enso

The enso, a simple circle drawn with a single, broad brushstroke, is the zen symbol of infinity. It represents the infinite void, the 'no-thing,' the perfect meditative state, and Satori (enlightenment.)

http://altreligion.about.com/library/glossary/symbols/bldefsenso.htm

The enso is one of the deepest symbols in Japanese zen: a symbolic representation of enlightenment, encompassing the universe in an endless, cyclical line. For the practitioner, the act of creating each single circle is a visual ko-an in which the state of that moment is revealed. As a daily practice, drawing Zen circles can act as a spiritual diary. No deception is possible in painting an enso, for the character of the artist is fully exposed in its nakedness. The more one contemplates it, the more profound its simplicity becomes...

http://wwwnew.towson.edu/theatremfa/artists/ensopage.html

The Zen symbol "supreme" is an enso, a circle of enlightenment. The Shinjinmei, written in the sixth century, refers to the Great Way of Zen as "A circle like vast space, lacking nothing, and nothing in excess," and this statement is often used as an inscription on enso paintings. The earliest reference to a written enso, the first Zen painting, occurs in the Keitokudento-roku, composed in the eighth century:

A monk asked Master Isan for a gatha expressing enlightenment. Isan refused saying, "It is right in front of your face, why should I express it in brush and ink?"
The monk then asked Kyozan, another master, for something concrete. Kyozan drew a circle on a piece of paper, and said, "Thinking about this is and then understanding it is second best; not thinking about it and understanding it is third best." (He did not say what is first best.)

Thereafter Zen circles became a central theme of Zen art. Enso range in shape from perfectly symmetrical to completely lopsided and in brushstroke (sometimes two brushstrokes) from thin and delicate to thick and massive. Most paintings have an accompanying inscription that gives the viewer a "hint" regarding the ultimate meaning of a particular Zen circle. The primary types of enso are: (1) Mirror enso: a simple circle, free of an accompanying inscription, leaving everything to the insight of the viewer. (2) Universe enso: a circle that represents the cosmos (modern physics also postulates curved space). (3) Moon enso: the full moon, clear and bright, silently illuminating all beings without discrimination, symbolizes Buddhist enlightenment. (4) Zero enso: in addition to being curved, time and space are "empty," yet they give birth to the fullness of existence. (5) Wheel enso: everything is subject to change, all life revolves in circles. (6)Sweet cake enso: Zen circles are profound but they are not abstract, and when enlightenment and the acts of daily life-"sipping tea and eating rice cakes"-are one, there is true Buddhism. (7) "What is this?" enso: the most frequently used inscription on Zen circle paintings, this is a pithy way of saying, "Don't let others fill your head with theories about Zen; discover the meaning for yourself!"

http://zenart.shambhala.com/browse-gallery.htm?selectedBrowseKey=2488

The Enso (Japanese for 'circle') is a Zen symbol of the absolute, the true nature of existence and enlightenment. It is a symbol that combines the visible and the hidden, the simple and the profound, the empty and the full. As an expression of infinity, it has links to the western lemniscate, and may be painted so that there is a slight opening somewhere in the circle, showing that the Enso is not contained in itself, but that it opens out to infinity. In Zen art, the space on the page is at least as important as the brushstrokes themselves. It is no coincidence that the feeling of satori, a state of spiritual enlightenment beyond the plane of discrimination and differentiation, is often described as that of infinite space.

The Enso is a popular subject in Zen painting, where the circle is drawn in a single brushstroke and the state of mind of the painter is said to be indicated by the resulting circle - a strong and balanced Enso can only be painted by someone who is in equilibrium and inwardly calm. The very imperfections and contours of the Enso, which must be painted by the human hand rather than constructed as a mathematically correct circle, make the Enso a manifestation of perfection - it is perfect just as it is.

http://www.byzant.com/symbols/enso.asp
An enso ('circle') is the symbol of Zen itself and so an important art subject. fig. 17 is by an influential monk named Inzan (1758-1817), a 'grand-disciple' of Hakuin who trained many teachers. All of Hakuin Rinzai Zen (there are other schools) today comes through either Inzan or Takuju.

The circle itself represents the material world with its endless cycles. The space in the middle represents the emptiness at the heart of Zen.

This enso was brushed in two strokes, though one is usual. Inzan was probably trying to show directly the duality of good and evil, male and female...that together make up the whole of our world.

Zen art is the only religious art that I know with a sense of humor and delight.

http://www.zenpaintings.com/collecting-new.htm