in the Islamic world. A practical example of this is the debate over the permissibility of the mawlid or commemoration of Muhammad's birthday. All scholars agree that such celebrations did not exist in the early period of Islamic history, and yet mawalid commemorations are a common element in Muslim societies around the world. Even so, Sunnis' scholars are divided between emphatic unconditional condemnation and conditional acceptance of the celebration with the former insisting it is a bidah and thus automatically unlawful, while the latter argues it nonetheless is contextually permissible. British historian Sadakat Kadri has noted the change over time in what is considered bidah. Hadith were not written down until the 9th century, at least in part because "traditionists such as Ibn Hanbal considered human literature to be an unholy innovation." This interpretation changed even for very conservative jurists such as Ibn Taymiyyah who wrote dozens of books. Ibn Taymiyyah however considered mathematics, a bidah, a false form of knowledge that "does not bring perfection to the human soul, nor save man from castigation of God, nor lead him to a happy life", and forbade its use in determining the beginning of lunar months. Very conservative Wahhabis allow the broadcast of television but Indian Deobandi forbid their followers from watching it, but make use of the more recent invention the internet to issue fatwas. Traditionally who died of plague and who did not was explained as simply the will of God based on al-Bukhari's al-Sahih hadith, but studying the progress of the Black Death (bubonic plague) in the 14th century, scholar Ibn al-Khatib noted those who died had the plague transmitted to them from "garments, vessels, ear-rings; ... persons ... by infection of a healthy sea-port by an arrival from an infected land" where as isolated individuals were immune. In the Muqaddimah, Ibn Khaldun defends the science of medicine from suggestions that it is an innovation going against the Sunna. "The medicine mentioned in religious tradition ... is in no way part of the divine revelation." It was simply part of "Arab custom and happened to be mentioned in connection with the circumstances of the Prophet, like other things that were customary in his generation." But was "not mentioned in order to imply that [it] is stipulated by the religious law." In his Book of Knowledge Al-Ghazali observed that many phenomena once thought bidah had come to be though