The word
太極, tàijí: 太 means "supreme" and 極 is the highest beam of a roof, the furthest point. It's often rendered "the supreme ultimate" or "the great pole." It's the single principle, still undivided, from which all differences come. Tai chi is the same word in the older romanization, the one that named the martial art.
In the I Ching
The idea comes from the Great Commentary, one of the Ten Wings: "In the changes there is taiji. It gives rise to the two forms; the two forms give rise to the four images; the four images give rise to the eight trigrams."
Step by step:
- From taiji come the two forms: yin and yang, the broken line and the solid one.
- Paired up, they make four images: old yin, young yang, young yin and old yang.
- Taken three at a time, they make the eight trigram figures.
It's a way of saying that all the book's complexity springs from a single source that keeps dividing.
Later on
In the 11th century, Zhou Dunyi placed taiji at the heart of his philosophy in a famous diagram. The symbol of two fish, one black and one white, that everyone now links with taiji spread later still.
In a reading
It doesn't appear in the hexagram texts, but it lies beneath each one: every change is a movement of that unity.