Si Xiang
The 4 Phenomena

Si Xiang
The 4 Phenomena are usually
depicted as Bigrams.
"The Wuji, the Limitless, produces the delimited, and this is The Taiji, the Absolute. The Taiji produces 2 Forms, named Yin and Yang. The 2 Forms produce 4 Phenomena, named the Lesser and Great Yang, and the Lesser and Great Yin. The 4 Phenomena act on the 8 Trigrams, the Ba Gua." - by Fu Xi
We are tempted to associate the 4 Phenomena with the classic 4 elements of Western esoteric philosophy. This is productive but much like the 3 Cycles of the Wu Xing or the 5 Elements, the 4 Phenomena are active, motive principles.
In the I Ching, or Classic Book of Changes, the nature of these phenomena are abstracted and depicted simply as broken and unbroken lines for Yin and Yang. A Great Yin or Yang line is so old and powerful that it changes to it's opposite. A Lesser line is too young and remains unchanged. This is depicted by placing an X in a broken line or an O on an unbroken line and form the basis of the divinatory aspect of the I Ching. The Early Heavn arrangement of the Ba Gua shows the oppositional arrangements of the Trigrams. These Trigrams oppose each other line by line.
Hygeia's Bowl Pattern

Hygeia's Bowl
The motions of the trigrams form
the Hygeia's Bowl Pattern.
The Later Heaven arrangement demonstrates the complementary dynamic behavior of the Trigrams along 4 poles. This is difficult to describe but the above animation shows the progression where three poles intersect a fourth in turn.
