OF SENSATION IN GENERAL. 389 



changes, of which the effects thus transiently remain upon the nerves of sense, 

 are more permanently impressed upon the Sensorium ; since, as formerly 

 shown ( 491), we can only in this manner account for the phenomena of 

 Memory, and for the effects produced upon this power, by material changes 

 in the brain. Hence, the diminution in the force of sensations, which is the 

 consequence of their habitual recurrence, may be considered as resulting from 

 these two general facts, the persistence of the impression made by them 

 upon the sensorium, and the consequent absence of a change in its state. 

 when a sensory impression is brought to it, which is of the same nature with 

 one already registered there : the degree in which the consciousness is ex- 

 cited, being dependent, as just stated, not upon the absolute degree of the 

 impressing cause, but upon the amount of change which it produces in the 

 sensorial apparatus. In this respect there is a perfect conformity between 

 the law of sensation, and that of muscular contraction ; for stimuli which ex- 

 cite the latter, usually lose their force in proportion to the frequency of their 

 repetition. Indeed, both may be considered as results of the more general 

 laws of vitality ; for the actions of other tissues follow the same rule, as is 

 shown by the tolerance, that may be gradually established in the system, of 

 medicinal agents, poisons, &c., which would have at first produced the most 

 violent effects, when given in the same amount. 



513. It is curious, also, that the feelings of Pain or Pleasure, which unac- 

 customed sensations excite, are often exchanged for each other, when the sys- 

 tem is habituated to them ; this is especially the case, in regard to impressions 

 communicated through the organs of smell and taste. There are many arti- 

 cles in common use among mankind, such as Tobacco, Fermented Liquors, 

 &c., the use of which cannot be said to produce a natural enjoyment, since 

 it is at first unpleasant to most persons ; and yet it first becomes tolerable, 

 then agreeable; and at last the want of them is felt as a painful privation, and 

 the stimulus must be applied in an increasing degree, in order to produce 

 the usual effect. 



514. It is through the medium of Sensation, that we acquire a knowledge 

 of the material world around us; and that its changes excite mental operations 

 in ourselves. The various kinds or modes of Sensation excite in us various 

 ideas regarding the properties of matter ; and these properties are known to 

 us, only through the changes which they produce in the several organs. Thus 

 a man totally blind from birth can form no idea of the nature of light or co- 

 lours; nor could one completely deaf have any just conception of musical 

 tones. It is well known that instances exist, in which, from some imperfection 

 of the organization, there is an incapacity for distinguishing colours or musi- 

 cal tones, whilst there is no want of sensibility to light or sound ; and that 

 some persons are naturally endowed with a much greater range of the sen- 

 sory faculties, than others possess. Hence it does not seem at all improbable, 

 that there are properties of matter, of which none of our senses can take 

 immediate cognizance ; and which other beings might be formed to perceive, 

 in the same manner as we are sensible to light, sound, &c. Thus, it is well 

 known, that many animals are affected by atmospheric changes, in such a 

 manner, that their actions are regarded by Man as indications of the probable 

 state of the weather ; and the same is the case in a less degree with some 

 of our own species ; who are peculiarly susceptible of the same influences. 

 Now the most universal of all the qualities or propensities of matter, 

 that, in fact, on which our notion of it is founded, is resistance ; and 

 it is this quality, of which the knowledge seems most universally diffused 

 throughout the Animal kingdom. In the lowest tribes, we find that contact, 

 between their surface and some material body, is required to produce sensa- 



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