190 Philosophy of the Human MiJid. [Chap. XIL 



rests, are those of ^vibration and association. 

 Newton had taught that the rays of hght, fall- 

 ing upon the bott£)m of the eye, excite vibra- 

 tions in the retinoy and that these vibrations, be- 

 ing propagated along the optic nerves into the 

 brain, produce the sensation of seeing*. Dr. 

 Hartley adopted this hypothesis, and applied it, 

 >vith ingenious additions and modifications of his 

 own, to the other senses. Mr. Locke had thrown 

 new light on the doctrine o^ association, and shown 

 its great influence and importance in the opera- 

 tions of the human mind. Dr. Hartley also adopt- 

 ed the leading ideas of this great metaphysician 

 on this subject, and by uniting them M^ith the 

 Newtonian opinions, formed a system on which 

 the praise of great ingenuity and plausibility has 

 been bestowed. 



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 



view to tlie clerical profession, in the church of England ; but feel- 

 ing some scruples about subscribing to the thirty-nine articles, he 

 relinquished that design, and devoted himself to the study and 

 practice of medicine, in which he was eminent. While he de- 

 parted from the public standards of his church in several impor- 

 tant particulars, he was much distinguished for the force of his 

 mind, the extent of his learning, the amiablencss and benevolence 

 of his disposition, and the purity of his moral character. He was 

 a firm believer in Revelation, and wrote, though not with ortho- 

 doxy, yet with great seriousness and ability in its defence. 

 * See Additional Notes— (KK.) 



