1$ Philosophy of the Human Mind. 



light, falling upon the bottom of the eye, excite 

 vibrations in the retina, and that these vibrations 

 being propagated along the optic nerves into the 

 brain, produce the sensation of seeing. Dr. Hart- 

 ley adopted this hypothesis, and applied it, with 

 ingenious additions and modifications of his own, 

 to the other senses. Mr. Locke had thrown new 

 light on the doctrine of association, and shown its 

 great influence and importance in the operations 

 of the human mind. Dr. Hartley also adopted 

 the leading ideas of this great metaphysician on this 

 subject, and by uniting them with the Newtonian 

 opinions, formed a system on which the praise of 

 great ingenuity and plausibility has been be- 

 stowed. 



He taught, that the white medullary substance 

 of the brain, spinal marrow, and the nerves pro- 

 ceeding from them, form the immediate instrument 

 of sensation and motion; that whatever changes 

 are produced in this substance, corresponding 

 changes take place in our ideas; that external ob- 

 jects impressed upon the nerves occasion, first in 

 the nerves on which they are impressed, and then 

 in the brain, vibrations of the small and infinitesi- 

 mal medullary particles, which vibration excites a 

 sensation in the percipient principle, which re- 

 mains as long as 4he vibration lasts, that is, as long 

 as the object continues to affect the organs of sense. 

 That the medullary substance having once vibrated 

 in a particular manner, does 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 being less vigorous than the 

 original one, is called a miniature vibration, or vi* 



