NATURE 



185 



THURSDAY, JUNE 30, i88i 



ILLUSIONS 



Illusions. By James Sully. International Science Series, 



Vol. xxxiv. (London : C. Kegan Paul and Co., 1881.) 



OF the many interesting subjects to which the pub- 

 lishers of the International Science Series have 

 hitherto devoted their volumes, few have presented so 

 formidable a test of the strength of their respective 

 authors as the one which has been assigned to ;\Ir. 

 James Sully. Occupying some of the most obscure 

 regions of physiology, and passing over into the re- 

 motest cloudlands of psychology, illusions furnish mate- 

 rial alike for the most laborious exploration and the 

 most keen-sighted analysis ; but for this very reason they 

 constitute a class of phenomena which invite failure of 

 treatment at the hands of any but the most accomplished 

 psychologist. Mr. Sully is alieady well known to stand 

 in the first rank as a writer of this class, and the able 

 manner in which he has handled the difficult subject 

 consigned to him shows that it could not have been con- 

 signed to a better man. The wide range of his reading, 

 the clearness and force of his style, as well as the sound- 

 ness of his judgment, give him what we may call excep- 

 tional advantages for undertaking a treatise of this kind, 

 while the methodical, not to say laborious, manner in 

 which he has executed the task, shows that he has thrown 

 all his strength into its performance. 



First we have presented a "Definition of Illusion," 

 which, standing in a scientific as distinguished from a 

 philosophical treatise, is justly framed so as to exclude 

 any question with the idealist or the sceptic. "For our 

 present purpose the real is that which is true for all. . . . 

 Human e.xperience is consistent ; men's perceptions and 

 beliefs fall into a consensus. From this point of view 

 illusion is seen to arise through some exceptional feature 

 in the situation 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. Whether the common experience 

 which men thus obtain is rightly interpreted is a question 

 which does not concern us here. For our present 

 purpose, which is the determination and explanation of 

 illusion as popularly understood, it is sufficient that there 

 is a general consensus of belief, and this may be pro- 

 visionally regarded as at least practically true." 



Next we have a very methodical and judicious " Classi- 

 fication of Illusions." As distinguished from hallucina- 

 tions, illusions " must always have a starting-point in 

 some actual impression, whereas a hallucination has no 

 such basis." Still the one shades off so gradually into 

 the other that no determinate line can be drawn between 

 them. Therefore this distinction, although recognised as 

 a distinction, is not constituted, as it has been constituted 

 by some technical writers, " the leading principle of 

 classification"; but a more popular or common-sense 

 principle is adopted. All immediate knowledge, or 

 knowledge not attained by any conscious process of 

 inference, may be divided into four principal varieties — 

 Internal Perception (Introspection), External Perception, 

 Vol. XXIV. — No. 609 



Memory, and Belief, including Unreasoned Expectation. 

 The difficult question as to the relation of Belief to Know- 

 ledge is expressly set aside — it being allowed by every one 

 that many of our beliefs are for all purposes of action as 

 good as knowledge. Each of these four sources of 

 immediate or uninferred knowledge is open to the con- 

 tamination of illusion. Such is notoriously the case 

 with sense-perception, which, as the best-marked variety, 

 is treated first. 



In the course of a clear analysis of Perception stress is 

 laid upon what the author calls a " stage of prepercep- 

 tion," during which the mind receives the impression of 

 sense, but has not yet interpreted the impression into a 

 coherent percept. " In many of our instantaneous per- 

 ceptions these two stages are indistinguishable to con- 

 sciousness . . . But in the classification of an object or 

 the identification of an individual thing there is often an 

 appreciable interval between the first impression and the 

 final stage of complete recognition." [The time, that is, 

 during which, in Mr. Spencer's language, the mind is 

 forming its "integrations" — a process which takes place 

 more rapidly in adults than in children, and in " quick- 

 witted" than in less "ready" individuals.] "And here 

 ft is easy to distinguish the two stages of preperception 

 and perception. The interpretive image is slowly built 

 up by the operation of suggestion, at the close of which 

 the impression is suddenly illumined as by a flash of light, 

 and takes a definite precise shape." Now illusions of 

 perception may arise in either of these two stages. Even 

 in limine sufficient attention may not be paid to the 

 original impression, and thus a timid man will readily fall 

 into the illusion of ghost-seeing, because too little atten- 

 tive to the actual impression of the moment. But next, 

 even if the sensation is properly attended to, " misappre- 

 hension may arise of what is actually in the mind at the 

 moment." Although this "may sound paradoxical," it 

 means nothing more than that " the incoming nervous 

 process may to some extent be counteracted by a power- 

 ful reaction of the centres." Thus, for instance, a sensation 

 of colour may be appreciably modified when there is a 

 tendency to regard it in one particular way. 



After giving parenthetically a number of illustrations 

 of errors of perception which have their root in the initial 

 processes of sensation and "preperception," the essay 

 passes on to a further consideration of the more impor- 

 tant class of illusions which are connected with the later 

 stages of perception, or the process of interpreting the 

 sense-impression. These misinterpretations of sense- 

 impressions fall into two classes, according as they are 

 connected with a process of suggestion or with the process 

 of preperception. The illusion of a second shouter in an 

 echo is given as an example of the former, while that of 

 seeing spectres in familiar objects after the imagination 

 has been excited by ghost-stories, supplies an illustration 

 of the latter. The first of these classes of illusions arises 

 from without, the second from within : the one is there- 

 fore called Passive, the other Active. Besides these there 

 are other "sub-divisions" which need not here be de- 

 tailed. Indeed we think that a desirable simplicity of 

 classification might have been attained by ignoring these 

 lesser ramifications, and restricting attention to the mam 

 divisions—/.^, illusions arising in the initial processes of 

 sensation, in those of preperception, and in those of 



