Idealism easily widens its doubt, to correspond with the dimensions of the wider nervous law. Does not an aptitude for special impressions, so stringently determined as to translate the antecedent "blow" into the consequent, "light" or "sound," disqualify our senses for giving evidence respecting supposed facts of the outer world? As for the "distinctive impressibility of the eye," as Mr. Bain[100] describes colour, it need not be held real except for our own sensorium,[w] and if colour be a questionable reality, other alleged realities become questionable too. The world we live in, may be a totally different world from what we are taught, generation after generation, to believe it. Who can lay down the limits of what our minds create for themselves outside us?[101] The mental disease of the madman causes his eye to see that which is not. Guilt and sickness fill bedchambers with unreal spectres. Putting disease aside, and taking the case of healthy eye and healthy mind, it is confessedly difficult to define the exact province of each. A boy couched by Cheselden[102] saw all things in one plane; there was no perspective, and objects in the room seemed to touch his eyeballs. The mind creates perspective, how much then may it not create? The mind also refuses to surrender its own associations at the bidding of optical laws. Mr, Wheatstone's ingenious instrument called the Pseudoscope, brings into play laws which reverse the impressions of solidity and hollowness. A person looking through it steadily at the face of a statue sees a hollow mask. The convexity of feature is gone, and a concave set of features (representing the bust reversed) is perceived in its stead. But, let the same person gaze through his pseudoscope ever so long at the face of a human being, and he will look for a like reversal in vain. The flesh and blood features refuse to change;--in other words, the mind refuses to yield its _long-accustomed_ impression.[103] If these things and others like them are fairly considered, what becomes of our readings in the unclosed book of Nature? The nature we see is our own thought reflected back again. Nature's answers take not only tone and compass, but meaning and utterance from our own interrogations. We think that we are assimilating knowledge, when we are actually engaged in manufacturing aliments to suit our own intellectual digestions. The most inward of all things,--our essential _self_,--at once retired into shadow when we pursued it; and now, in trying to show how _self_ is fed by substance from without, we have learned to suspect that all its food is unsubstantial.[x]