What's more, as the Eastern Chin author Ko Hung once wrote, "... fu are celestial signs derived from Lao Tzu .... the reason why they are seldom effective today is that many errors have crept in over the long history of their transmission ...." Since these words were written over 1,500 years ago, the amount of "depreciation" the fu have suffered down through the years can only be imagined.
今人用之少驗者，由於出來歷久，傳寫多誤故也，… … 今符上字不可讀，誤不可覺，故莫知其不定也，… …」說這話時，已距今一千五百年以上，至於這千年來「傳寫多誤」的「折舊率」究竟又有多少，更是只有天知道了。