Criticism of the Quran is an interdisciplinary field of study concerning the factual accuracy of the claims and the moral tenability of the commands made in the Quran, the holy book of Islam. The Quran is viewed to be the scriptural foundation of Islam and is believed by Muslims to have been sent down by God (Allah) and revealed to Muhammad by the archangel Jabreel, also spelt Jibraeel (Gabriel). It has been subject to criticism both in the sense of being studied by mostly secular Western scholars and in being found fault with. In "critical-historical study" scholars (such as John Wansbrough, Joseph Schacht, Patricia Crone, Michael Cook) seek to investigate and verify the Quran's origin, text, composition, history, examining questions, puzzles, difficult text, etc. as they would non-sacred ancient texts. The most common criticisms concern various pre-existing sources that Quran relies upon, internal consistency, clarity and ethical teachings. According to Toby Lester, many Muslims find not only the religious fault-finding but also Western scholarly investigation of textual evidence "disturbing and offensive". Historical reliability of the QuranHistory of the Quran and Historicity of Muhammad According to Islamic tradition, the Quran is the literal word of God as recited to the Islamic prophet Muhammad through the angel Gabriel. Muhammad, according to tradition, recited perfectly what the archangel Gabriel revealed to him for his companions to write down and memorize. The early Arabic script transcribed 28 consonants, of which only 6 can be readily distinguished, the remaining 22 having formal similarities which means that what specific consonant is intended can only be determined by context. It was only with the introduction of Arabic diacritics some centuries later, that an authorized vocalization of the text, and how it was to be read, was established and became canonical. Prior to this period, there is evidence that the unpointed text could be read in different ways, with different meanings.