Textual criticism is a branch of textual scholarship, philology, and literary criticism that is concerned with the identification of textual variants in either manuscripts or printed books. Ancient scribes made alterations when copying manuscripts by hand. Given a manuscript copy, several or many copies, but not the original document, the textual critic might seek to reconstruct the original text (the archetype or autograph) as closely as possible. The same processes can be used to attempt to reconstruct intermediate versions, or recensions, of a document's transcription history. The ultimate objective of the textual critic's work is the production of a "critical edition" containing a scholarly curated text. 

Many ancient works, such as the Bible and the Greek tragedies,[citation needed] survive in hundreds of copies, and the relationship of each copy to the original may be unclear. Textual scholars have debated for centuries which sources are most closely derived from the original, hence which readings in those sources are correct.[citation needed] Although biblical books that are letters, like Greek plays, presumably had one original, the question of whether some biblical books, like the Gospels, ever had just one original has been discussed. Interest in applying textual criticism to the Qur'an has also developed after the discovery of the Sana'a manuscripts in 1972, which possibly date back to the 7–8th centuries.
Whose manuscript had been discovered?
 Sana'a manuscripts