Chapter XIV: Section 7: ) (1)

Is not this good-humoured? Is it not a piece of pleasant bantering, to be equalled only by certain French philosophers? The real conclusion, however, winds up his First Book and runs as follows:--"A true sceptic will be diffident of his philosophical doubts, as well as of his philosophical conviction; and will never refuse any innocent satisfaction, which offers itself, upon account of either of them.

"Nor is it only proper we should in general indulge our inclination in the most elaborate philosophical researches, notwithstanding our sceptical principles, but also that we should yield (sic) to that propensity, which inclines us to be positive and certain in _particular points_, according to the light, in which we survey them in any _particular instant_. 'Tis easier to forbear all examination and inquiry, than to check ourselves in so natural a propensity, and guard against that assurance, which always arises from an exact and full survey of an object. On such an occasion we are apt not only to forget our scepticism, but even our modesty too; and make use of such terms as these, _'tis evident_, _'tis certain_, _'tis undeniable_; which a due deference to the public ought, perhaps, to prevent. I may have fallen into this fault after the example of others; but I here enter a _caveat_ against any objections, which may be offered on that head; and declare that such expressions were extorted from me by the present view of the object, and imply no dogmatical spirit, nor conceited idea of my own judgment, which are sentiments that I am sensible can become nobody, and a sceptic still less than any other.[121]

* * * * *

It is obvious to remark that no amount of easiness would maintain most minds in this balanced position of the pleasant know-nothing man. The general tendency would be to acknowledge the negative side alone. And it would be well if an absence of serious convictions, seriously asserted, and acted on, did not gradually weaken the sense of Responsibility by making Truth appear indifferent because unattainable.

We, however, are just now more concerned with two other equally obvious comments. One, that Hume appears to take for granted the point at issue. Suppose it for argument's sake to be true that impressions and ideas (as described by him) make up our whole ordinary consciousness; does this shew that no latent power or entity exists by which we _become_ conscious of those passing trains? When impressed by colours, are we conscious of an optic nerve, retina, crystalline lens and other instrumental powers of vision? Can we, if we try, perceive by sense the nerve-currents brainwards, or the sensory which receives and compares them? In both cases (eye and inward eye) pathology affords an evidence of consciousness which happy health refuses us. The brainsick sense sees colours and phantoms which are not--the disordered mind dwells on impressions and ideas absolutely unreal, and acts on them as stern realities. And thus our own purely subjective states reveal to us our own subjectivity. 'Tis so in fevers, in lunacies, in vices--'tis so to the drowning or the desperate man. These mournful changes which pass over ourselves issue from an interior activity of self-ness and form one of its commonest verifications.

This first comment admits of extension. If we endeavour to introduce experiment (as well as experience) into Mental Science, must we not ask a previous question:--Shall this or that experiment be tried? In other words, by what inner law shall we shape our inquiries so as to gain useful facts for our intended induction?--Nay, we may further ask: What inner Being is to settle the questions, criticize them, and judge the final issue? And if we seem to see our way on these topics, we may feel pretty sure that whenever our psychology comes to practical trial, we proceed as being sure of a Self, more or less _self-conscious of Self_, and are quite confident that its self-ness will continue during the whole time of our investigations.

Our second comment may be simply summed, but the consideration given to it ought to be minute and careful. Suppose instead of successive perceptions, impressions, or ideas, we substitute a succession of phenomena, and then apply to them Hume's line of thought, we have an acute statement of the modern teachings which relegate the noblest part of our Nature, our reasonings and our beliefs to the territory of the Unknowable. In a word, all knowledge thus seems to be gained by "looking on," none by "looking in." Truth within ourselves especially if it manifests a Truth above ourselves is made to appear hopeless. And so far does the process of Elimination extend, that principles involved even in our "looking on" must not be drawn out of their latency, for fear they should become accepted parts of knowledge. Let any thinker repeat with this substitution the Personal Identity argument in his own mind, and he will soon see what a shadow is cast over an infinitely wider world of thought.[122]

The same process of repetition ought in fairness to produce another effect. Are not these philosophic _argutiæ_, these Pyrrhonic subtilties closely akin to the difficulties raised against _all_ first principles; and more particularly all Theistic principles? But does anybody on their account doubt his own Self-ness or Identity? Or does any one refuse to act on the supposition of _other-ness_, and _outer-ness_, or ignore his world of fellow-men and hard objectivities which press upon him from every side? Why then should anybody ignore on their account the great First-Cause?

In the text of Chapter III. the elements of our reasonable belief in our own Personal self-ness and sameness have been shortly mentioned;--of such work-day belief, that is to say, as suffices for actual life, and gains from it, and throughout it, a perpetual verification. If any one wishes to go deeper than this, he must inquire upon what evidence first principles are accepted by reasoning men; what difficulties attach to such principles; and under what conditions these difficulties are held to be nugatory. This inquiry is troublesome but promises real satisfaction. We have not, therefore, declined it, as may be seen in the ensuing Chapter. One fact is manifest beforehand--that whatever evidence is presupposed valid by those first principles of every-day knowledge, may be safely presupposed, accepted, and reasoned upon, in the ground-work of Natural Theology.

It was Hume's object to push his scepticism to its most extreme verge. Thus pushed, it "so wrought upon" him that he was "ready to reject all belief and reasoning" till a return to every-day life made his speculations appear in his own eyes "cold and strained and ridiculous." What then was the inference Hume himself intended? Which was really groundless--every-day belief or scepticism? Will his useful dilemma induce the reader to receive Kant's excuse for the celebrated doubter, when he bids us let the man alone because he is but trying the strength of human reason? At all events, Hume's way of stating his case seems to justify the old remark, that, while Superstition is refuted by Reason, Nature itself refutes the Sceptic.

B.--EXTRACTS FROM POPULAR LECTURES, BY PROFESSOR HELMHOLTZ, ON THE RECENT PROGRESS OF THE THEORY OF VISION.

"If now we compare the eye with other optical instruments, we observe the advantage it has over them in its very large field of vision. This for each eye separately is 160° (nearly two right angles) laterally, and 120° vertically, and for both together somewhat more than two right angles from right to left. The field of view of instruments made by art is usually very small, and becomes smaller with the increased size of the image.

"But we must also admit, that we are accustomed to expect in these instruments complete precision of the image in its entire extent, while it is only necessary for the image on the retina to be exact over a very small surface, namely, that of the yellow spot. The diameter of the central pit corresponds in the field of vision to an angular magnitude which can be covered by the nail of one's forefinger when the hand is stretched out as far as possible. In this small part of the field our power of vision is so accurate that it can distinguish the distance between two points, of only _one minute_ angular magnitude, _i.e._ a distance equal to the sixtieth part of the diameter of the finger-nail. This distance corresponds to the width of one of the cones of the retina. All the other parts of the retinal image are seen imperfectly, and the more so the nearer to the limit of the retina they fall. So that the image which we receive by the eye is like a picture, minutely and elaborately finished in the centre, but only roughly sketched in at the borders. But although at each instant we only see a very small part of the field of vision accurately, we see this in combination with what surrounds it, and enough of this outer and larger part of the field, to notice any striking object, and particularly any change that takes place in it. All of this is unattainable in a telescope.

"But if the objects are too small, we cannot discern them at all with the greater part of the retina.

'When, lost in boundless blue on high,
The lark pours forth his thrilling song,'

the 'ethereal minstrel' is lost until we can bring her image to a focus upon the central pit of our retina. Then only are we able to _see_ her.

"To _look_ at anything means to place the eye in such a position that the image of the object falls on the small region of perfectly clear vision. This we may call _direct_ vision, applying the term _indirect_ to that exercised with the lateral parts of the retina--indeed with all except the yellow spot.

"The defects which result from the inexactness of vision and the smaller number of cones in the greater part of the retina are compensated by the rapidity with which we can turn the eye to one point after another of the field of vision, and it is this rapidity of movement which really constitutes the chief advantage of the eye over other optical instruments....

"A great part of the importance of the eye as an organ of expression depends on the same fact; for the movements of the eyeball--its glances--are among the most direct signs of the movement of the attention, of the movements of the mind, of the person who is looking at us." _Popular Lectures on Scientific Subjects_, pp. 212-214.

The great German next proceeds to catalogue some principal defects of the Eye. 1. Chromatic aberration connected with 2. spherical aberration and defective centering of the cornea and lens, together producing the imperfection known as astigmatism, and 3. irregular radiation round the images of illuminated points. "Now," adds Helmholtz, "it is not too much to say that if an optician wanted to sell me an instrument which had all these defects, I should think myself quite justified in blaming his carelessness in the strongest terms, and giving him back his instrument. Of course, I shall not do this with my eyes, and shall be only too glad to keep them as long as I can--defects and all. Still, the fact that, however bad they may be, I can get no others, does not at all diminish their defects, so long as I maintain the narrow but indisputable position of a critic on purely optical grounds." (p. 219.)

He then goes on to other faults. 4. Defective transparency. 5. Floating corpuscules (Muscæ Volitantes). 6. The "blind spot" with other gaps in the field of vision. "So much," he concludes, "for the physical properties of the Eye. If I am asked why I have spent so much time in explaining its imperfection to my readers, I answer, as I said at first, that I have not done so in order to depreciate the performances of this wonderful organ, or to diminish our admiration of its construction. It was my object to make the reader understand, at the first step of our inquiry, that it is not any mechanical perfection of the organs of our senses which secures for us such wonderfully true and exact impressions of the outer world. The next section of this inquiry will introduce much bolder and more paradoxical conclusions than any I have yet stated. We have now seen that the eye in itself is not by any means so complete an optical instrument as it first appears: its extraordinary value depends upon the way in which we use it: its perfection is practical, not absolute.... Wherever we scrutinise the construction of physiological organs, we find the same character of practical adaptation to the wants of the organism; although, perhaps, there is no instance which we can follow out so minutely as that of the eye.

"For the eye has every possible defect that can be found in an optical instrument, and even some which are peculiar to itself; but they are all so counteracted, that the inexactness of the image which results from their presence very little exceeds, under ordinary conditions of illumination, the limits which are set to the delicacy of sensation by the dimensions of the retinal cones....

"The adaptation of the eye to its function is, therefore, most complete, and is seen in the very limits which are set to its defects. Here the result which may be reached by innumerable generations working under the Darwinian law of inheritance, coincides with what the wisest Wisdom may have devised beforehand. A sensible man will not cut firewood with a razor, and so we may assume that each step in the elaboration of the eye must have made the organ more vulnerable and more slow in its development. We must also bear in mind that soft, watery animal textures must always be unfavourable and difficult material for an instrument of the mind....

"But, apparently, we are not yet come much nearer to understanding sight. We have only made one step: we have learnt how the optical arrangement of the eye renders it possible to separate the rays of light which come in from all parts of the field of vision, and to bring together again all those that have proceeded from a single point, so that they may produce their effect upon a single fibre of the optic nerve.

"Let us see, therefore, how much we know of the sensations of the eye, and how far this will bring us towards the solution of the problem." P. 226, seq.

From the Professor's mention of "much bolder and more paradoxical conclusions," the final result of his next inquiry may be anticipated. Sensation is so far from making evident the truth of our visual knowledge that it increases our perplexities tenfold. "The inaccuracies," he tells us, "and imperfections of the eye as an optical instrument, and those which belong to the image on the retina, now appear insignificant in comparison with the incongruities which we have met with in the field of sensation. One might almost believe that Nature had here contradicted herself on purpose, in order to destroy any dream of a pre-existing harmony between the outer and the inner world.

"And what progress have we made in our task of explaining Sight? It might seem that we are farther off than ever; the riddle only more complicated, and less hope than ever of finding out the answer. The reader may perhaps feel inclined to reproach Science with only knowing how to break up with fruitless criticism the fair world presented to us by our senses, in order to annihilate the fragments." (p. 269.)

How triumphant does Idealism now appear! How little trustworthy that boasted sense of which mankind have constantly said, "seeing is believing," although an apostle and philosophers innumerable have put the two in opposition!

Perhaps, however, instead of leading to a "triumph of Idealism," the paradoxes and incongruities--in a word, the vast accumulation of the Unknowable--belonging to eyesight _considered as a Sensation_, must be allowed to land us on the shore of a far-stretching Scepticism illimitable to the mind's eye. And this seems to be the eminent writer's own final opinion.[123] So, too, it will always appear when the case is fairly argued out; and that for the reasons adduced in our text. The course of argument there pursued was adopted before the Professor's book came to hand; but we have now added some extracts from his pages in the shape of footnotes, and have given references to other interesting topics touched upon by him.

For our purpose, however, it is necessary in some degree to disregard the variety of those topics, and fix our attention upon the conclusive issue. It is plain, that respecting our senses, as well as our other primary sources of information, the limits of what we can completely explain are very narrow. Yet each for himself and all of us for our race must needs every day accept and act upon this limited and imperfect kind of knowledge about what most essentially concerns our actions as well as our speculations.

Several strong examples of such incompleteness are given by Helmholtz in his scientific inquiry into the rationale of the visual sense-impressions. We observe, for instance, in his chapter on Sensation (p. 236 seq.) that all light-waves are the same in kind of movement, but differ in size as widely as the ripples on a sea-beach (round which happy children play) differ from the vast Atlantic ship-engulfing billows sixty or a hundred feet apart. All these undulations are similar in respect of reflection, refraction, interference, diffraction, and polarisation, as well as in their production of heat.[124] Now, it is the interpretation of such movements into its own language by which our eye gives us the sensation of colour. Yet this power of interpretation is curiously limited--it does not appreciate the gentler ripples of the light-waves--it does not reach to their mightier undulations. Consequently, there may be tender colour-delicacies adorning the Universe, completely incognisable by us, and there may be also glows and intensities of light-beams magnificently resplendent, and unspeakably grand in tone, of which we can through our visual apparatus form no possible conception. Thus, our eye translates some waves into a language which we call colour, but its scholarship is limited. A certain number of signs it catches and interprets, the rest lie altogether outside its ken. The Sun's softer light-harmonies, and his most awful emanations of beauty remain equally unknown.

And another limitation has been imposed upon our optical apparatus. For a perception of heating powers belonging to colour-waves the eye refers us to the skin;--and as to their chemical powers we are only just now discovering the instruments fitted for their true appreciation.

Skilful, too, and yet at the same time very skill-less, is the divination into sunlight given us by our human eyes;--sunlight, that is to say, as a general resultant in its whiteness. For, if our eyes, keen and susceptible to us perfect clearness, attempt to analyze white light into its factors and elements, their resolving faculty manifests still more blank inabilities. And they fail also in examining certain colours:--

"The most striking difference," writes Helmholtz, "between the mixture of pigments and that of coloured light is, that while painters make green by mixing blue and yellow pigments, the union of blue and yellow rays of light, produces white.... In general, then, light, which consists of undulations of different wave-lengths, produces different impressions upon our eye, namely, those of different colours. But the number of hues which we can recognise is much smaller than that of the various possible combinations of rays with different wave-lengths which external objects can convey to our eyes. The retina cannot distinguish between the white which is produced by the union of scarlet and bluish-green light, and that which is composed of yellowish-green and violet, or of yellow and ultramarine blue, or of red, green, and violet, or of all the colours of the spectrum united. All these combinations appear identically as white; and yet, from a physical point of view, they are very different. In fact, the only resemblance between the several combinations just mentioned is, that they are indistinguishable to the human eye. For instance, a surface illuminated with red and bluish-green light would come out black in a photograph; while another lighted with yellowish-green and violet would appear very bright, although both surfaces alike seem to the eye to be simply white. Again, if we successively illuminate coloured objects with white beams of light of various composition, they will appear differently coloured. And whenever we decompose two such beams by a prism, or look at them through a coloured glass, the difference between them at once becomes evident.

"Other colours, also, especially when they are not strongly pronounced, may, like pure white light, be composed of very different mixtures, and yet appear indistinguishable to the eye, while in every other property, physical or chemical, they are entirely distinct." (pp. 239-241.)

We may speak of visual Sensation, then, as a _limited_ power of _translating_ light. And what relation does visual _Perception_ bear to this Power? Probably the simplest way of expressing it, is to say that it is neither more nor less than the _translation of a translation_. The mind thus construes to itself what the visual sense is every moment busied with expressing in its own special language--the interpretation of movement, into colour, light and shadow. And from these data--these colours, lights and shadows, the mind draws its own inferences.

Now these inferences thus drawn from preceding Sense inferences,--limited in range, as we have seen, and defective in analytic power;--these inferences, such as they are, constitute the boasted certainty of eyesight; and of all things apprehended by its means,--all

--quæ sunt oculis subjecta fidelibus et quæ
Ipse sibi _tradit_ spectator.

It needs but a statement of the mode in which our final mind-interpretations are constructed,--of these translated translations,--obscure in grammar and imperfect in vocabulary--to prove how very difficult is the position of the Realist. In view of this Empire of the Unknowable proclaimed by Science over the surest of our perceiving powers, the firmest foundations of our experimental knowledge, Helmholtz suggests that his reader "may feel determined to stick fast to the 'sound common sense' of mankind, and believe his own senses more than physiology." (p. 270.)

And such, no doubt, is the conclusion of the matter to the greater part of mankind. But we will in the first place prefer hearing the last word of the physiologist. From page 270 to page 313 of his work, he argues out the great question of _how_ we perceive under the full impression of its vast importance to psychology, metaphysics, and the first principles upon which all science and all reasonings repose. "We have," he says (p. 281), "already learned enough to see that the questions which have here to be decided are of fundamental importance, not only for the physiology of sight, but for a correct understanding of the true nature and limits of human knowledge generally."

The Physiologist's last word is this--Sense impressions are signs, the meaning of which we learn inductively by a process of self education. "Illusions obviously depend upon mental processes which may be described as false inductions.... There appears to me to be in reality only a superficial difference between the 'conclusions' of logicians and those inductive conclusions of which we recognise the result in the conceptions we gain of the outer world through our sensations. The difference chiefly depends upon the former conclusions being capable of expression in words, while the latter are not; because, instead of words, they only deal with sensations and the memory of sensations. Indeed, it is just the impossibility of describing sensations, whether actual or remembered, in words, which makes it so difficult to discuss this department of psychology at all." (pp. 307, 8.) And again (p. 314), "There is a most striking analogy between the entire range of processes which we have been discussing, and another System of Signs, which is not given by nature but arbitrarily chosen, and which must undoubtedly be learned before it is understood. I mean the words of our mother tongue.

"Learning how to speak is obviously a much more difficult task than acquiring a foreign language in after-life. First, the child has to guess that the sounds it hears are intended to be signs at all; next, the meaning of each separate sound must be found out, by the same kind of induction as the meaning of the sensations of sight or touch; and yet we see children by the end of their first year already understanding certain words and phrases, even if they are not yet able to repeat them. We may sometimes observe the same in dogs.

"Now this connection between Names and Objects, which demonstrably must be _learnt_, becomes just as firm and indestructible as that between Sensations and the Objects which produce them. We cannot help thinking of the usual signification of a word, even when it is used exceptionally in some other sense; we cannot help feeling the mental emotions which a fictitious narrative calls forth, even when we know that it is not true; just in the same way as we cannot get rid of the normal signification of the sensations produced by any illusion of the senses, even when we know that they are not real.

"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?

For an examination of the ground principle of Induction, the reader must be referred to our next chapter. But it is at once clear that no human _experience_ can possess the attribute of universality, otherwise it would cease to be human. We have then in this present appeal to the veracity of Experience, no _absolute_ knowledge to deal with, only knowledge as relative to mankind. Nay, we must go a little further still in our limitation, and say to the _generality_ of mankind. For our eyes do not all see perfectly alike--a North-American Indian sees what a Cockney cannot discover; the trained eye discerns differently from the untrained. On the differences of power in eye and ear rest the differences in many kinds of theorising--amongst which art-perceptions yield an obvious and familiar set of examples. And if we try for a more precise estimate of the value of our limited human relativity, and proceed by way of comparison between _our own_ diverse endowments, who shall venture to say that the eye of our body interpreted by our understanding, tells our inmost self more truly than the eye of our human soul, informing us directly of the facts of its intuitive vision? So far as our actual means of valuing these two modes of beholding can go, there is no knowledge so perfect as the product of pure intuition, the glorious fabric of Mathematical Science. And to pure Science it matters not whether the requisite Schematism is drawn upon a sheet of white paper or on the clear tablet of the imagining faculty of a philosopher. The purely inward view is in truth generally the farthest reaching, and the most unclouded. When, therefore, it is, and has been for centuries, apparent to the inmost eye of the generality of our race that there _really_ exists a spiritual world within themselves--above them, and in the far distant future beyond us all, permanent while we change, and the evidence of our own ultimate permanency,--such knowledge may undeniably be human, the very flower and distinction of our human nature; and it may on that account be received by us as true.

If, again, our ordinary human soul is so far a Christian as to exclaim with Tertullian, "O good God," by what logical process shall we confute its utterance, while we maintain the utterance of our commonest sense-perceptions?

That we all see in frames, that we all think in frames, no rational thinker or perceiver will deny. If, however, any of us chooses to be an Idealist or Nihilist, let him at least be consistent;--if he will assert the necessity of Doubt, let him maintain its empire by doubting his own assertion. But let no man think that Doubt leads him any whither except to an abnegation of thought, a mistrust alike of Sense and Soul, and an abdication of every human prerogative:--

"Thy hand, great Anarch, lets the curtain fall,
And universal Darkness buries all."

So sang the witty rhymer, but we may add in prose that Doubt if thoroughly real, invariably commits suicide, and becomes first doubtful, after that, a non-entity at last.

C.--HELMHOLTZ ON SPECIALTIES OF SENSIBILITY.

The following passages from this interesting writer will be found in his Chapter "on the Sensations of Sight," between pp. 232 and 236. They will, it is hoped, be thoroughly intelligible if read in connection with the part of our last Chapter (pp. 158, 9) where a reference to this note was made.

"The nerve-fibres have been often compared with telegraphic wires traversing a country, and the comparison is well fitted to illustrate this striking and important peculiarity of their mode of action. In the network of telegraphs we find everywhere the same copper or iron wires carrying the same kind of movement, a stream of electricity, but producing the most different results in the various stations according to the auxiliary apparatus with which they are connected. At one station the effect is the ringing of a bell, at another a signal is moved, and at a third a recording instrument is set to work.... Nerve-fibres and telegraphic wires are equally striking examples to illustrate the doctrine that the same causes may, under different conditions, produce different results.... As motor nerves, when irritated, produce movement, because they are connected with muscles, and glandular nerves secretion, because they lead to glands, so do sensitive nerves, when they are irritated, produce sensation, because they are connected with sensitive organs.... Whether by the irritation of a nerve we produce a muscular movement, a secretion or a sensation depends upon whether we are handling a motor, a glandular, or a sensitive nerve, and not at all upon what means of irritation we may use. It may be an electrical shock, or tearing the nerve, or cutting it through, or moistening it with a solution of salt, or touching it with a hot wire. In the same way (and this great step in advance was due to Johannes Müller) the _kind_ of sensation which will ensue when we irritate a sensitive nerve, whether an impression of light, or of sound, or of feeling, or of smell, or of taste, will be produced, depends entirely upon which sense the excited nerve subserves, and not at all upon the method of excitation we adopt.

"Let us now apply this to the optic nerve, which is the object of our present enquiry. In the first place, we know that no kind of action upon any part of the body except the eye and the nerve which belongs to it, can ever produce the sensation of light. The stories of somnambulists, which are the only arguments that can be adduced against this belief, we may be allowed to disbelieve. But, on the other hand, it is not light alone which can produce the sensation of light upon the eye, but also any other power which can excite the optic nerve. If the weakest electrical currents are passed through the eye they produce flashes of light. A blow, or even a slight pressure made upon the side of the eyeball with the finger, makes an impression of light in the darkest room, and, under favourable circumstances, this may become intense. In these cases it is important to remember that there is no objective light produced in the retina, as some of the older physiologists assumed, for the sensation of light may be so strong that a second observer could not fail to see through the pupil the illumination of the retina which would follow, if the sensation were really produced by an actual development of light within the eye. But nothing of the sort has ever been seen. Pressure or the electric current excites the optic nerve, and therefore, according to Müller's law, a sensation of light results, but under these circumstances, at least, there is not the smallest spark of actual light.

"In the same way, increased pressure of blood, its abnormal constitution in fevers, or its contamination with intoxicating or narcotic drugs, can produce sensations of light to which no actual light corresponds. Even in cases in which an eye is entirely lost by accident or by an operation, the irritation of the stump of the optic nerve while it is healing is capable of producing similar subjective effects. It follows from these facts that the peculiarity in kind which distinguishes the sensation of light from all others, does not depend upon any peculiar qualities of light itself. Every action which is capable of exciting the optic nerve is capable of producing the impression of light; and the purely subjective sensation thus produced is so precisely similar to that caused by external light, that persons unacquainted with these phenomena readily suppose that the rays they see are real objective beams.

"Thus we see that external light produces no other effects in the optic nerve than other agents of an entirely different nature. In one respect only does light differ from the other causes which are capable of exciting this nerve: namely, that the retina, being placed at the back of the firm globe of the eye, and further protected by the bony orbit, is almost entirely withdrawn from other exciting agents, and is thus only exceptionally affected by them, while it is continually receiving the rays of light which stream in upon it through the transparent media of the eye.

"On the other hand, the optic nerve, by reason of the peculiar structures in connection with the ends of its fibres, the rods and cones of the retina, is incomparably more sensitive to rays of light than any other nervous apparatus of the body, since the rest can only be affected by rays which are concentrated enough to produce noticeable elevation of temperature.

"This explains why the sensations of the optic nerve are for us the ordinary sensible sign of the presence of light in the field of vision, and why we always connect the sensation of light with light itself, even where they are really unconnected. But we must never forget that a survey of all the facts in their natural connection puts it beyond doubt that external light is only _one_ of the exciting causes capable of bringing the optic nerve into functional activity, and therefore that there is no exclusive relation between the sensation of light and light itself."

Some of the quotations just made direct attention to illusions of Sight which (as we have seen in our last note) Helmholtz elsewhere calls "false inductions." Now one curious fact relative to these impressions is that in many instances the objective consequent is due to a subjective antecedent. Some readers may like to peruse a short account of five variously caused sight-illusions taken from an Oration on _Positivism_ delivered by the present writer at St. George's Hall in May 1871. The particulars here given of the fifth illusion should be compared with the foot-note on page 158 _ante_.

"I will mention five instances in which people believe they see something, and do not see it; in other words, the objective antecedent is wanting, and the impression is produced partly by the sensory apparatus, partly by the mind itself. As I describe these instances one by one, let my hearers ask themselves, How does this illusion come about? Is it produced by our optic instrument or by our mental activity?

"First, then, Take a lighted stick, and whirl it rapidly round and round. You believe you see a circle of sparks--in reality it is no more than a simple train, and on a like illusion the Catherine-wheel is constructed. Again, put yourself in the hands of an optically inclined friend, and let him operate upon you thus. He shall place a cardboard down the middle axis of your face, quite close against your nose--one side of his board, say the right, coloured a brilliant red, the left a vivid green. After an instant or two let him suddenly substitute another board, white on both sides. Do my young friends guess what will follow? Your right eye will see green, your left red--the reverse of what they saw before; yet neither will see correctly, for both eyes are looking at uncoloured surfaces.

"Thirdly, Watch the full moon rising--how large and round she looks, resting as it were upon that eastern hill, and seen amidst the tops of its forest trees! How much larger and broader than when she hangs aloft in upper sky! Has every one here learned the true reason why? If not, look at her through a slit in a card, and her diameter will be the same.

"Fourthly, A schoolboy is crossing his bedroom in the deep dark night, anxiously hoping that his head may not come into collision with the bed-post. Though carefully and successfully avoiding it, he imagines of a sudden that the blow is imminent. Quick as thought he stops to save his head, and, behold, the room is as quickly filled with sparks or flames of fire. Another moment, and all becomes dark once more. I have heard many a schoolboy exclaim over this phenomenon, but never knew one who could explain it. Finally, did you ever, on opening your eyes in a morning, close them quickly again, and keep them shut, directing them as if to look straight forwards? Most persons of active nervous power, after a few trials--say a dozen, or a score--are surprised to see colours appear and flit before the sight. Some years ago, Germany's greatest poet tried, at the suggestion of her greatest physiologist, a series of experiments on these coloured images. He found that by an effort of will he could cause them to come and go, govern their movement, march, and succession. And this took place under no conditions of impaired sensation, nor any hallucination of a diseased mind. A thoroughly healthy will succeeded in impressing itself upon physical instruments, controlling their law, and creating at its own pleasure an unfailingly bright phantasmagoria.

"Some here may, others may not, have apprehended the distinctions between our five cases. The first two are due to the sensory apparatus, its optical laws of continued impression and complementary colour. In the latter three, mind intervenes. The enlarged size of the moon occurs through rapid comparison, the fiery lights in a dark room through instinctive apprehension, both influences of mind on the sensory system. The fifth and most interesting of all is no bad example of interference between moral and material law. The will truly causative (you may remark) overrules the natural process of physical impression, alters it, and creates a designed effect. I wish I could induce my young friends to devise a number of experiments on similar mixed cases, and, having tried them, to dissect out their real laws. These sharpenings of the critical faculty are exceedingly useful--they cultivate clearness; and most people know that two-thirds among our mistakes in life are caused by confusion of thought.

"Besides all other uses, such lessons teach at once the necessity, as we said before, of observing your own observations. And as, first, the real witness of every observation is our mind; every fact which comes through our bodily senses being to us a mental impression, it seems but common sense to hear above all things what mind has to say for and about itself. Then, secondly, where would be the benefit derived from our observations, if we could not reason upon them, or could place no confidence in our own reasonings? Yet the art of reasoning is so purely a mental process, that it can be represented by symbols as abstract and free from material meaning as if they were bare algebraic signs. Thirdly, in the most accurate of sciences mind extends our knowledge far beyond the circle of observation, and gives us axiomatic assurance of its own accuracy. Who ever saw, or ever can see, all straight lines in all conceivable positions, yet who doubts that throughout the whole universe no two straight lines ever did inclose or can inclose a space? And, fourthly, can it be a matter of indifference to any of us what evidence the mind offers concerning its own moral nature, and what is the value of that evidence, and the laws deducible therefrom? How true it thus appears that 'know thyself' lies at the root of all knowledge, and that the man who receives no witness from within can know nothing as he ought to know it!"

D.--POPULAR ACCOUNT OF PURE IDEALISM WITH CRITICAL REMARKS.

"A classification of systems of philosophy according to the cosmological conceptions governing them has actually been made. It is founded on a consideration of the differences among philosophers as to what that totality of existence is which is to be accepted as really vouched for by Mind. All agree, as we have said, that Mind is the sole voucher for anything; but philosophers are divisible into schools according to the various views they have taken of the constitution of that phenomenal Universe, that Cosmos, that total round of things, of which we have a recurring assurance in every act of perception, and which is orbed forth more or less fully for each man in his wider contemplations.

"The popular or habitual conception of mankind in general is that there are two distinct worlds mixed up in the phenomenal Cosmos--a world of Mind, consisting of multitudes of individual minds, and a world of Matter, consisting of all the extended immensity and variety of material objects. Neither of these worlds is thought of as begotten of the other, but each of them as existing independently in its own proper nature and within its own definite bounds, though they traffic with each other at present. Sweep away all existing minds, and the deserted Earth would continue to spin round all the same, still whirling its rocks, trees, clouds, and all the rest of its material pomp and garniture, alternately in the sunshine and in the depths of the starry stillness. Though no eye should behold, and no ear should hear, there would be evenings of silver moonlight on the ocean-marge, and the waves would roar as they broke and retired. On the other hand, suppose the entire fabric of the material Universe abolished and dissolved, and the dishoused population of spirits would still somehow survive in the imaginable vacancy. If this second notion is not so easy or common as the first, it still virtually belongs to the popular conception of the contents or constitution of the Cosmos. The conception is that of a Natural Dualism, or of the contact in every act of perception of two distinct spheres, one an internal perceiving mind, and the other an external world composed of the actual and identical objects which this mind perceives.

"On the first exercise of philosophic thought, however, this conception is blurred. An immense quantity of what we all instinctively think of as really existing out of ourselves turns out, on investigation, not to exist at all as we fancy it existing, but to consist only of affections of the perceiving mind. The redness of the rose is not a real external thing, immutably the same in itself; it is only a certain peculiar action on my physiology which the presence of an external cause or object seems to determine. Were my physiology different, the action would be different, though the cause or object remained the same. Indeed, there _are_ persons in whom the presence of a rose occasions no sensation of redness such as is known to me, but a much vaguer sensation, not distinguishable from what I should at once distinguish as greenness. And, as colour is thus at once detected as no external independently-existing reality, but only a recurring physiological affection of myself and other sentient beings like myself, so with a thousand other things which, by habit or instinct, I suppose as externally and independently existing. When I imagine the depopulated Earth still wheeling its inanimate rotundity through the daily sunshine and the nocturnal shadow, or one of its bays still resonant in moonlit evenings with the roar of the breaking waves, it is because, in spite of myself, I intrude into the fancy the supposition of a listening ear, and a beholding eye analogous to my own. It is only by a strong effort that I can realize that a great deal at least of what I thus think of as the goings-on of things by themselves is not and cannot be their goings-on by themselves, but consists at the utmost of effects interbred between them and a particular sentiency in the midst of them. But the effort may be made; and, when it is made repeatedly, in a great many directions, and with reference to a great many of the so-called properties of matter, the inevitable result for the philosophic mind is that the popularly-imagined substance of a real external world finds itself eaten away or corroded, at least to a certain depth. So far philosophers are agreed. It is when they proceed to consider to _what_ depth the popularly-imagined substance of the real external world is thus eaten away, or accounted for, that they begin to differ.

"Some philosophers, departing as little as may be from the popular judgment, suppose that, however much of the apparent external world may be resolved into affections of the subjective sentiency, there still remains an objective residue of such primary qualities as extension, figure, divisibility, mobility, etc., belonging to external matter itself, and by the direct and immediate cognizance of which the mind is brought face to face with external substance, and knows something of its real goings-on. Philosophers of this school are known generally as REALISTS. More numerous, however, are those who, not allowing an objective and independent reality even to the so-called primary qualities of matter, but believing them as well as colour, odour, or savour, to be only affections of the sentiency, deny that the mind is in any sense brought face to face with real external things such as they seem in the act of perception. To thinkers of this school there has been given the general name of IDEALISTS. This broad distinction of Philosophers cosmologically into REALISTS and IDEALISTS is so far convenient enough. Cosmologically, or in respect of this present Universe of ours, with its dualism of Mind and Matter, every man must declare himself either a REALIST or an IDEALIST, if he understands the meanings attached to these terms. The distinction has reference solely to his notion of the so-called external or material world in its relations to the perceiving mind. If he abides, though only in part, by the popular conception, and regards the material world as a substantial reality independent of the perceiving mind, and which the mind, according to its powers, presses against and directly apprehends in every act of perception, then he is a REALIST. If, on the other hand, he cannot see that there need be asserted any external material world with such characters as we attribute to it, but supposes that our unanimous agreement in the imagination of such an external world is merely a habit of our own sentiency, projecting its own ideas or affections outwards, and giving them a body, then he is an IDEALIST." Masson, "_Recent British Philosophy_," pp. 58-64. Again p. 69, seq., "There is the system of _Constructive Idealism_. It may be so called to distinguish it from the more developed and extreme Idealism presently to be spoken of. According to this system, we do not perceive the real external world immediately, but only mediately--that is, the objects which we take as the things actually perceived are not the real objects at all, but only vicarious assurances, representatives, or nuntii of real unknown objects. The hills, the rocks, the trees, the stars, all the choir of heaven and earth, are not, in any of their qualities, primary, secondary, or whatever we choose to call them, the actual existences out of us, but only the addresses of a 'something' to our physiology, or eductions by our physiology out of a 'something.' They are all Thoughts or Ideas, with only this peculiarity involved in them, that they will not rest in themselves, but compel a reference to objects out of self, with which, by some arrangement or other, they stand in relation. Difficult as this system may be to understand, and violently as it wrenches the popular common sense, it is yet the system into which the great majority of philosophers in all ages and countries hitherto are seen, more or less distinctly, to have been carried by their speculations. While the Natural Realists among philosophers have been very few, and even these have been Realists in a sense unintelligible to the popular mind, quite a host of philosophers have been Constructive Idealists. These might be farther subdivided according to particular variations in the form of their Idealism. Thus, there have been many Constructive Idealists who have regarded the objects rising to the mind in external perception, and taken to be representative of real unknown objects, as something more than modifications of the mind itself--as having their origin without. Among these have been reckoned Malebranche, Berkeley, Clarke, Sir Isaac Newton, Tucker, and possibly Locke. But there have been other Constructive Idealists, who have supposed the objects rising in the mind in external perception to be only modifications of the mind itself, but yet, by some arrangement, vicarious of real unknown objects, and intimating their existence. Among such have been reckoned Descartes, Leibnitz, Condillac, Kant, and most Platonists. The general name 'Idealists' it will be seen properly enough includes both the classes as distinct from the Natural Realists, inasmuch as both classes hold that what the mind is directly cognizant of in external perception is only ideas. But, inasmuch as these ideas are held by both classes, though under divers hypotheses, to refer to real existences beyond themselves, and distinct from the perceiving mind, the thinkers in question may also properly enough be called Realists or Dualists, though not 'Natural' Realists or Dualists. They occupy a midway place between the Natural Realists and the philosophers next to be mentioned.