14G THE PRINCIPLES OF SCIENCE. 



The Third Requisite Conformity with Facts. 



Before we accept a new hypothesis, it must furnish us 

 with distinct credentials, consisting in the deductive anti- 

 cipation of a series of facts, which are not already con- 

 nected and accounted for by any equally probable hypo- 

 thesis. We cannot lay down any precise rule as to thel 

 number of accordances which can establish the truth of 

 an hypothesis, because the accordances will vary much inj 

 value. While, on the one hand, no finite number of 

 accordances will give entire certainty, the probability of 

 the hypothesis will increase very rapidly with the number 

 of accordances. Seldom, indeed, shall we have a theory 

 free from difficulties and apparent inconsistency with facts. 

 Though one real and undoubted inconsistency would be 

 sufficient to overturn the most plausible theory, yet there 

 is usually some probability that the fact may be misin- 

 terpreted, or that some supposed law of nature, on which 

 we are relying, may not be true. Almost every problem 

 in science thus takes the form of a balance of probabilities. 

 It is only when difficulty after difficulty has been success- 

 fully explained away, and decisive experimenta crucis 

 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 

 and rejected, 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 oi 

 vortices, are mere child's play compared with those of 

 gravitation and the undulatory theory already described. 

 The vortices are on the whole plausible suppositions ; for 

 the planets and satellites bear at first sight much re- 

 semblance to objects carried round in whirlpools, ar 



