But there were other influences to come. With the fall of the Umayyads and the rise of the Abbasids, the intellectual centre of the empire moved to the basin of the Euphrates and the Tigris. The story of the founding of Baghdad there, in 145, we have already heard. We have seen, too, that the victory of the Abbasids was, in a sense, a conquest of the Arabs by the Persians. _Græcia capta_ and the rest came true here; the battles of al-Qadisiya and Nahawand were avenged; Persian ideas and Persian religion began slowly to work on the faith of Muhammad. At the court of the earliest Abbasids it was fashionable to affect a little free thought. People were becoming enlightened and played with philosophy and science. Greek philosophy, Zoroastrianism, Manichæism, the old heathenism of Harran, Judaism, Christianity—all were in the air and making themselves felt. So long as the adherents and teachers of these took them in a purely academic way, were good subjects and made no trouble, the earlier Abbasids encouraged their efforts, gathered in the scientific harvest, paid well for translations, instruments, and investigations, and generally posed as patrons of progress. But a line had to be drawn somewhere and drawn tightly. The victory of the Abbasids had raised high hopes among the Persian nationalists. They had thought that they were rallying to the overthrow of the Arabs, and found, when all was done, that they had got only another Arab dynasty. So revolts had begun to break out afresh, and now, curiously enough, they were of a marked religious character. They were an expression of religious sects, Buddhistic, Zoroastrian, Manichæan, and parties with prophetic leaders of their own; all are swept together by Muslim writers as _Zindiqs_, probably literally, “initiates,” originally Manichæans, thereafter, practically non-Muslims concealing their unbelief. For when not in open revolt they must needs profess Islam. In 167, we find al-Mahdi, who was also, it is true, much more strict than his father, al-Mansur, appointing a grand inquisitor to deal with such heretics. Al-Mansur, however, had contented himself with crushing actual rebellion; and Christian, Jew, Zoroastrian, and heathen of Harran were tolerated so long as they brought to him the fruits of Greek science and philosophy.