PHYSIOLOGY. 49 



the sensations of debility and difficulty are also uneasy. (" See 

 page 59-1 



LXXV. In sensations of impression, their being pleasant or 

 painful often depends on the degree of force in the impression, 

 allowance being made for the sensibility of the system. 



LXXVI. As impressions, by being repeated, produce weak 

 sensations (LIV.), impressions, at first painful, may, by repe- 

 tition, be changed into pleasant, and the pleasant into insipid 

 and uneasy. Hence arises, with regard to moderate impres- 

 sions, the pleasure of novelty, the desire of variety, and the 

 desire of increasing the force of pleasant impressions. 



LXXVI I. There is a condition of impressions rendering 

 them objects of desire or aversion, that cannot, with certainty, 

 be referred to their force. " For instance, it is not yet deter- 

 mined whether the fragrant and fetid smells differ merely by the 

 force of impression ; and the same applies to sweet and bitter 

 tastes. I suppose, therefore, that there may be other circum- 

 stances determining the desire or aversion. 11 This condition we 

 call the quality of impressions. 



LXXVIII. Impressions are often rendered objects of desire 

 or aversion, by combination, succession, and relation. 



LXXIX. No sensations arise originally in the mind without 

 a previous change in the state of the body. " For this, I refer 

 to Mr. Locke. The import of it is, that we have no innate 

 ideas. 11 



LXXX. Certain impressions, and certain states of the bo- 

 dy, like to those which produce the sensations of consciousness, 

 may both of them act upon the nervous system without pro- 

 ducing any sensation. " I repeat it again, that the ordinary 

 course of the communication of the different parts of the nerv- 

 ous system, is by sensation and volition ; but this is not univer- 

 sally the case. In how many cases it was so at first, and the con- 

 sciousness of it has been lost by repetition, I will not determine ; 

 but, I say, it is probable there are certain cases where there 

 never was any sensation, as in the instance of a purgative ; this 

 perhaps a man takes for the first time of his life ; he has a sen- 

 sation of the effect, the contents being propelled towards the ex- 

 tremity of the rectum ; he is sensible of them when there, but 

 the previous operation he is absolutely insensible of. There are, 



VOL. I. D 



