"There is one other point of comparison which is worth notice. The elementary signs of language are only twenty-six letters, and yet what wonderfully varied meanings can we express and communicate by their combination! Consider, in comparison with this, the enormous number of elementary signs with which the machinery of sight is provided. We may take the number of fibres in the optic nerves as two hundred and fifty thousand. Each of these is capable of innumerable different degrees of sensation of one, two, or three primary colours. It follows that it is possible to construct an immeasurably greater number of combinations here than with the few letters which build up our words. Nor must we forget the extremely rapid changes of which the images of sight are capable. No wonder, then, if our senses speak to us in language which can express far more delicate distinctions and richer varieties than can be conveyed by words." Finally (pp. 315, 16), "The correspondence, therefore, between the external world and the Perceptions of Sight rests, either in whole or in part, upon the same foundation as all our knowledge of the actual world,--on _experience_, and on constant _verification_ of its accuracy by experiments which we perform with every movement of our body. It follows, of course, that we are only warranted in accepting the reality of this correspondence so far as these means of verification extend, which is really as far as for practical purposes we need. "Beyond these limits, as, for example, in the region of Qualities, we are in some instances able to prove conclusively that there is no correspondence at all between sensations and their objects. "Only the relations of time, of space, of equality, and those which are derived from them, of number, size, regularity of co-existence and of sequence--'mathematical relations' in short, are common to the outer and the inner world, and here we may indeed look for a complete correspondence between our conceptions and the objects which excite them. "But it seems to me that we should not quarrel with the bounty of nature because the greatness, and also the emptiness, of these abstract relations have been concealed from us by the manifold brilliance of a system of signs; since thus they can be the more easily surveyed and used for practical ends, while yet traces enough remain visible to guide the philosophical spirit aright, in its search after the meaning of sensible Images and Signs." Let therefore this account of visual Perception be accepted by us, as it will probably be by three-fourths of scientific men throughout Europe. And, next, let us ask, as every real thinker will proceed to ask, on _what_ grounds of _certitude_ rests our assurance as regards the daily and hourly information received through this avenue of perception, reasoned and acted upon with unswerving confidence by us all?