HYPOTHESES. 9 



if admitted, to explain such facts as are found capable of being 

 deduced from it. And this explanation is the purpose of 

 many, if not most, hypotheses. Since explaining, in the 

 scientific sense, means resolving an uniformity which is not a 

 law of causation, into the laws of causation from which it 

 results, or a complex law of causation into simpler and more 

 general ones from which it is capable of being deductively 

 inferred ; if there do not exist any known laws which fulfil this 

 requirement, we may feign or imagine some which would 

 fulfil it ; and this is making an hypothesis. 



An hypothesis being a mere supposition, there are no other 

 limits to hypotheses than those of the human imagination ; 

 we may, if we please, imagine, by way of accounting for an 

 effect, some cause of a kind utterly unknown, and acting 

 according to a law altogether 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 fictitious, but is supposed to 

 produce its effects according to laws similar to those of some 

 known class of phenomena. An instance of the first kind is 

 afforded by the different suppositions made 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 also suggested itself to 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 respecting the nature of 

 light, the one ascribing the phenomena to a fluid emitted from 

 all luminous bodies, the other (now generally received) attri- 



