Toward the end of the 11th century, the gates of ijtihad closed for political reasons. The fragile Muslim empire--from Iraq in the East to Spain in the West--was experiencing a series of internal convulsions. Dissident denominations were popping up and declaring their own runaway governments. So the main Muslim leader, known as the caliph, cracked down politically. Within a few generations, Islam saw the closing of something else--the gates of ijtihad. The 135 schools of thought were whittled down to only four, in which conservative Sunni teachings reigned. This in turn produced a rigid reading of the Quran as well as a series of legal opinions known as fatwas that scholars could no longer overturn or even question, but only imitate. With some glorious exceptions, that's what Muslim scholars have been doing to this day--imitating each other's medieval prejudices, without much introspection. In fact, after the gates of ijtihad were closed, innovation was deemed a crime. Tolerance took a severe beating as result. One of the enduring lessons of history is that whenever an empire becomes insular to "protect" itself, intellectual decline and cultural intolerance are sure to follow.