In the _Koran_ God is brought in saying, "We have given you a book." By this it appears that the impostor published early, in writing, some of his principal doctrines, as also some of his historical relations. Thus, in his life of himself we find his disciples reading the twentieth chapter of the _Koran_, before his flight from Mecca; after which he pretended many of the revelations in other chapters were brought to him. Undoubtedly, all those said to be revealed at Medina must be posterior to what he had then published at Mecca; because he had not yet been at Medina. Many parts of the _Koran_ he declared were brought to him by the angel Gabriel, on special occasions, of which we have already met with several instances in his biography. Accordingly, the commentators on the _Koran_ often explain passages in it by relating the occasion on which they were first revealed. Without such a key many of them would be perfectly unintelligible. There are several contradictions in the _Koran_. To reconcile these, the Mussulman doctors have invented the doctrine of abrogation, _i.e._, that what was revealed at one time was revoked by a new revelation. A great deal of it is so absurd, trifling, and full of tautology that it requires no little patience to read much of it at a time. Notwithstanding, the _Koran_ is cried up by the Mussulmans as inimitable; and in the seventeenth chapter of the _Koran_ Mahomet is commanded to say, "Verily if men and genii were purposely assembled, that they might produce anything like the _Koran_, they could not produce anything like unto it, though they assisted one another." Accordingly, when the impostor was called upon, as he often was, to work miracles in proof of his divine mission, he excused himself by various pretences, and appealed to the _Koran_ as a standing miracle.[65] Each chapter of the _Koran_ is divided into verses, that is, lines of different length, terminated with the same letter, so as to make a different rhyme, but without any regard to the measure of the syllables.