32 PHYSIOLOGY. 



of metaphysics will know the distinction pointed at here between 

 the qualities of bodies as primary and secondary. Those we 

 call primary which we perceive by the sense of touch, viz. so- 

 lidity, motion, Sec., which are inseparable from the idea of body, 

 and upon which all the secondary are founded. Whether these 

 distinctions be well or ill founded, it is not my business to in- 

 quire ; but it is agreed among all philosophers, that, with re- 

 gard to the first four senses, we have no notion of the nature of 

 the bodies acting upon them ; we can perceive no necessary con- 

 nexion between these bodies and the sensations that they produce, 

 as, why the tremor of bodies produces sound, or the impulse of 

 light the sensation of vision, far less why a refrangibility of the 

 rays of light produces red, &c. ; for all that we can perceive, 

 this might have been inverted, and what now occasions the 

 sensation of red, might have occasioned that of blue. I mean 

 to take hold of the fact, that we see no necessary connexion be- 

 tween the operation of bodies, and the sensations they produce, so 

 that we must not be very rash in concluding from sensations as to 

 the nature of the actions occasioning them. But, though it is true 

 that such are our sensations of certain modes of touch, that we 

 have the idea of body, of an extended solid from them, I would 

 not have this applied to all the sensations which we receive from 

 touch. Thus, the appending a weight to my leg as a mechan- 

 ical power to resist my moving so easily, gives a sensation of 

 weight ; but, at the same time, that very sensation can arise 

 without any weight appended, and entirely from a particular 

 state of the nervous system ; it is merely a sense of resistance 

 to the motion of the nervous power in the crural nerve. I have 

 no doubt that all pain arises from the force of mechanical im- 

 pulse ; but if a man, in the gout for example, has a sensation as 

 if a dog was gnawing at his toe, or as if a chord was con- 

 stricting it, we must not infer that in this case, the pain arises 

 from analogous modes of action." 



XLII. To produce any sensation of impression, a certain 

 force of impression is necessary ; and, from a lesser force, no 

 sensation arises. The degree of force is likewise so limited, on 

 the other hand, that, in a high degree, it destroys the organ ; 

 and, in degrees approaching to this, a general sensation of pain, 

 rather than the sensation of any particular object, is produced. 



