THE NATURE OF THE SOUL. 99 



perfected, the desire has grown to make them exact; 

 that is, to make the study of phenomena as purely 

 empirical as possible, and to formulate the resultant 

 laws as clearly as the circumstances permit — if 

 possible, mathematically. The latter is, however, only 

 feasible in a small province of human knowledge, 

 especially in those sciences in which there is a question 

 of measurable quantities ; in mathematics, in the first 

 place, and to a greater or less extent in astronomy, 

 mechanics, and a great part of physics and chemistry. 

 Hence these studies are called "exact sciences" in 

 the narrower sense. It is, however, productive only 

 of error to call all the physical sciences exact, and 

 oppose them to the historical, mental, and moral 

 sciences. The greater part of physical science can 

 no more be treated as an exact science than history 

 can ; this is especially true of biology and of its sub- 

 sidiary branch, psychologj 7 . As psychology is a part 

 of physiology, it must, as a general rule, follow the 

 chief methods of that science. It must establish the 

 facts of psychic activity by empirical methods as much 

 as possible, by observation and experiment, and it 

 must then gather the laws of the mind by inductive 

 and deductive inferences from its observations, and 

 formulate them with the utmost distinctness. But, 

 for obvious reasons, it is rarely possible to formulate 

 them mathematically. Such a procedure is only 

 profitable in one section of the physiology of the 

 senses ; it is not practicable in the greater part of 

 cerebral physiology. 



One small section of physiology, which seems 

 amenable to the " exact " method of investigation, 

 has been carefully studied for the last twenty years 

 and raised to the position of a separate science under 



