30 MOTIONS OF THE RETINA. SECT. III. 6. 



pie retain the ideas they had learned early in life, 

 but find great difficulty in acquiring new trains of 

 memory ; infomuch that in extreme old age we 

 frequently fee a forgetfulnefs of the bufinefs of yef- 

 terday, and at the fame time a circumftantial re- 

 membrance of the amufements of their youth ; till 

 at length the ideas of recollection and activity of 

 the body gradually ceafe together fuch is the con- 

 dition of humanity ! and nothing remains but the 

 vital motions and fenfations. 



VI. i. In oppofition to this doctrine of the pro- 

 duction of our ideas, it may be afked, if fome of 

 our ideas, like other animal motions, are volunta- 

 ry, why can we not invent new ones, that have 

 not been received by perception ? The anfwer will 

 be better underftood after having perufed the fuc- 

 ceeding lection, where it will be explained, that 

 the mufcular motions likewife are originally excited 

 by the ftimulus of bodies external to the moving 

 organ ; and that the will has only the power of re- 

 peating the motions thus excited. 



2. Another objector may alk, Can the motion of 

 an organ of fenfe referable an odour or a colour ? 

 To which I can only anfwer, that it has not been 

 dernonflrated that any of our ideas refemble the ob- 

 jects that excite them : it has generally been be- 

 lieved that they do not ; but this ftiall be difcuffed 

 at large in Seel. XIV. 



5. There is another objection that at firft view 

 would feem lefs eafy to furmount. After the am- 

 putation of a foot or a finger, it has frequently hap- 

 pened, that an injury being offered to the (lump of 

 the amputated limb, whether from cold air, too 

 great preffure, or other accidents, the patient has 

 complained of a fenfation of pain in the foot or 

 finger, that was cut off. Does not this evince that 

 all our ideas are excited in the brain, and not in 

 the organs of fenfe ? This objection is anfwered, 



by 



