THE SENSES. 625 



nerves; but the mind is accustomed to interpret these 

 modifications in the state of the nerves produced by external 

 influences as properties of the external bodies themselves. 

 This mode of regarding sensations is so habitual in the case 

 of the senses which are more rarely affected by internal 

 causes, that it is only on reflection that we perceive it to be 

 erroneous. In the case of the sense of feeling, on the con- 

 trary, where many of the peculiar sensations of the nerves 

 perceived by the sensorium are excited as frequently by 

 internal as by external causes, we more readily apprehend 

 the truth. For it is easily conceived that the feeling of 

 pain or pleasure, for example, is due to a condition of the 

 nerves, and is not a property of the things which excite it. 

 What is true of these is true of all other sensations ; the 

 mind perceives conditions of the optic, olfactory, and other 

 nerves specifically different from that of their state of rest ; 

 these conditions may be excited by the contact of external 

 objects, but they may also be the consequence of internal 

 changes : in the former case the mind, having knowledge 

 of the object through either instinct or instruction, re- 

 cognises it by the appropriate changes which it produces 

 in the state of the nerves. 



The special susceptibility of the different nerves of sense 

 for certain influences, as of the optic nerve, or rather its 

 centre, for light ; of the auditory nerve, or centre, for 

 vibrations of the air, etc., and so on, is not due entirely 

 to those nerves having each a specific irritability for such 

 influences exclusively.. For although, in the ordinary 

 events of life, the optic nerve is excited only by the undu- 

 lations or emanations of which light may consist, the audi- 

 tory only by vibrations of the air, and the olfactory only by 

 odorous particles yet each of these nerves may have its 

 peculiar properties called forth by other conditions. In 

 fact, in whatever way and to whatever degree a nerve of 

 special sense is stimulated, the sensation produced is 

 essentially of the same kind ; irritation of the optic nerve 



