The indication of such a period in the development of the human mind brings clearly into view the dependence of the higher mental processes upon the basis furnished them by the experiences of sensation; it strongly suggests a rational order and proportion in the training of the several faculties of the child's mind; and finally, it prevents the formation and survival of false notions, by substituting certain definite though incomplete knowledge for much indefinite though very systematic speculation. FOOTNOTES: [13] A noted blind teacher of the blind says: "Wenn wir ... den Einfluss der Blindheit auf die geistige Thätigkeit des Blinden beobachten, so haben wir Blindgeborene und Blindgewordene ... streng auseinander zu halten." [14] This applies mainly to intellectual acquirements. The emotional life of those who have lost their sight is often, and with much truth, regarded as sadder and more dreary than that of the congenitally blind; the former regretfully appreciate what they have lost; the latter live in a different and more meagre world, but have never known any other. It is interesting in this connection to trace the influence of the age of "blinding" (_sit venia verbo_) on the mental development of eminent blind men and women. Of a list of 125 blind persons of very various degrees of talent, which I have been able to collect, the age of blinding was (approximately) ascertainable in 114 cases. Of these about 11 are really very distinguished, and 10 of them (the exception is the wonderful mathematician, Nicholas Saunderson) became blind either in advanced youth, middle life, or still later; of the group next in eminence (about 25) the average age of the onset of blindness is in early youth (at nine or ten years); and those earliest blind are generally musicians, who least of all require sight for their calling. The average age of blinding of the rest of the list--whose achievements would for the most part not have been recorded had they not been those of blind persons--is as low as seven years, while that of the musicians (about 15 in the group) is little over three years. All this speaks strongly for the permanent intellectual importance of sight in early education. [15] Dr. Heermann's observations also enable us to trace the anatomical conditions underlying the power of dream-vision. From ten cases in which post-mortem examinations were held, he concludes that, allowing for much individual difference, after about twenty years the optic nerves degenerate, and often as far back as the chiasma. This shows that the nerve is not necessary for dream-vision, and thus goes to prove that the process is dependent on cerebral organs--a valuable piece of evidence fifty years ago. Esquirol records a case of sight-hallucinations in a blind woman, again indicating the same conclusion.