Attention has already been called to the difference in the religious fervor of the Moslem tribes. Many of the Arabs even went so far as to deny the existence of God, although the vast body of them, perhaps, were indifferentists. The Berbers and the mass of the people generally were very enthusiastic Mohammedans, so that it was unsafe to express one’s opinions contrary to the faith or even to engage openly in certain philosophical studies, for these were regarded as heretical. Among the religious themselves there were varying interpretations of the Koran and differences of rite. Religious toleration existed to such an extent that not only were the Mozárabes allowed to retain their churches, their priesthood, and their councils, but also some of their holy days were celebrated by Christians and Moslems alike. There was one instance where the same building served as a Mohammedan mosque and a Christian church. Christian clergymen from foreign lands frequently visited Moslem Spain, while native churchmen went forth from the caliphate to travel in the Christian countries, returning later to the peninsula. [Sidenote: The wealth of Cordova.] [Sidenote: Economic prosperity.] In the tenth century Moslem Spain came to be one of the richest and most populous lands in Europe. The wealth of Cordova was astounding, although some allowance has to be made for the exaggerations of the chroniclers. At one time the Moslem capital was said to have 200,000 houses, 600 mosques, and 900 bath-houses, besides many public buildings. It was well paved, had magnificent bridges across the Guadalquivir, and contained numerous palaces of the caliphs and other great functionaries. The most famous of all was that of Az-Zahra, which was a palace and town in one, erected by Abd-er-Rahman III for one of his wives. The great mosque of Cordova, which is in use today as a Catholic cathedral, was equally luxurious. This was begun in the reign of Abd-er-Rahman I, and was continued and enlarged by later Moslem rulers. It came to have nineteen aisles one way, and thirty another, with twenty-one gates, and 1293 columns of porphyry and jasper with gilded capitals. In its adornment it was a wealth of marble, silver, and precious stones. Travellers came to Cordova from all parts of the world, but it is worthy of note as an evidence of the lack of complete security, even in the greatest days of the caliphate, that it was the practice to come in great bodies, for the roads were infested with bandits. One measure of the advance of Moslem Spain is in the revenues of the government, which were eighteen times greater in the reign of Abd-er-Rahman III than they were in the reign of Abd-er-Rahman I.[16] This wealth depended on economic well-being, which was especially in evidence in the tenth century. The Arabs were not innovators in agriculture, but they had already learned much from others, and they assimilated Hispano-Roman and Mozárabic methods, with