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NATURE 



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perception, passing on to illusions of introspection, 

 memory, and belief. 



Taking first the illusions of sensation, preperception, 

 and perception, the following is a brief sketch of Mr. 

 Sully's analysis of their nature and causes. 



I. The Limitation of Sensibility ; the amount of sensa- 

 tion is not always a fair measure of the amount of stimu- 

 lation, which may be either inadequate or over-adequate 

 in relation to the excitability of the sense-organs. 2. Tlie 

 Variation of Sensibility ; changes of organic state — 

 whether temporary, as those arising from fluctuating 

 nutrition or fatigue, &c., or permanent, as those arising 

 from age or disease — by supplying us with a variable 

 index of objective phenomena, lead to illusory misrepre- 

 sentations of these phenomena, unless such variations are 

 duly recognised and allowed for. 4. Exceptional Rela- 

 tion of Stimulus to Organ J a man crunching a biscuit 

 can scarcely believe that others do not hear the sound 

 more loudly than he does, or, on rubbing his nose with 

 the points of his third and fourth fingers crossed, that this 

 organ has not become split into two ; in such cases the 

 sense perceptions are interpreted by the help of more 

 familiar relations, and so illusion arises. J. Exceptional 

 External Arrangements ; unless the fact that we are 

 ou -selves moving is clearly presented to consciousness, 

 we instinctively conclude that surrounding objects are 

 moving in the opposite direction, and under similar cir- 

 cumstances are apt to suppose that a train which is just 

 shooting ahead of our own train is moving but slowly ; 

 on this principle depends the illusion of the stereoscope, 

 misjudging distances in the clear atmosphere of Switzer- 

 land, &c. 6. Devices of Art; perspective, effects of light 

 and shade, &c., are all so many devices to ensnare visual 

 perception into a misinterpretation of marks on a flat 

 surface for objects situated in space of three dimensions. 

 7. Misconception of Local Arrangement j the examples 

 given under this head appear to involve exactly the same 

 principle as 5, and nothing more. 8. Misinterpretation 

 of Forms the same remark applies to this head — 7 and 8 

 are really species of the genus 5, and ought to have been 

 considered under it. 9. Illusions of Recognition; as in 

 general we attend only to what is essential and constant 

 in objects, to the disregard of what is variable or acci- 

 dental, opportunity is thus furnished for a large class of 

 illusions ; imagination and expectant attention likewise 

 play an important part in producing illusions of this 

 kind. 10. Voluntary Selection of Interpretation. So 

 far the enumeration has been concerned with what 

 the author calls "Passive Illusions," i.e. illusions in 

 which the imagination is inactive, or comparatively 

 so; now we pass to the "Active Illusions," and as 

 an example of voluntary selection of interpretation we 

 may notice that in looking at a geometrical drawing of a 

 truncated pyramid the figure may by a voluntary act be 

 seen to represent alternately a solid upstanding form, or 

 a hollow receding box. 11. Involuntary Mental P re- 

 adjustment ; this resembles the last case, save that the 

 illusion is not due to an act of volition. " The whole past 

 mental life . . . serves to give a particular colour to new 

 impressions. . . . There is a personal equation in per- 

 ception as in belief." 12. Sub-expectation; this has 

 already been alluded to, and is obviously a potent cause 

 of illusion. 13. Vivid Expectation ; still more obviously 



so. Indeed vivid expectation may "produce something 

 like a counterfeit of a real sensation." An anxious mother 

 may fancy that she actually hears her child cry in an 

 adjoining room, &c. 14. Transition to Hallucination; 

 clearly but a step farther, and illusion passes into halluci- 

 nation, where imagination has become altogether detached 

 from present surroundings, and has entered on the 

 stage of highest activity. But hallucinations are not in- 

 variably of central origin ; they may also be of peripheral, 

 and do not always betoken pathological conditions, though 

 they usually reach their highest perfection in the insane 

 Thus there is an apparently unbroken continuity from 

 the scarcely noticeable illusions of normal life leading up 

 to the most startling hallucinations of abnormal. This 

 consideration leads to the following pretty piece of 

 speculation : — 



" We may, perhaps, express this point of connection 

 between the illusions of normal life and insanity by help 

 of a physiological hypothesis. If the nervous system has 

 been slowly built up, during the course of human history, 

 into its present complex form, it follows that those nervous 

 structures and connections which have to do with the 

 higher intellectual processes, or which represent the 

 larger and more general relations of our experience, have 

 been most recently evolved. Consequently, they would 

 be the least deeply organised, and so the least stable ; 

 that is to say, the most liable to be thrown hors de combat. 

 This is what happens temporarily in the case of the sane, 

 when the mind is held fast by an illusion. And, in states 

 of insanity, we see the process of nervous dissolution 

 beginning with these same structures, and so taking the 

 reverse order of the process of evolution. • And thus, we 

 may say that throughout the mental life of the most sane 

 of us these higher and more delicately balanced structures 

 are constantly in danger of being reduced to that state of 

 inefficiency which in its full manifestation is mental 

 disease." 



Next there follows an interesting chapter on Dreams, 

 in which the mechanism of thought in sleep is ably and 

 suggestively laid bare, so far as the complex and difficult 

 nature of the subject permits. Want of space however 

 prevents our entering upon this chapter, and therefore we 

 shall pass on at once to the Illusions of Introspection. 

 This and the next division of the work is perhaps the 

 part that displays most originality. At first sight it seems 

 almost impossible that the mind could be subject to illu- 

 sion in its consciousness of its present state or contents ; 

 but yet it is clearly shown that such is very frequently the 

 case. " No such clearly-defined mosaic of feelings pre- 

 sents itself in the internal region " as that which is pre- 

 sented in the external when interpreted by sensuous 

 perception ; " our consciousness is a closely-woven texture 

 in which the mental eye often fails to detect the several 

 threads or strands." Moreover, "many of these ingredients 

 are exceedingly shadowy, belonging to that obscure i-egion 

 of sub-consciousness which it is so hard to penetrate with 

 the light of discriminative attention." Thus numberless 

 illusions of introspection become possible. All cases of 

 " self-deception " fall into this category, whether they 

 arise from a wrong intellectual focussing of the attention, 

 so as to give undue prominence to some feelings over 

 others, or from a mere emotional bias. As examples we 

 may take the self-deception of a man who is really 

 " bored " by a social entertainment, yet making himself 



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