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.