HYPOTHESIS 137 



plain&quot; the known phenomena, that it was pretty generally (and 

 erroneously) regarded, for centuries, as a fully verified or estab 

 lished system. But, as St. Thomas pointed out with his character 

 istically prudent reserve, some other hypothesis might perhaps 

 account after all equally well or even better for the apparent 

 motions of the heavenly bodies. 1 And so after-events proved, 

 culminating in the substitution of the Copernican for the Ptolemaic 

 astronomy. 



Or again, two conflicting hypotheses may appear to account 

 equally well for certain phenomena. These latter will be differ 

 ently described on either hypothesis. And both descriptions will 

 appear to be equally accurate. But evidently at least one of the 

 descriptions must be de facto inaccurate : the assumptions involved 

 in the very language used based as this is on the supposition that 

 the phenomena are of such and such a nature, due to such and 

 such a cause, while they are quite otherwise in reality such as 

 sumptions necessarily falsify the whole description. 



We may conclude, then, that there is no fundamental differ 

 ence between working, descriptive, and explanatory hypotheses, 

 hypotheses of law, and hypotheses of cause, provided only and 

 always that they are suppositions which have for their objects the 

 REAL CAUSES of the observed phenomena, and the REAL LAWS accord 

 ing to which the changes wrought by those causes in the observed 

 phenomena actually take place. 



228. NATURE AND VERIFICATION OF CAUSAL OR EXPLA 

 NATORY HYPOTHESES. The first essential, then, of a scientific 

 hypothesis, is that it be a supposition of something real, equally 

 real with the phenomenon it is to explain. This at once disposes 

 of the class of suppositions referred to above (226), to which we 

 have recourse merely in order the better to realize some pheno 

 menon. But, furthermore, even when the object of our supposi 

 tion is not a mere fancied possibility, when we mean the cause or 

 law we are supposing, to be real, to be a fact, even then the ques 

 tion arises : Have we always or necessarily a scientific hypothesis, as 

 distinct from what some writers, for want of a better name, call 

 systematic or synthetic conceptions ? 2 What sort, in other words, 



1 &quot; Licet enim talibus suppositionibus factis apparentia salvarentur, non tamen op- 

 portet dicere has suppositiones esse veras, quia forte secundum aliquem alium modum, 

 nondum ab hominibus comprehensum, apparentia circa Stellas salvantur.&quot; In Lib. 

 i. De Ccelo et Mundo, 1. xvii. 



MERCIER, op. cit., p. 338. 



