Things are yin or yang depending on the criteria you apply. We can\'t say that something is yin or yang in itself, only in relation to something else. And something can be both yin and yang, depending on the criterion of comparison being used. A black ball, for instance, is yin in colour (relative to white, yang) but yang in shape (a sphere relative to a cube, a circle being yang and a square yin).

The chill of the valley when the sun is no longer overhead is a dynamic representation of the bright sunny midday yang energy changing later in the day to a cold yin energy and preparation for nightfall. Similarly, night is yin, day is yang, moon is yin, sun is yang. Yin and yang change into each other when they have grown \'tired\' and \'old\', essentially when they have reached an extremity of themselves and can no longer go on being themselves. So the system of yin and yang is one of constant change and relativity of states. The waxing and waning of the moon dynamically represents the advance of yang (and decline of yin) followed by the advance of yin (and decline of yang), which is illustrated in my notes on the bigua sequence of hexagrams.

Yin and yang are characterisations of changing states to highlight different natures and what causes one thing to change into another. Earth itself is yin and heaven yang. This is represented on a Chinese coin, the square hole in the middle is seen as yin and earth, the round circumference of the coin is yang and heaven. At first, it is best to simply accept these ideas rather than try to \'understand\' them. After all, this system is not the world itself but an arbitrary overlay to understand certain things about it in simple terms. When you think of things being either \'woody\' or \'tinny\', \'antique\' or \'reproduction\', you are doing much the same thing, discerning qualities.[Page]

And we have learnt also to expect change as a result of judging atmospheres; after a long period of sunny weather has grown tired and flowers are wilting we easily sense in a build-up of humidity that there will be a tremendous thunderstorm that will render everything fresh and clean again. This is yang becoming old and tired and discharging itself to become young and fresh yin. Yin becoming old and tired can be seen in the long drawn-out winter days suddenly greeting the first sunshine, not quite spring, there is still a chill in the air, but the insects are awakening, flowers beginning to bloom, and this feeling is young and vibrant yang being aroused. The dragon (very yang) indeed begins his ascent in the spring, coming out of winter hibernation. And in the span of a single day the cycle of yin and yang can be witnessed, perhaps the morning will start off dour, the sun swathed in clouds, but by afternoon the clouds have dispersed and the sun is out. As it says in Daodejing 23: \'Gusts of wind do not blow all morning. The downpour will not last all day.\'