A curious question here offers itself for our consideration. In the times of Robert Castrensis alchemy was as yet unknown to the Latins. Michael Scot, as we shall presently see, described it in one of his works as meeting with but a poor reception at its first introduction among them.[124] How then did it come to pass that in a few years the theory of Rases became so popular in the West, and continued for so many ages to direct the progress of chemical study among the European nations with enduring power? We find the explanation of this sudden change in the fact that human thought has always been subject to the tyranny of ruling ideas. In our own day the place of direction is filled by a doctrine of development which is eagerly made use of in every department of knowledge. In those earlier ages the same place seems to have been held by a doctrine of _transformation_. This idea ruled the thoughts of men like an obsession, in whatever direction they turned their minds. We see it in their superstitions, suggesting the wild tales of were-wolves and of other animal forms assumed at will by wizard and witch. We find it in religion, infusing a new meaning into the hyperbolical language of still earlier times, till, under this direction, there came to be fastened upon the Church a full-formed doctrine of Transubstantiation.[125] It is the operation of the same idea then that we are to remark also in the scientific sphere. As soon as the first shock of their surprise was over, the Latins greedily embraced a theory of chemical change which related itself so naturally to the prevailing habit of their minds, and which promised to show as operative in the mineral kingdom a law already conceived to hold good in the world of organic life. The Riccardian Library of Florence possesses another of those volumes to which we have already referred: a collection of alchemical treatises formed in the end of the thirteenth or beginning of the fourteenth century.[126] Among these appears one called the _Liber Luminis Luminum_. It is said to have been translated by Michael Scot, and, as there is no reason to doubt this ascription, we have now the means of determining with some fulness and accuracy the lines on which the philosopher proceeded in his chemical researches. The book opens with a preface somewhat scholastic,[127] and one which, on this ground as well as on others, is probably to be ascribed to Scot himself. In this part of the work he informs us that he took as his basis in the following compilation a text called the _Secreta Naturae_. To it he added material derived from other sources, which seemed necessary in order to complete the doctrine of chemistry contained in the _Secreta_. In this way he endeavoured to present his readers with a full and practical body of Alchemy according to the teaching of the school to which he belonged.