For many years (12 centuries to be precise), what Muslims believe about their God, their Prophet, and the Universe has been based upon a trilogy of texts. These are the Quran (what Allah said), the Hadith (what Muhammad said), and the Seerah (Muhammad's biography, or what others said about him).
The trilogy is based upon a hierarchy of authority. Every single letter and word of the Arabic Quran is considered to be without error. Not only what it says about Allah, but also what it relates about history, the natural and supernatural universe, and everything else is absolute perfection.
The authority of the Hadith is based upon a "science" that early Muslim scholars devoted years to develop. Tens of thousands of alleged sayings of Muhammad were carefully examined. Their authenticity was determined, not by whether or not a particular hadith made any historical, theological, or scientific sense, but by who related the information. If Aisha said that Muhammad said this or that, it was considered much more authentic than if someone less reliable reported what he said (comment: Muslims even argue the reason Allah commanded his Prophet to initiate a sexual relationship with this nine-year-old child was to give her those added years to hear and remember what he said!). At the top of the chain are the "Hadith Qudsi", or the "Holy Hadith". These are utterances given directly to Muhammad by God that for one reason or another did not make it into the Quran. Next in authority are the Hadiths found in books named after the men who collected them, of whom Bukhari and Muslim are the best known.
Muhammad's first extant biographies were only written about two centuries after his birth. Earlier writers such as al-Waqidi had given accounts of his raids, or "Maghazi", but these are not considered full biographies. The most authentic and earliest Seerah is that of Ibn Ishaq (translated in English as The Life of Muhammad by Guillaume but also available as an abridged version here).
Ibn Ishaq often noted that even he was unsure about the authenticity of material that he related. He began many sentences with "zaama" (it is alleged), and ended many others with "Allah yaalam" (only God knows if this is really true). Still others began with the sceptical phrase, "fi ma dhukira li" (it was told me...). This was particularly true when he recounted miraculous events associated with the life of Muhammad. As one example among many, Ibn Ishaq wrote, "It is alleged that (a woman who wanted Muhammad's father Abdallah to marry her instead of Amina, Muhammad's mother) used to say that a light blazed between Abdallah's eyes, but after he had sex with Amina (and she conceived the Prophet) the light went away." What Ibn Ishaq was saying was, "OK...I'm going to tell it the way I heard it, but I don't really believe that's the way it happened."
Muslims have always acknowledged these exaggerations in the Seerah, but until recently have accepted the historical events recounted therein as true. What I find fascinating is that the recent shift away from believing the Seerah has nothing to do with new historical or archeological evidence that might cast doubt upon the former accounts, but is simply that more non-Muslims are becoming familiar with what the Seerah actually says. In an Arabic interview, Wafa Sultan stated that it was "impossible to read the biography of Muhammad and really believe it, and emerge a mentally healthy person". Modern Muslims, particularly those living in the West, simply respond by stating they do not believe those elements of the Seerah that do not correspond with their wishful, mystical conception of their Prophet.
I've had a learning experience over the past few months since I posted Muslims and Muhammad and listed a dozen or so reasons why I did not believe anyone who did the things Muhammad did could be a Prophet of God. Initially Muslims, most of whom know nothing of the darker side of Muhammad, predictably responded that I was lying. One described me as a "bad person" who must be influenced by anti-Islamic literature. So I spent the next few weeks presenting the historical evidence from Islamic sources for the things I had mentioned, ending here. The response of Muslims suddenly changed; they no longer accused me of lying, but now said they did not believe material just because it was in the Seerah. To be valid, information about Muhammad had to be not only in the Seerah, but also in the Hadith. Muhammad could not have killed the poetess Asma bint Marwan, because her name does not appear in the Hadith. He could not have tortured Kinana to death at Khaybar to steal his wealth, because the Hadith does not mention that.
I noted here that Tariq Ramadan in his book In the Footsteps of the Prophet deliberately misrepresented material from the Seerah to present an image of Muhammad and his followers that Ibn Ishaq never intended. Now I'm beginning to understand how it works. If the Seerah says something positive about Muhammad (he was faithful to his first wife Khadijah), as a Muslim you believe it. If it says something that can be manipulated into something good (Abu Bakr exchanged non-Muslims slaves for Muslim slaves) you write (as Tariq did) that Abu Bakr set slaves free as an example of human rights. But if the Seerah says something that can in no way be seen as good, such as the murder of Asma bint Marwan, the Western-educated Muslim intellectual simply denies it because her name is not mentioned by al-Bukhari or Sahih Muslim (comment: back in the heartland, Muslims have no problem at all with Kinana of Khaybar or Asma bint Marwan).
The lengths to which Muslims go to protect their Prophet are amazing. First, they declare that all Prophets are maasum, protected from al-Kabair (major sins). That means that anything Muhammad did cannot be considered even sinful, much less evil. Next they accuse the Hebrew Scriptures of being corrupted because they honestly present the failures of Biblical characters (comment: the Prophet David, according to Muslims, could never have committed adultery with Bathsheba, and Lot could never have committed incest with his daughters. The fact that Jews never considered Lot a Prophet at all has no relevance in the strange discipline of Islamic scholarship). Finally, Muslims deny the evidence even from their own sources when it indicates that Muhammad was not the person they want him to be.