Character amnesia
It is difficult to establish exactly how many different Chinese characters are in use today; the new HSK, a widely used proficiency test for Standard Chinese as a second language, tests over 2,600 different characters, whilst in 1988 the People's Republic of China published the "List of Commonly Used Characters in Modern Chinese" which contains 7,000 different characters. In Japan, where a smaller set of characters are in general usage, the Japanese Ministry of Education prescribes the teaching of 2,136 kanji in primary and secondary school in a list called the jōyō kanji, meaning "regular-use Chinese characters".
Xu Shen
In the postface, Xu gives a brief account of the history of writing. According to legend, Chinese characters were first invented by Cang Jie, who was inspired by footprints to create a system of signs that refer to the natural world. These original graphs could then be combined to make meaningful characters with referents distinct from their component graphs. This was important for Xu Shen, who emphasized the productivity, even fertility, of Chinese writing. Despite the novel meanings of compound characters, Xu Shen believed that true understanding of composite characters was contingent on an understanding of their components. Providing clear explanation of these relationships lay at the heart of his motivation.
Central to Xu Shen's thought is the contrast between "wen" (文 patterns) and "zi" (字 characters), indeed these contrasting categories of graphs receive separate mention in the work's title. Even today, there is disagreement over the exact definitions of the two terms. The Song dynasty scholar Zheng Qiao (郑樵) first presented the interpretation that "wen" and "zi" are the difference between non-compound and compound characters. More recently, other scholars, such as Françoise Bottéro (2002), have argued that the addition of a specifically phonetic element (and thus not simply compounding) marks the principal difference between "wen" and "zi". From this original binary contrast, Xu Shen formally delineated for the first time the six categories (六书) of Chinese characters.
To organize the thousands of headwords, Xu Shen established 540 radicals, and ordered them from least to greatest complexity. Each radical then headed its own group, which in turn subsumed all composite characters which incorporated the specific radical. The total number of 540 has a cosmological weight, as it can be derived from the product of 6, 9, and 10. 6 and 9 are the numbers of Yin and Yang, and 10 a number signifying completion. This number symbolizes the exhaustive scope of the dictionary.