HYPOTHESES. 15 



of it. But in order that this may be the case, I conceive it 

 to be necessary, when the hypothesis relates to causation, that 

 the supposed cause should not only be a real phenomenon, 

 something actually existing in nature, but should be already 

 known to exercise, or at least to be capable of exercising, an 

 influence of some sort over the effect. 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 hypothesis, 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 hypothesis 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 possibly terminate in obtaining real proof. For 

 this purpose, as is justly remarked by M. Comte, it is indis- 

 pensable that the cause suggested by the hypothesis should 

 be in its own nature susceptible of being proved by other 

 evidence. This seems to be the philosophical import of 

 Newton's maxim, (so often cited with approbation by sub- 

 sequent 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 Dr. 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 his 

 own rule. It is certainly not necessary that the cause assigned 

 should be a cause already known ; else how could we ever 

 become acquainted with any new cause ? But what is true in 

 the maxim is, that the cause, though not known previously, 

 should be capable of being known thereafter ; that its existence 

 should be capable of being detected, and its connexion with 

 the effect ascribed to it should be susceptible of being proved, 



* Philosophy of Discovery, pp. 185 et seqq. 



