4. _Zoroastrianism_ was known to the Arab tribes in the north-east, but does not seem to have exercised any influence in Mecca or Medina except indirectly through Judaism in its angelology. As soon, however, as the armies of Islam conquered Mesopotamia it began to penetrate the thought and practices of Islam (see below). _Sources of Authority._--Islam, as we have said, is founded on: (1) the Koran; (2) the tradition or rather the _sunna_ (manner of life of Mahomet) contained in the tradition (_Hadith_); (3) _ijma`_; the universal agreement; (4) _qiyas_ (analogy). 1. The _Koran_[1] (properly _Qur'an_ from _qara'a_ to collect, or to read, recite) is the copy of an uncreated original preserved by God (see below), sent down from the seventh heaven to Gabriel in the first heaven, and revealed to Mahomet in sections as occasion required. These revelations were recited by the Prophet and in many cases written down at once, though from ii. 100 it would seem that this was not always the case. God is the speaker throughout the revelations. It seems probable that the whole Koran was written in Mahomet's lifetime, but not brought together as a whole or arranged in order. As it exists now the Koran consists of 114 chapters called _suras_ (from _sura_, a row of bricks in a wall, a degree or step). The first is the _Fatiha_ (opening), which occupies the place of the Lord's Prayer in Christianity. The others are arranged generally in order of length, the longest coming first, the shortest (often the earliest in date) coming at the end. Certain groups, however, indicated by initial unvowelled letters, seem to have been kept together from the time of the Prophet. At the head of each _sura_ is a title, the place of its origin (Mecca or Medina) and the number of its verses (_ayat_) together with the formula, "In the name of God the Merciful, the Compassionate" (except in _sura 9_). For liturgical purposes the whole book is divided into 60 sections (_ahzab_) or into 30 divisions (_ajza_), each subdivided into a number of prostrations (_ruk`a_ or _sajda_). The origin of the collected and written Koran is due to Omar, who in the caliphate of Abu Bekr pointed out that many possessors of _suras_ were being slain in the battles of Islam and their property lost, that there was a danger in this way that much of the revelation might disappear, and that men were uncertain what was to be accepted as genuine revelation. Accordingly Zaid ibn Thabit who had been secretary to Mahomet, was commissioned to collect all he could find of the revelation. His work seems to have been simply that of a collector. He seems to have done his work thoroughly and made a copy of the whole for Abu Bekr. The collection was thus chiefly a private matter, and this copy passed after Abu Bekr's death into the hands of Omar, and after his death to Hafsa, daughter of Omar, a widow of Mahomet.