth circling around.

In the yin-yang symbol there is also a light spot in the middle of the darkness, and a dark spot in the middle of the lightness. This is to show that at the very end of things there is always a new beginning, that when things reach their extreme they begin to turn into their opposite. Just when the year reaches its lowest and darkest point in midwinter, the days slowly begin to get longer and there is hope for the new year.

As Lao Tzu says:

"The Tao begets the one,
The one begets the two,
The two beget the three and
The three beget the ten thousand things.

All things are backed by the shade,
Faced by the light
And harmonised by the immaterial breath."