The mediæval booksellers were not all permitted to ply their trade in their own way. Since the supply of books for the students depended on them, the Universities of Paris, Oxford, and elsewhere deemed it their duty to keep them under control, having in view the maintenance of pure texts and the interests of the students, at whose expense the booksellers were not to be permitted to fatten. By the rules of the University of Paris the bookseller was required to be a man of wide learning and high character, and to bind himself to observe the laws regarding books laid down by the University. He was forbidden to offer any transcript for sale until it had been examined and found correct; and were any inaccuracy detected in it by the examiner, he was liable to a fine or the burning of the book, according to the magnitude of his error. The price of books was also fixed by the University, and the vendor forbidden to make more than a certain rate of profit on each volume. Again, the bookseller could not purchase any books without the sanction of the University, for fear that he might be the means of disseminating heretical or immoral literature. Later, it was made obligatory on him to lend out books on hire to those who could not afford to buy them, and to expose in his shop a list of these books and the charges at which they were to be had. The poor booksellers, thus hedged about with restrictions, often joined some other occupation to that of selling manuscripts in order to make both ends meet, but when this practice came to the notice of the University they were censured for degrading their noble profession by mixing with it "vile trades." But presumably no such rules as the above hampered the booksellers of non-university towns, such as London. The control assumed by the Universities over the book trade presently extended to interference with original writings and a censorship of literature. With the introduction of printing and the consequent increase of books and of the facilities for reproducing them this censorship was taken up by the Church.