A philosophy originated in ancient China, a graphical representation of the Yin-Yang theory.
It attempts to capture the essence of everything on a conceptual level: those that are active are defined as Yang (white), and the opposite are defined as Yin (black).
The Taiji symbolizes opposing concepts such as day and night, light and darkness, male and female. At the same time, you may also call it a condensed version of the ever-changing, dynamic World.
Furthermore, there is a dot of Yin in Yang, and conversely a dot of Yang in Yin. This indicates that the distinction between Yin and Yang is not absolute. There is darkness even in light.
Taiji is the “one” at the beginning, the Yin and Yang that divide the “one” are called Liangyi1.
As an aside, the magi in this story were all practitioners of Western magecraft who were incompatible with Chinese philosophies.

Garden of sinners Pamphlet: Kara no Kyoukai Settings Glossary
Translator's Notes
  1. ^ Liangyi is read as Ryougi in Japanese. Shiki's name alludes to this concept of Yin/Yang duality.