464 SPECIAL METHODS OF DISCOVERY. 



exceptional and residuary instances requires, again, a dif- 

 ferent process. 



And 4. With respect to its dependence upon the state 

 of knowledge at the time, and upon the chronological 

 order of the sciences, it is evident that as our mental 

 faculties have only a finite degree of power, we can only 

 use a method of discovery successfully, provided the sub- 

 ject is ripe, or has arrived at that stage at which discovery 

 is possible, i.e. at which the intellectual and other labour 

 necessary to effect it has come within the limit of our 

 means. 



In the practical discovery of new truths, and in the in- 

 vestigation of those already known, every known variety, 

 combination, and permutation of mental and physical me- 

 thod is employed, those methods being selected which are 

 best suited to the particular case; for instance, the senses, 

 with instrumental aids, for discovering sensible things ; 

 comparison in detecting similarities and differences ; 

 analysis in finding the constituent parts, either simple or 

 compound, of phenomena and substances ; division and 

 exclusion in discovering causes, coincidences, and their 

 relations ; induction and inference in disclosing abstract 

 qualities, and general laws and principles ; and synthesis 

 and deduction in discovering new compounds and effects. 



In most cases of discovery we imagine new hypotheses, 

 and test them by experiment and observation, or by the 

 latter alone, as in astronomy ; but we do not always 

 imagine an hypothesis before we make an original research. 

 Many discoveries are evolved by means of study and rea- 

 soning, i.e. by classifying and comparing known truths, or 

 by drawing conclusions from facts already observed ; some 

 are effected by the invention of new experiments, and 

 especially by new methods of examining a force or phe- 

 nomenon ; others, by the employment of more powerful 



