36 PHYSIOLOGY. 



physic, that of two impressions giving pain, the stronger oblit- 

 erates the weaker. According to Hippocrates, the corporeal im- 

 pression is equally made, whether the mind is attentive or not ; 

 hut the mind can attend to one sensation only at a time. The 

 difficulty that occurs here, arises from the power the mind has of 

 making so very surprisingly quick transitions from one sensa- 

 tion or idea to another ; so that it is difficult to perceive that we 

 do not attend to several at once. But considering volition as a 

 subject of sensation, the difficulty is still greater. If there are a 

 certain number of motions under the power of the will and vo- 

 lition arises from sensation, and, if we are conscious both of 

 the sensation and the volition, if this is necessary, as the 

 Stahlians suppose, we attend to many volitions at once, for we 

 make at once a number of motions that may be called volunta- 

 ry. Thus, I am speaking, moving my eyes and my hands 

 which you may think I do too much and many other motions 

 are going on at the same time. It would appear, therefore, that 

 the mind does attend to more than one sensation at a time ; 

 but I shall solve that difficulty by and by. 1 ' 



L. Though the mind admits but of one sensation at one 

 time, several impressions may act at the same time, if they be 

 such as can unite in producing a single sensation ; and such is 

 the case of many of the impressions which produce the particu- 

 lar sensations of the same genus, as in those especially of colour, 

 sound, smell, and taste. " A sound and a colour, presented at 

 the same time, cannot unite in one sensation ; and if the one is 

 observed, the other must be neglected. But different sounds or 

 colours can very readily unite in producing one sensation : thus, 

 rays of light being thrown on surfaces suited to reflect portions 

 of them, some will give a yellow colour, others will, in the same 

 circumstances, give the sensation at least of a blue colour ; but 

 both, yellow and blue, being by means of a prism thrown toge- 

 ther upon the same spot, we find that the effect of these two im- 

 pressions acting together, is to give the sensation of green. So 

 there are certain tones in sound, which, sounded upon different 

 instruments, or on different chords of the same instrument, will 

 unite, and give the expression of one single sound : this we call 

 a concord. The same applies to the union of tastes and smells.*" 



LI. In each of these genera, many impressions, which sepa- 



