Chap. XII.] Philosophy of the Human Mind. I9I 



the nerves on which they are impressed, and then 

 in the brain, vibrations of tlie small and infmitesi- 

 mal medullary particles, ^\'hich vibration excites a 

 sensation in the percipient princij)lc, which remains 

 as long as the vibration kists, tliat is, as long as 

 the object continues to affect the organs of sense. 

 Tliat the medidlaiy substance having once vibrated 

 in a particular manner, docs not return entirely to 

 its natural state, but continues disposed to vibrate 

 in that manner rather than any other, which ten- 

 dency of the brain to the renewal of the vibration 

 is the cause of the retention of the idea in the ab- 

 sence of the archetype. That whatever renews 

 the vibration, renews also the perception ; but the 

 renewed vibration beino; less vigorous than the 

 original one, is called a miniature 'cibration, or vi- 

 bratiuncle, and the renewed perception correspond- 

 ing with it is called an idea. That vibrations may 

 be revived not only by the repetition of external 

 impressions, but by their association with each 

 other; and th.at, of vibrations which have been 

 associated together a sufficient number of times, 

 either synchronously, or in succession, if one be 

 excited, it Avill excite the miniatures of all tlie rest. 

 This is supposed to furnish a solution to all the 

 phenomena of the association of ideas. 



According to this theory the nerves are divided 

 into two classes, sensory and mot 01^ ; the former 

 beino- the immediate instruments of sensation, the 

 latter of motion. Both originate in the medullary 

 substance of the brain, and their vibrations influ- 

 ence and modify each other. In short, every sen- 

 sation, idea, muscular motion, affection, and in- 

 ternal feeling, whatever, is su])posed, by Dr. liait- 



