INTRODUCTION. 



In the second part of this work, I have endeavoured to throw Ught on 

 the physical causes of life, on the conditions necessary for its existence, 

 and on the origin of that exciting force of vital movements, without 

 which no body could actually possess life. 



I now propose to enquire what feeling may be, how the special 

 organ giving rise to it (the nervous system) produces the wonderful 

 phenomenon of sensations, how sensations themselves produce ideas 

 through the medium of the brain, and how ideas cause in that organ 

 the formation of thoughts, judgments and reasoning ; in short, of acts 

 of intelligence that are still more wonderful than sensations. 



" But," it is said, "the functions of the brain are of a different order 

 from those of the other viscera. In the latter causes and effects are 

 of the same nature (physical nature). . . . 



" The functions of the brain are of quite a different order : they 

 consist in receiving sense impressions through the nerves, in transmit- 

 ting them, immediately^to. ,the mind, in preserving the traces of these 

 impressions, and in reproducing them with varying rapidity, clearness 

 and Julness whenever the mind needs them in its operations or the 

 laws of association of ideas recall them ; lastly, in transmitting to 

 the^nuscles again through the medium of nerves the commands of 

 the will. 



" Now these three functions involve a mutual influence, which has 

 always remained incomprehensible, between divisible matter and the 

 indivisible ego. This has always constituted an impassable hiatus in 

 the system of our ideas, and the stimabhng block of all philosophies ; 

 they involve us moreover in a further difi&culty that has no necessary 

 connection with the first : not only do we not imderstand nor ever 

 shall understand how impressions on the brain can be perceived by the 

 mind and produce images in it ; but however refined our means of 

 investigation, these traces cannot be made visible in any way ; and we 

 are entirely ignorant of their nature, although the effect of age and 

 diseases on the memory leave us in no doubt either as to their 



