SCIENCE. 



485 



SCIENCE: 



A Weekly Record of Scientific 

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ILLUSIONS.* 

 In reality this work might have been styled an 

 essay on error, for the author deals, in his clear and 

 masterly way, with other errors of the human judg- 

 ment than those which are termed illusions in the 

 narrower sense of that term. His essay loses nothing, 

 and gains much by thus occupying a much broader 

 field than the one, furnished by the sensory illusion, 

 would constitute per se. Perhaps the most unfortu- 

 nate part of the work, is the opening passage : " Com- 

 mon sense, knowing nothing of fine distinctions, is 

 wont to draw a sharp line between the region of ill- 

 usion and that of sane intelligence. To be the victim 

 of an illusion is, in the popular judgment, to be ex- 

 cluded from the category of rational men. The term 

 at once calls up images of stunted figures with ill-de- 

 veloped brains, half-witted creatures, hardly distin- 

 guishable from the admittedly insane 



The nineteenth century intelligence plumes itself on 

 having got at the bottom of mediaeval visions and 

 church miracles, and it is wont to commiserate the 

 feeble minds that are still subject to these self-decep- 

 tions." 



We say tnis passage is an unfortunate one, and this 

 particularly because of its position in the opening 

 chapter of a book which, as we must particularly em- 

 phasize, is throughout one of the clearest and most 

 readable psychological treatises that we have found in 



* Illusions, a psychological study. By James Sully. New 

 York, D. Appleton and Co.— Volume XXXIII. of the In- 

 ternational Scientific Series. 



the English language ; this passage on the other hand, 

 is as full of wrong assumptions, misconstructions, 

 and errors as a single paragraph can well be. The 

 popular mind fails to contemn the bearer of an illu- 

 sion, as it does the bearer of a delusion ; the mediaeval 

 visions were not, even in popular parlance illusions, 

 but hallucinations, and indeed the popular sense in 

 which the term illusion is used, that is, the one em- 

 ployed by poets and classical writers, is anything but 

 a reflection on the bearer of the illusion. The day- 

 dream, the poetic illusion, and the constructions of a 

 sanguine temperament, are the objects associated in 

 the lay-mind with that term. 



On the fourth page is further evidence that the 

 author has failed to discriminate practically between 

 delusions, hallucinations, and illusions. After stating 

 that alienists have good reason to limit the word illu- 

 sion to illusory perceptions, he adds " such illusions 

 of the senses are the most palpable and striking evi- 

 dences of mental disease." Inasmuch as illusions are 

 common with the sane, it is incorrect to lay greater 

 stress on the not very frequent illusions of the insane, 

 than on the marked and characteristic hallucinations 

 and the still more universal delusions of that class. 



The author defines an illusion as a species of error 

 which counterfeits the form of immediate, self-evident, 

 or intuitive knowledge whether as a sense perception 

 or otherwise. Further on he discriminates between 

 the illusion and the fallacy, by characterising the 

 former as a falsification of primary or intuitive know- 

 ledge, and the latter as a falsification of secondary or 

 inferential knowledge. It must be admitted that the 

 author is happier in his discrimination than in his 

 definition, and an illustration of the difficulty under 

 which definers labor recurs in the peroration of the 

 same chapter, where he says that the illusion is seen 

 to arise through " some exceptional feature in the sit- 

 uation or condition of the individual, which, for the 

 time, breaks the chain of intellectual solidarity which 

 under ordinary circumstances binds the single member 

 to the collective body." The greater portion of this 

 passage would constitute an excellent nucleus for a 

 definition of insanity, but at the same time it seems to 

 us that it fails to cover those common illusions, which 

 involve the visual apparatus, and of which familiar il- 

 lustrations are furnished, in most physiological text 

 books. The dividing line between the delusion, the 

 -hallucination, and the illusion, should have been 

 strictly drawn at the outset, by our author. We 

 have offered the following as showing the difference 

 between the hallucination and the illusion : While a 

 hallucination is a subjective perception of an object 

 as a real presence, without a real presence to justify 

 the perception, and a memory is the subjective per- 



