One day, while bathing in the Yellow River, he noticed the patterns on the shell of a tortoise; this led him to observe the patterns of nature, and thus, understand the very structure of reality itself.
Fu Hsi developed a system of trigrams, which he called the Book of Changes.Two thousand years later, the good King Wen was falsely imprisoned by the last heir to the tyranical Shang dynasty. While incarcerated, Wen studied The Book of Changes, and by dividing and doubling Fu Hsi's original trigrams, created the hexagrams we know today.
The I-Ching is based on the concept of Yin and Yang- the balance of polarized opposites, the two primary forces of the Universe. Yang is creative, light and active, while Yin is receptive, dark and passive.
Together, they form a complimentary pair which is always seeking balance.