The Book of Changes is one of the basic Confucian Classics. It is also much cherished by the Taoists. It is divided into the texts and commentaries. The texts consist of sixty-four hexagrams and judgments on them. These hexagrams are based on the Eight Trigrams, each of which consists of three lines, divided or undivided, the divided representing the weak, or yin, and the undivided representing the strong, or yang. Each of these eight corresponds to a direction, a natural element, a moral quality, etc. For example, ch'ien is heaven, k'un is earth, chen (activity) is thunder, sun (bending) is wind, k'an (pit) is water, li (brightness) is fire, ken (to stop) is mountain, and tui (pleasure) is a collection of water. Each trigrarn is combined with another, one upon the other, thus making sixty-four hexagrams. These hexagrams symbolize all possible situations. For example, the hexagram with the water trigram over the fire trigram symbolizes conquest, success, etc.
Each hexagram is followed by two texts, namely (1) the kua-tz'u or the explanation of the text of the whole hexagram and (2) the yao-tz'u or the explanation of the component lines. The commentaries number seven. First is (3) the tuan-chuan or the commentary on (1) and then there is (4) the hsiang or abstract meaning of (1) and (2). For the first two hexagrams (the ch'ien or Heaven and k'un or Earth), there are in addition (5) the wen-yen or commentary on the first two texts to stress their philosophical or ethical meaning. Following these sixty-four hexagrams and their discussions, there are (6) the hsi-tz'u or the appended re- marks, (7) the remarks on certain trigrams, (8) the remarks on the order of the hexagrams, and (9) the random remarks on the hexagrams. Nos. 3, 4, and 6, each in two parts, and nos. 5, 7, 8, and 9 form the "ten wings" of the book.
The most important parts are the texts (1 and 2) and discussions (5) on the first two hexagrams, the appended remarks (6), and the remarks on the trigrams (7). It is here that much of Chinese philosophical speculation has been based.
Tradition has ascribed the Eight Trigrams to legendary Fu-hsi, the sixty-four hexagrams to King Wen (r. 1171-1122 B.c.), the two texts (1 and 2) to him or Duke Chou (d. 1094 B.C.) and the "ten wings" to Confucius. Most modern scholars have rejected this attribution, but they are not agreed on when and by whom the book was produced. Most probably it is a product of many hands over a long period of time, from the fifth or sixth century B.c. to the third or fourth century B.C.
Source : Wing-Tsit Chan, in Chinese Philosophy