sources claiming to know more about the time of Muhammad than earlier ones (to add embellishments and exaggeration common to an oral storytelling tradition). Discrepancies compared to non-Muslim sources. But there are also similarities and agreements both in information specific to Muhammad, and concerning Muslim tradition at large. Some parts or genres of sīra, namely those dealing with miracles, are not fit as sources for scientific historiographical information about Muhammad, except for showing the beliefs and doctrines of his community. Nevertheless, other content of sīra, like the Constitution of Medina, are generally considered to be authentic by both Muslim and non-Muslim historians. Henri Lammens complains of contradictions in the Traditions about Muhammad's life, including on the number of his children and wives. Some accounts have him having one child, others two, and still another claimed he had twelve children, including eight boys. While most accounts state he had nine wives, "some passages of the sira speak of twenty three wives." Muhammad is thought to have lived between 60 and 65 years according to tradition. Hadith The hadith collections include traditional, hagiographic accounts of verbal and physical traditions attributed to Muhammad, and for many, often explain what a verse in the Quran is referring to in regards to Muhammad. Unlike the Quran, hadiths are not universally accepted by Muslims. Early Muslim scholars were concerned that some hadiths (and sīra reports) were fabricated, and thus they developed a science of hadith criticism (see Hadith studies) to distinguish between genuine sayings and those that were forged, recorded using different words, or were wrongly ascribed to Muhammad. In general, the majority of western academics view the hadith collections with considerable caution. Bernard Lewis states that "The collection and recording of Hadith did not take place until several generations after the death of the Prophet. During that period the opportunities and motives for falsification were almost unlimited." The main feature of hadith is that of Isnad (chains of transmission), which are the basis of determining the authenticity of the reports in traditional Islamic scholarship. According to Stephen Humphreys, while a number of "very capable" modern scholars defended the general authenticity of isnads, most modern scholars regard isnads with "deep suspicion". Jonathan A. C. Brown, a Sunni who follows the Hanbali school of Islamic jurisprudence, asserts that the hadith tradition is a "common sense science" or a "common sense tradition" and is "one of the biggest