INTRODUCTION. 53 



As, for example, when he has ascertained that sensation and 

 motion occupy distinct situations in every part of the nervous 

 system. 



This " Law," however, it must be remembered, is not a discovery 

 by itself, nor does it give him any new information, but is simply 

 the expression, in convenient and comprehensive language, of the 

 facts with which he was already previously acquainted. It is very 

 dangerous, therefore, to make these laws or general principles the 

 subjects of our study instead of the vital phenomena, or to suppose 

 that they have any value, except as the expression of previously 

 ascertained facts. Such a misconception would lead to bad prac- 

 tical results. For if we were to observe a phenomenon in discord- 

 ance with a " law" or " principle," we might be led to neglect or 

 misinterpret the phenomenon, in order to preserve the law. But 

 this would be manifestly incorrect. For the law is not superior to 

 the phenomenon, but, on the contrary, depends upon it, and derives 

 its whole authority from it. Such mistakes, however, have been 

 repeatedly made in Physiology, and have frequently retarded its 

 advance. 



IY. There is only one means by which Physiology can be 

 studied : that is, the observation of nature. Its phenomena cannot 

 be reasoned out by themselves, nor inferred, by logical sequence, 

 form any original principles, nor from any other set of phenomena 

 whatever. 



In Mathematics and Philosophy, on the other hand, certain truths 

 are taken for granted, or perceived by intuition, and the remainder 

 afterward derived from them by a process of reasoning. But in 

 Physiology, as in all the other natural sciences, there is no such 

 starting point, and it is impossible to judge of the character of a 

 phenomenon until after it has been observed. Thus, the only way 

 to learn what action is exerted by nitric acid upon carbonate of 

 soda is to put the two substances together, and observe the changes 

 which take place ; for there is nothing in the general characters of 

 these two substances which could guide us in anticipating the result. 



Neither can we infer the truths of Physiology from those of 

 Anatomy, nor the truths of one part of Physiology from those of 

 another part ; but all must be ascertained directly and separately 

 by observation. 



For, although one department of natural science is almost always 

 a necessary preliminary to the study of another, yet the facts of 



