418 ACTUAL WORKING IN ORIGINAL RESEARCH. 



like a house ; the maker of a watch is not a watch ; nor is 

 the contriver of a steam-engine, a steam engine himself ; 

 and even a parent is not necessarily exactly like his children. 



The determination of causes is one of the most difficult 

 parts of original research, because it requires the greatest 

 combination of human powers, and it is consequently the 

 one in which an investigator is the most apt to err. It 

 requires, more than any other scientific occupation, an 

 extensive and sound knowledge of general scientific prin- 

 ciples and their relative degrees of importance, an acquaint- 

 ance with the natural order of dependence of the sciences, 

 and the forces of which they treat ; the principles of con- 

 servation of matter and of force, of action and reaction, of 

 the correlation, mutual convertibility, and equivalence of 

 the various forces ; the inseparable connection between 

 stored-up powers and molecular states, and between active 

 forces' and molecular changes; the general property which 

 matter possesses of enabling one force to simultaneously 

 produce many effects, and of several forces to produce 

 one effect ; the rhythmical action of forces, and the nume- 

 rous other great general principles already enumerated in 

 Chapter XIV. As the method of determining causes, 

 effects, conditions, coincidences, and other abstruse rela- 

 tions, are processes of inference, this part of research 

 especially requires a logical habit of mind, and a consider- 

 able power of reasoning correctly. Sometimes, in conse- 

 quence of the investigator's mind being engrossed with the 

 details of his research, the most obvious cause of the phe- 

 nomena he is investigating is overlooked, and either no 

 cause is discovered or a wrong one assigned. 



It is a wise rule, not to assume the existence or opera- 

 tion of occult or new causes, when ordinary ones are suffi- 

 cient to account for all the effects, or when they will best 

 explain them ; and on the other hand, it is also a good 



