It is a common prejudice to treat Voltaire as if he had done nothing save write the Pucelle and mock at Habakkuk. Every serious and instructed student knows better. Voltaire's popularisation of the philosophy of Newton (1738) was a stimulus of the greatest importance to new thought in France. In a chapter of this work he had explained with his usual matchless terseness and lucidity Berkeley's theory of vision. The principle of this theory is, as every one knows, that figures, magnitudes, situations, distances, are not sensations but inferences; they are not the immediate revelations of sight, but the products of association and intellectual construction; they are not directly judged by vision, but by imagination and experience. If this be so, neither situation, nor distance, nor magnitude, nor figure, would be at once discerned by one born blind, supposing him suddenly to receive sight. Voltaire then describes the results of the operation performed by Cheselden (1728) on a lad who had been blind from his birth. This experiment was believed to confirm all that Locke and Berkeley had foreseen, for it was long before the patient could distinguish objects by size, distance, or shape.[61] Condillac had renewed the interest which Voltaire had first kindled in the subject, by referring to Cheselden's experiment in his first work, which was published in 1746.[62] It happened that in 1748 Reaumur couched the eyes of a girl who had been born blind. Diderot sought to be admitted to the operation, but the favour was denied him, and he expressed his resentment in terms which, as we shall see, cost him very dear. As he could not witness the experiment, he began to meditate upon the subject, and the result was the _Letter on the Blind for the Use of those who See_. published in 1749--the date, it may be observed in passing, of another very important work in the development of materialistic speculation, David Hartley's _Observations on man, his frame, his duty, and his expectations_. Diderot's real disappointment at not being admitted to the operation was slight. In a vigorous passage he shows the difficulties in the way of conducting such an experiment under the conditions necessary to make it conclusive. To prepare the born-blind to answer philosophical interrogatories truly, and then to put these interrogatories rightly, would have been a feat, he declares, not unworthy of the united talents of Newton, Descartes, Locke, and Leibnitz. Unless the patient were placed in such conditions as this, Diderot thinks there would be more profit in questioning a blind person of good sense, than in the answers of an uneducated person receiving sight for the first time under abnormal and bewildering circumstances.[63] In this he was undoubtedly right. If the experiment could be prepared under the delicate conditions proper to make it demonstrative evidence, it would be final. But the experiment had certainly not been so prepared in his time, and probably