HYPOTHESES. 17 



thing actually existing in nature, but should be already 

 known to have some influence upon the supposed 

 effect ; the precise degree and manner of the influence 

 being the only point undetermined. In any other 

 case, it is no evidence of the truth of the hypothesis 

 that we are able to deduce the real phenomena 

 from it. 



Is it, then, never allowable, in a scientific hypo- 

 thesis, to assume a cause ; but only to ascribe an 

 assumed law to a known cause? I do not assert this. 

 I only say, that in the latter case alone can the hypo- 

 thesis be received as true merely because it explains 

 the phenomena : in the former case it is only useful 

 by suggesting a line of investigation which may pos- 

 sibly terminate in obtaining real proof. For this 

 purpose, as is justly remarked by M. Comte (who 

 of all philosophers seems to me to have approached 

 the nearest to a sound view of this important 

 subject), it is indispensable that the cause suggested 

 by the hypothesis should be in its own nature sus- 

 ceptible of being proved by other evidence. This 

 seems to be the philosophical import of Newton's 

 maxim (so often cited with approbation by subsequent 

 writers) , that the cause assigned for any phenomenon 

 must not only be such as if admitted would explain 

 the phenomenon, but must also be a vera causa. 

 What he meant by a vera causa Newton did not indeed 

 very explicitly define ; and Mr. Whewell, who dissents 

 from the propriety of any such restriction upon the 

 latitude of framing hypotheses, has had little difficulty 

 in showing* that his conception of it was neither 

 precise nor consistent with itself: accordingly his 

 optical theory was a signal instance of the violation of 



* Philosophy of the Inductive Sciences, ii. 441 6. 

 VOL. IJ. C 



