In the 1970s the Revisionist School of Islamic Studies raised fundamental doubts about the reliability of traditional Islamic sources and applied the historical-critical methods to the early Islamic period, including the veracity of the conventional account of Muhammad. A major source of difficulty in the quest for the historical Muhammad is the modern lack of knowledge about pre-Islamic Arabia. According to Harald Motzki, "On the one hand, it is not possible to write a historical biography of the Prophet without being accused of using the sources uncritically, while on the other hand, when using the sources critically, it is simply not possible to write such a biography."