RELATIONS BETWEEN STIMULUS AND SENSATION 453 



sides and yet the two have the same task in common. 1 The former, which 

 considers the psychical side, seeks to eliminate from the cognitive and per- 

 ceptive processes everything which proceeds from the effects of the objective 

 world, in order to obtain in its purity that which is proper to the mind itself. 

 Natural science on the contrary seeks to remove the relative and the formal 

 elements of thought, definition, notation, forms, hypothesis, etc., in order to 

 secure what belongs to the world of actuality, the laws of which it seeks to 

 know. In order to give a theoretical explanation of sensations from the scien- 

 tific standpoint we must bear in mind the following propositions. 



1. There are two different degrees of distinction among sensations. The 

 one most essential is the distinction between those belonging to the different 

 senses, as between the sensations of blue, sweet, warm, and loud. This differ- 

 ence is designated as the difference in modality of sensation, and is so complete 

 that it precludes any transition from one to the other or any relation of 

 greater or less similarity between them. For example, one cannot say whether 

 sweet is more like blue or red. The second difference, which Helmholtz limits 

 to a difference in quality between sensations belonging to the same sense, is 

 less exclusive. Within the same sense transition and comparison are possible. 

 From blue we can pass through violet and carmine red to scarlet red and can 

 say, e. g., that yellow is more like orange than like blue. 



We distinguish the following modalities : pressure and touch ; heat and cold ; 

 taste; smell; hearing; and sight. (With regard to pain cf. Chapter XVII, 4.) 



2. Experiment has shown that the profound difference between the senses 

 does not depend in any wise upon the kind of external stimuli by which the 

 sensations are aroused, but is determined solely by the kind of sensory nerve 

 affected. 



For illustration, physics considers light as extremely rapid vibrations of a 

 hypothetical, imponderable medium, the ether, which is distributed throughout 

 all space. When these vibrations of the ether strike the retina, the latter is 

 excited and in its turn produces through the optic nerve an excitation in the 

 brain, which gives rise in consciousness to a sensation of light. But this sen- 

 sation of light has not the least resemblance to the vibrations which constitute 

 the objective phenomenon of light. This itself should be fairly convincing evi- 

 dence that the sensation cannot agree in kind with its external cause. Con- 

 clusive proof is found in considerations such as the following: If the eyeball be 

 pressed upon, we receive even in pitch darkness a sensation which is characterized 

 by a brilliant play of colors. A blow upon the eye produces a flash of light. 

 Here we have a perfectly typical sensation, and yet no light at all has reached 

 the eye. The sensation is unquestionably due to an excitation of the optic 

 nerve produced by the mechanical pressure on the eyeball. The same thing is 

 experienced when an electric current is conducted through the eye ; we get a 

 sensation of light, even though no objective light may be present. 



3. Since therefore sensations of exactly the same nature are aroused by 

 three wholly different kinds of stimuli light, mechanical pressure, and elec- 



1 The following discussion is essentially a repetition of the views of Helmholtz as set 

 forth in " Die Tatsachen in der Wahrnehmung," Berlin, 1879. 



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