Our remarks have carried us over the borders of the twilight territory,—a circumstance we may ascertain by putting into words what we think we know, and our reasons for thinking that we know it. If the eye be in focus, (but not otherwise), a line of light—that is to say moving imponderable matter of extreme tenuity—so passes through its transparent liquids as to strike a sensitive spot, and there produce what is called an image. We apprehend in our minds this image-producing function as a relation between light and the effect realized. A relation definite and exact,—in scientific language a "constant"; which we can formulate into optical laws, and thus express with useful nicety. Taking advantage of the laws thus obtained, and employing that light-power which everywhere blesses our world, we reproduce the like, image upon a screen. Its likeness we gather from comparison, by looking into an eye from without. Both images, thus seen by us, are in point of fact similar sensations.
A philosophic reader may at once perceive what the Idealist will infer respecting this act of comparison. Neither image—on retina or on screen—exists apart from the eye. So far as we know, if there were no eyes there would be no images; and some writers (e.g., Schleiden) have positively affirmed that without eyes all would be, not only to us, but in itself, darkness;—the world absolutely void of Light. But the truth may be summed in a sentence. Light is not for the eye in the same sense that the eye is for light. Light is for other things besides. It exerts its activity on life, animal and vegetable;—on inorganic substances;—and in other ways likewise.—Going no further than our screen, we can so manage matters as to engrave and otherwise fix the image thrown upon it;—in other words our moving line of imponderable matter will produce further effects, chemical and mechanical, visible and palpable.
Proceeding to a cross-examination of the knowledge with which we have credited ourselves, our next business is to try whether we can verify the objectivity of our optical image. Now it impresses sight in two respects,—as superficial form—and as colour. The family of forms is, we are aware widely connected. Sound evokes them. Draw a violin bow across a string stretched over finely silted sand, and the different notes will be correlated by a diversity of shapes,[96] into which the sand will arrange itself. Therefore, we ought to find means of verifying Form without much difficulty. Indeed we do so every day satisfactorily; our hands are perpetually demonstrating the general accuracy of our eyes, and even those delicate instruments our finger-ends, do not always add much to the information sight has given us.
But about colour? Distinct colour-waves have (as we said before) distinct velocities, and are therefore objectively distinguished even in the inorganic universe. They also act differently upon the growth of animals and plants,—and other distinctions might be added. The sensation is, however, our point,—the special thing called colour both by careful speakers and in child parlance,—what do we really know about this? Little indeed except as an impression received by sight. The man born in complete blindness taking a piece of red cloth to examine, described the fabric minutely; but, when asked if he could say anything about its redness, likened that "hue angry and brave" to the sound of a trumpet. A simile most conclusive,—suggested probably by his having often heard of certain "scarlet-coated gentry";—and proving beyond doubt that colour is non-existent in the sensory of a person affected from birth by a deep-seated lesion. To one less thoroughly blind, spectra are possible, and red light may be produced under pressure. It thus appears, that colour must be perceived by a nervous substratum, called the rod and cone layer; and hence we explain our power of distinctly seeing the blood-vessels of the retina lying immediately before that structure.[97]
These curiosities, of vision shew that our powers of verifying shape are superior to our powers of verifying colour;[98] add, too, that the latter sensation, (as an idealist might maintain,) is known to be sometimes unreal, since it occurs without a coloured object. We can produce it, for instance, by gazing at the sun—a phenomenon mentioned by Aristotle. But then, this ideal sense-affection ranges with a variety of others, which taken together constitute a very much wider law. Not to mention many superinduced mental states, we see light under the influence of a touch or blow,—of electricity,—of chemicals, such as narcotic medicines, which attack the nervous system. We hear sound under like appliances stimulating the auditory nerve. And the whole of these affections are to be explained by another Aristotelian doctrine, extended and pushed to its consequences. Special senses have their own proper faculties, and when called into action each exerts its power within its special province. Had Aristotle dissected out nerve-fibres, he might have discovered the larger empire of specialty now known to our anatomists.[99]
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]
We may henceforth consider ourselves face to face with Sphinx; and it is well to take the true measure of her lineaments. If the above reasoning be sound, to know, is to make a mirror and reflect ourselves back from it. To verify, is to put ourselves in new postures before our infallible mirror. Each fresh item of induction, is a freshly reflected phantom. At all events, the contrary position will never be established. Ignorant as we are, respecting the true centre of our mental firmament, we must necessarily be always more ignorant respecting all possibilities which seemingly outlie its glowing horizon. No one who rationally weighs the worth of a fact, or who decomposes it into its elementary constituents, will ever be absurd enough to imagine that he can disprove the ideal theory by proving the truth of its opposite.
The strongest strain of Idealism comes upon the last sentence. Some years ago, English philosophers had agreed in the conclusion that all debates must for the future be settled by an appeal to facts. Could there be a more happily chosen ground for arbitration?—or one better suited to the calibre of everybody concerning whose business-like reflections we might say, with King Henry,—
"His thinkings are below the moon"?