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 never will be.[64]

Read in the light of the rich and elaborate speculative literature which England is producing in our own day, Diderot’s once famous Letter on the Blind seems both crude and loose in its thinking.  Yet considering the state of philosophy in France at the time of its appearance, we are struck by the acuteness, the good sense, and the originality of many of its positions.  It was the first effective introduction into France of these great and fundamental principles; that all knowledge is relative to our intelligence, that thought is not the measure of existence, nor the conceivableness of a proposition the test of its truth, and that our experience is not the limit to the possibilities of things.  That is an impatient criticism which dismisses the French philosophers with some light word as radically shallow and impotent.  Diderot grasped the doctrine of Relativity in some of the most important and far-reaching of all its bearings.  The fact that he and his allies used the doctrine as a weapon of combat against the standing organisation, is exactly what makes their history worth writing about.  The standing organisation was the antagonistic doctrine incarnate.  It made anthropomorphism and the absolute the very base and spring alike of individual and of social life.  No growth was possible until this speculative base had been transformed.  Hence the profound significance of what looks like a mere discussion of one of the minor problems of metaphysics.  Diderot was not the first to discover Relativity, nor did he establish it; but it was he who introduced it into the literature of his country at the moment when circumstances were ripe for it.

Condillac, as we have said, had published his first work, the Essay on the Origin of Human Knowledge, three years before (1746).  This was a simple and undeveloped rendering of the doctrine of Locke, that the ultimate source of our notions lies in impressions made upon the senses, shaped and combined by reflection.  It was not until 1754 that Condillac published his more celebrated treatise on the Sensations, in which he advanced a stride beyond Locke, and instead of tracing our notions to the double source of sensation and reflection, maintained that reflection itself is nothing but sensation “differently transformed.”