THE USE OF HYPOTHESIS. 617 



Almost every problem in science thus takes the form of 

 a balance of probabilities. It is only when difficulty 

 after difficulty has been successfully explained away, and 

 decisive experimenta cntcis have, time after time, resulted 

 in favour of our theory, that we can venture to assert the 

 falsity of all objections. 



The sole real test of an hypothesis is its accordance 

 with fact. Descartes' celebrated system of vortices is 

 exploded, not because it was intrinsically absurd and 

 inconceivable, but because it could not give results in 

 accordance with the actual motions of the heavenly bodies. 

 The difficulties of conception involved in the apparatus 

 of vortices, are child's play compared with those of gravi- 

 tation and the undulatory theory already described. 

 Vortices are on the whole plausible suppositions ; for 

 planets and satellites bear at first sight much resemblance 

 to objects carried round in whirlpools, an analogy which 

 doubtless suggested the theory. The failure was in the 

 first and third requisites ; for, as already remarked, the 

 theory did not allow of precise calculation of planetary 

 motions, and was thus incapable of rigorous verification. 

 But so far as we can institute a comparison, facts are en 

 tirely against the vortices. Newton did not ridicule the 

 theory as absurd, but showed l that it was " pressed with 

 many difficulties." He carefully pointed out that the 

 Cartesian theory was inconsistent with the laws of Kepler, 

 and would represent the planets as moving more rapidly 

 at their aphelia than at their perihelia. 2 The rotatory 

 motion of the sun and planets on their own axes is in 

 striking conflict with the revolutions of the satellites 

 carried round them ; and comets, the most flimsy of bodies, 

 calmly pursue their courses in elliptic paths, irrespective 

 of the vortices which they pass through. We may now 

 also point to the interlacing orbits of the minor planets 

 as a new and insuperable difficulty in the way of the 

 Cartesian ideas. 



Newton, though he established the best of theories, was 

 also capable of proposing one of the worst ; and if we 

 want an instance of a theory decisively contradicted by 



1 Principia, bk. iii. Prop. 43. General Scholium. 



2 Ibid. bk. ii. Sect. ix. Prop c^. 



