METHODS OF DISCOVERING CAUSES. 423 



relations between them. Two things may however be 

 inseparable and vary equally together, and yet be only 

 coincident. 



The Method of Residues is Having allowed for the 

 effect of all known causes, whatever remains of the effect 

 must be due to an unknown cause. The proof of a com- 

 plete analysis of a phenomenon is, that there is nothing 

 left unaccounted for. This method is continually used in 

 chemical discovery ; it is specially suited to the discovery 

 of new elementary substances and of some astronomical 

 phenomena. 



The causes of some phenomena are far more difficult to 

 ascertain than those of others ; those of some phenomena 

 are perceived at once, but those of time and space are far 

 beyond our deepest conceptions. In a research, in order 

 to suggest likely causes, we compare the phenomena we 

 have found with those already known. They either re- 

 semble some known phenomena or they do not ; in the 

 former case they are probably produced by some known 

 cause or causes ; in the latter, they may be either due to 

 some unknown cause, or to some peculiar combination, or 

 mode of action, of known ones. The degree of probability 

 that they are due to an unknown cause is however very 

 small, because the discovery of a new physical agency, or 

 even of a new relation between known physical powers, is 

 a very rare occurrence. 



As the number of physical and chemical phenomena 

 already known is immense, and our mental powers are 

 so limited as only to enable us to think of a few things 

 simultaneously, it is evident that the new phenomena can 

 only be compared with a portion of the old ones at a time. 

 Well-classified knowledge of science, and familiarity with 

 the great groups of scientific truths, and with the orders 

 of forces, substances, and relations of substances, in phy- 



