In 213 BCE, on the advice of his chancellor Li Si, the First Emperor of Qin ordered the destruction of historical records of the conquered states and of privately held copies of the Songs, the Documents and the writings of the philosophers. Works on medicine, divination and agriculture were exempt, which is why the Book of Changes came through unharmed. The scale of the burning has been debated, since a copy of everything was retained in the imperial library, but that library burned in the fighting at the end of the dynasty, and the effect on transmission was real.
When the Han set about reconstructing the canon, texts came back by two routes, and the difference became a century-long controversy. New Text versions were dictated or written out from memory by aged scholars and recorded in the current Han script; they were the first in the field and their masters held the official academic chairs. Old Text versions were manuscripts said to have been recovered from concealment, most famously from a wall of Confucius's house in Qufu, written in the pre-Qin script. The two often differed in wording, in chapter count, and in interpretation, and the argument was never purely philological: chairs, salaries and the right to define orthodoxy hung on it.
The Book of Documents carries the sharpest result. An Old Text version in fifty-eight chapters was presented at court by Mei Ze in the early fourth century CE and became the standard. Doubts accumulated for centuries, and in the seventeenth century Yan Ruoqu, who died in 1704, demonstrated in a systematic evidential study that twenty-five of its chapters were a later fabrication, assembled from quotations in other works. His conclusion is now generally accepted, and those chapters are read as forgery.