With regard to the blind, much of what has been said above is entirely irrelevant. However intimately we appreciate the function of sight in our own mental development, it is almost impossible to imagine how different our life would have been had we never seen. But here, at the outset, a fundamental distinction must be drawn between those blind from birth or early infancy, and those who lose their sight in youth or adult life.[13] "It is better to have seen and lost one's sight than never to have seen at all," is quite as true as the sentiment which this form of statement parodies. Expressed physiologically, this means, that to have begun the general brain-building process with the aid of the eye insures some further self-development of the visual centre, and thus makes possible a kind of mental possession of which those born blind are inevitably deprived.[14] A fact of prime importance regarding the development of the sight-centre is the age at which its education is sufficiently completed to enable it to continue its function without further object-lessons on the part of the retina. If we accept as the test of the independent existence of the sight-centre its automatic excitation in dreams, the question can be answered by determining the age of the onset of blindness, which divides those who do not from those who still retain in their dream-life the images derived from the world of sight. The data that enable me to answer this question were gathered at the Institutions for the Blind in Philadelphia and Baltimore. Nearly 200 persons of both sexes were personally examined, and their answers to quite a long series of questions recorded. All dates and ages were verified by the register of the institution, and the degree of sight was tested.