HYPOTHESES. 353 



to causation, that the supposed cause should not only be a real phenomenon, 

 something actually existing in nature, but should be already known to ex- 

 ercise, or at least to be capable of exercising, an influence of some sort over 

 the effect. In any other case, it is no sufiicient 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 

 may be very useful by suggesting a line of investigation which may possi- 

 bly terminate in obtaining real proof. But for this purpose, as is justly 

 remarked by M. Comte, it is indispensable that the cause suggested by the 

 hypothesis should be in its own nature susceptible of being pi-oved 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 as- 

 signed 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; according- 

 ly 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 al- 

 ready known ; otherwise we should sacrifice our best opportunities of be- 

 coming acquainted with new causes. 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 connection with the effect ascribed to it should be susceptible of 

 being proved, by independent evidence. The hypothesis, by suggesting 

 observations and experiments, puts us on the road to that independent ev- 

 idence, if it be really attainable ; and till it be attained, the hypothesis 

 ought only to count for a more or less plausible conjecture. 



§ 5. This function, however, of hypotheses, is one which must be reckon- 

 ed absolutely indispensable in science. When Newton said, " Hypotheses 

 non fingo," he did not mean that he deprived himself of the facilities of 

 investigation afforded by assuming in the first instance what he hoped ul- 

 timately to be able to prove. Without such assumptions, science could 

 never have attained its present state ; they are necessary steps in the prog- 

 ress to something more certain ; and nearly every thing which is now theo- 

 ry was once hypothesis. Even in purely experimental science, some in- 

 ducement is necessary for trying one experiment rather than another ; and 

 though it is abstractedly possible that all the experiments which have been 

 tried, might have been produced by the mere desire to ascertain what 

 would happen in certain circumstances, without any previous conjecture 

 as to the result ; yet, in point of fact, those unobvious, delicate, and often 

 cumbrous and tedious processes of experiment, which have thrown most 

 light upon the general constitution of nature, would hardly ever have been 

 undertaken by the persons or at the time they were, unless it had seemed 

 to depend on them whether some general doctrine or theory which had 

 been suggested, but not yet proved, should be admitted or not. If this be 



* Philosophy of Discovery, p. 185 et seq. 

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