The circle, a round enclosure, written as a zero or the alphabet O, is used to represent many things by various cultures in the world. In the study of metaphysics, the simple circle is often used to signify completion and/or perfection. Of course the circle can also mean zero or nothing.
One recalled a question from an elderly Chinese gentleman about the circle and what it actually symbolized in Chinese thoughts. At the time, he was having an animated conversation with a mutual friend, AK about the bagua (the eight trigrams) and about how much his professor friend knows about the I Ching when one happened to drop in to pay a visit. The elderly man oblivious of my presence emphasized that the professor has even written a book on bagua and that according to him the best book out of China is the Book of Changes. AK, whom I had given a W/B translated copy of the Zhouyi years earlier, introduced me and said that I know something about the Book of Changes.
Thereupon one was tested about what the circle represents when people say that the circle appears now empty and now filled; the elderly man had cupped a circle with both thumbs and fingers to emphasize the point. I cannot recall the exact answer given back then but it seemed to have satisfied him and he left shortly thereafter. It pays not to claim to know too many things. One had browsed through the bagua book of his friend in a bookstore. The contents were rather basic and the author could not clearly explain the concepts. With seemingly endless changes, the bagua is not easy even for masters to understand.(Think Lu Hsun of Wu who was trapped by Kungming's Eight Arrays.)
In Zhouyi studies, the circle is taken to represent Heaven and the square to represent Earth. Heaven is round and Earth is square and we can see these representations in the old Chinese coins. The yarrow stalks (and coins) are round, while the hexagrams are square. In line with this understanding, inner alchemy (neidan) adepts use the circle to represent Heaven and a few other things such as the Gold Pill (Jindan – the Golden Elixir), and the Center (Zhong, a bisected circle later symbolized by the Taiji - the Supreme Ultimate).
The Circulation of the Light meditation also evolves around a circle of breath and light. The breath when inhaled is made to flow down the front part of the body to Kun / Earth (bottom) and reversed up the back to Qian / Heaven (top). This is known as the backward flow. Upon exhalation, remnants of breath will continue downwards to Kun to complete a circle. Once a circular connection has been made, the rest is easy and simple through continual practice and spontaneity.
If we discuss the Wuji tu (Chart of the Infinite) we can say that the circle was originally empty and silent. Then after an eternity, there was chaos and the circle became filled with yin yang (Earth and Heaven) and the ten thousand things. Therefore to return to Tao we have to go back the same way through the empty circle (Wuji). But firstly we have to reach and pass through the center (the filled circle - Taiji) before we can proceed further. The center is what the great sages pointed to in their Daoist texts, Confucian books and Buddhist sutras.
If you can see the circle filled with yin yang, it is full as depicted in the Taiji; when yin yang is no longer present the circle is empty (Wuji). In the absence of duality, the silvery moon is the empty circle.
Meanwhile, if some readers think that the simple circle is nothing spectacular or important, perhaps they would understand why all the planets in Heaven are round or circular in shape. Planet Earth is round too. Of course we have to listen to the scientists, since science is also important. And my knowledge of science is minimal.
If only the Hubble space telescope was invented before the Tao Te Ching was written. Probably after seeing the pictures, Laozi can be a bit more descriptive in some of the chapters. Ha, what can be proven to people if things can only be witnessed within? Even if Laozi had explained the sights in full, there will still be scholars and Daoists who would dispute what they have not seen or cannot see. (A joke often shared between my Daoist friend and me.) If only they follow the right path and cultivate, they may be able to see for themselves the simple circles within. Perhaps, only then will there be some lasting peace.