HYPOTHESES. 11 



of accounting for an effect, some cause of a kind 

 utterly unknown, and acting according to a law alto- 

 gether fictitious. But as hypotheses of this sort 

 would not have any of the plausibility belonging to 

 those which ally themselves by analogy with known 

 laws of nature, and besides would not supply the want 

 which arbitrary hypotheses are generally invented to 

 satisfy, by enabling the imagination to represent to 

 itself an obscure phenomenon in a familiar light; 

 there is probably no hypothesis in the history of 

 science in which both the agent itself and the law of 

 its operation were fictitious. Either the phenomenon 

 assigned as the cause is real, but the law according to 

 which it acts, merely supposed; or the cause is ficti- 

 tious, but is supposed to produce its effects according 

 to laws similar to those of some known class of phe- 

 nomena. An instance of the first kind is afforded by 

 the different suppositions respecting the law of the 

 planetary central force, anterior to the discovery of the 

 true law, that the force varies as the inverse square 

 of the distance ; which was itself suggested by 

 Newton, in the first instance, as an hypothesis, and 

 was verified by proving that it led deductively to 

 Kepler's laws. Hypotheses of the second kind are 

 such as the vortices of Descartes, which were ficti- 

 tious, but were supposed to obey the known laws of 

 rotatory motion ; or the two rival hypotheses respect- 

 ing the nature of light, the one ascribing the pheno- 

 mena to a fluid emitted from all luminous bodies, the 

 other (now more generally received) attributing them 

 to vibratory motions among the particles of an ether 

 pervading all space. Of the existence of either fluid 

 there is no evidence, save the explanation they are cal- 

 culated to afford of some of the phenomena ; but they 

 are supposed to produce their effects according to 



