CHAPTER IL 



OF THE NERVOUS FLUID. 



A SUBTLE substance, remarkable for the rapidity of its movements and 

 receiving little attention because it cannot be directly observed, 

 collected, nor experimentally examined ; a substance of this character 

 is the very strange and wonderful agent that nature employs for 

 producing the muscular movement, feeling, inner emotions, ideas, and 

 acts of intelligence, which many animals are able to carry out. 



Now since we can only know this substance through the effects 

 that it produces, we must begin by discussing it at the outset of the 

 third part of this work ; this fluid is the only substance capable of 

 causing the phenomena which so much excite our wonder ; and if 

 we refuse to admit its existence and powers, we shall be forced to 

 abandon all search for physical causes for these phenomena and to 

 have recourse once more to vague and baseless theories for the satis- 

 faction of our curiosity. 



With regard to the necessity for investigating this fluid by means 

 of its effects, is it not now an admitted fact that there exist in nature 

 various kinds of substances, imperceptible to our senses, that we cannot 

 take hold of, nor collect and examine as we should like ; substances 

 so attenuated and so subtle that they can only manifest their existence 

 under certain circumstances, and through the medium of some of their 

 results, which we succeed by careful attention in identifying ; sub- 

 stances, in short, whose nature we can only ascertain up to a certain 

 point by means of inductions and analogies, guided by a large number 

 of observations ? The existence of these substances is however proved 

 by certain effects which can be produced in no other way ; effects 

 which we have to study carefully in various phenomena whose causes 

 we seek. 



It may be said by some that since we possess so few means for 

 determining with precision the nature and qualities of these sub- 

 stances, every wise man who is concerned only with exact knowledge 

 should leave them out of account. 



