These six books, the two _Sahihs_ and the four _Sunans_, came to be regarded in time as the principal and all-important sources for traditional science. This had already come about by the end of the fifth century, although even after that voices of uncertainty continued to make themselves heard. Ibn Maja seems to have been the last to secure firm footing, but even he is included by al-Baghawi (d. 516) in his _Masabih as-sunna_, an attempted epitome into one book of what was valuable in all. Still, long after that, Ibn Khaldun, the great historian (d. 808), speaks of five fundamental works; and others speak of seven, adding the _Muwatta_ of Malik to the six above. Others, again, especially in the West, extended the number of canonical works to ten, though with varying members; but all these must be regarded as more or less local, temporary, and individual eccentricities. The position of the six stands tolerably firm. So much it has been necessary to interpolate and anticipate with regard to the students of tradition whose interest lay in gathering up and preserving, not in using and applying. From the earliest time, then, there existed these two classes in the bosom of Islam, students of tradition proper and of law proper. For long they did not clash; but a collision was inevitable sooner or later. Yet, if the circle of the Muslim horizon had not widened beyond the little market-town of al-Madina, that collision might have been long in coming. Its immediate causes were from without, and are to be found in the wave of conquest that carried Islam, within the century, to Samarqand beyond the Oxus and to Tours in central France. Consider what that wave of conquest was and meant. Within fourteen years of the Hijra, Damascus was taken, and within seventeen years, all Syria and Mesopotamia. By the year 21, the Muslims held Persia; in 41 they were at Herat, and in 56 they reached Samarqand. In the West, Egypt was taken in the year 20; but the way through northern Africa was long and hard. Carthage did not fall till 74, but Spain was conquered with the fall of Toledo in 93. It was in A.D. 732, the year of the Hijra 114, that the wave at last was turned and the mercy of Tours was wrought by Charles the Hammer; but the Muslims still held Narbonne and raided in Burgundy and the Dauphiné. The wealth that flowed into Arabia from these expeditions was enormous; money and slaves and luxuries of every kind went far to transform the old life of hardness and simplicity. Great estates grew up: fortunes were made and lost; the intricacies of the Syrian and Persian civilizations overcame their conquerors. All this meant new legal conditions and problems. The system that had sufficed to guard the right to a few sheep or camels had to be transformed before it would suffice to adjust the rights and claims of a tribe of millionnaires. But it must not be thought that these expeditions were only campaigns of plunder. With the Muslim armies