The defence of the new doubt belonged on the other hand to the school of Al Kindi. This chemist lived and died during the ninth century. He was probably the earliest Arabian commentator on Aristotle, and seems to have paid special attention to the _Meteora_ of that author. The treatise _De Mineralibus_, so often appended to the _Meteora_ as a supplement, is ascribed to Al Kindi in the Paris manuscript.[114] It represents the alchemy of the time. Between these two contending parties stood the school of Avicenna, which now occupied an intermediate position and doubted of the doubt. That this had not always been the opinion of Avicenna himself is plain, however, from a passage which occurs in his _Sermo de generatione lapidum_, where the author unhesitatingly pronounces against the theory of transmutation. ‘Those of the chemical craft,’ he says, ‘know well that no change can be effected in the different species of things, though they can produce the appearance of them: tinging that which is ruddy with yellow till it looks like gold, and that which is white with colour at their pleasure till the same effect is in great measure produced. Nay, they can also remove the impurity from lead, so that it looks like silver, though it be lead still, and can endue it with such strange qualities as to deceive men’s senses, and this by the use of salt and sal ammoniac.’[115] Avicenna was evidently well acquainted with the secrets of art and held them at their proper value. Had his followers in the eleventh century done the same they would have supported the school of Al Kindi instead of taking a less definite position. This view of the later Arabian schools and their differences is forced upon us by the fact, that works are extant under the names of Rases, Al Kindi, and Avicenna, which evidently belong to the eleventh century, the period when they first appeared, and could not therefore have been written by authors who lived at an earlier date. They are plainly the production of later chemists who followed more or less intelligently the doctrine of these great masters in alchemy. The artifice involved in this ascription of authorship is one which has always been common in Eastern literature.