I have referred above to the sense of touch; it is, I think, clear that the first impression a child can have of sight must take the form of feeling the image on its retina, as though the object were actually inside the head, and it could have no idea that it was outside until, by touching with the hand, it would gradually learn by experience that the tangible outside object corresponded with the image located in the head; this is fully borne out by the testimony of men who, born blind, have, by an operation, received their sight late in life; in each case their first experience of seeing gave the impression that the object was touching the eye, and they were quite unable to recognise by sight an object such as a cup or plate or a round ball which they had commonly handled and knew perfectly well by touch; in fact, the idea of an object formed by the sense of touch is so absolutely different to that formed by the sense of sight that it would be impossible without past experience to conclude that the two sensations referred to one and the same object. The image formed on the retina has nothing in common with the sense of hardness, coldness, and weight experienced by touch, the only impression on the retina being that of colour or shade, and an outline; it is, however, hardly conceivable that even the outline of form would be recognised by the eye until touch had proved that form comprised also solidity and that the two ideas had certain motions in common both in duration in Time and extension in Space. Again, our senses of sight and hearing are alike based on the appreciation of frequencies of different rapidity; brightness and colour in light are equivalent to loudness and pitch in sound, but in sound we have no equivalent to perception of form or situation in space; it gives us no knowledge of the existence of objects when situated at great distances, nor can movements be followed even at short distances without having material contact, by means of the air, with the object; sight indeed appears to have to do with Space- and sound with Time-perception. In examining Nature by means of our senses we find we are so hemmed in by what we have always taken for granted and so bound down by modes of reasoning derived from what we have seen, heard, or felt in our daily life, that we are sadly hampered in our search after the truth. It is difficult to sweep the erroneous concepts aside and make a fresh start. In fact the great difficulty in studying the Reality underlying Nature is analogous to our inability to isolate and study the different sounds themselves which fall upon the ear, if our own language is being uttered, without being forced to consider the meaning we have always attached to those sounds.