Although Sale in his “Preliminary Discourse” to his translation of the Qoran endeavours to contribute to a fair estimation of Mohammed and his work, of which his motto borrowed from Augustine, “There is no false doctrine that does not contain some truth” ("nulla falsa doctrina est quae non aliquid veri permisceat"), is proof, still the prejudicial view remained for a considerable time the prevalent one.  Mohammed was branded as imposteur even in circles where Christian fanaticism was out of the question.  Voltaire did not write his tragedy Mahomet ou le fanatisme as a historical study; he was aware that his fiction was in many respects at variance with history.  In writing his work he was, as he himself expresses it, inspired by “l’amour du genre humain et l’horreur du fanatisme.”  He wanted to put before the public an armed Tartufe and thought he might lay the part upon Mohammed, for, says he, “is not the man, who makes war against his own country and dares to do it in the name of God, capable of any ill?” The dislike that Voltaire had conceived for the Qoran from a superficial acquaintance with it, “ce livre inintelligible qui fait fremir le sens commun a chaque page,” probably increased his unfavourable opinion, but the principal motive of his choice of a representative must have been that the general public still regarded Mohammed as the incarnation of fanaticism and priestcraft.

Almost a century lies between Gagnier’s biography of Mohammed and that of the Heidelberg professor Weil (Mohammed der Prophet, sein Leben and seine Lehre, Stuttgart, 1843); and yet Weil did well to call Gagnier his last independent predecessor.  Weil’s great merit is, that he is the first in his field who instituted an extensive historico-critical investigation without any preconceived opinion.  His final opinion of Mohammed is, with the necessary reservations:  “In so far as he brought the most beautiful teachings of the Old and the New Testament to a people which was not illuminated by one ray of faith, he may be regarded, even by those who are not Mohammedans, as a messenger of God.”  Four years later Caussin de Perceval in his Essai sur l’histoire des Arabes, written quite independently of Weil, expresses the same idea in these words:  “It would be an injustice to Mohammed to consider him as no more than a clever impostor, an ambitious man of genius; he was in the first place a man convinced of his vocation to deliver his nation from error and to regenerate it.”

About twenty years later the biography of Mohammed made an enormous advance through the works of Muir, Sprenger, and Noldeke.  On the ground of much wider and at the same time deeper study of the sources than had been possible for Weil and Caussin de Perceval, each of these three scholars gave in his own way an account of the origin of Islam.  Noldeke was much sharper and more cautious in his historical criticism than Muir or Sprenger.  While the biographies written by these two men have now only historical value, Noldeke’s History of the Qoran is still an indispensable instrument of study more than half a century after its first appearance.