ON COMMON NOTIONS OF PROBABILITY. 113 



turn those which precede to account, in examining 

 opinions of various kinds, whether on this subject at 

 large, or on particular cases of its application. 



The doctrine of probabilities seems to some to assume 

 a sort of power of prophesying,, or of predicting the 

 run of events; to others, it appears that unless such a 

 power of prophesying be attained, the theory can be of 

 no use. Both notions are correct in one sense and in- 

 correct in another : there is prophecy, but not of par- 

 ticular events, and derived, not from inspiration, but 

 from observation. The astronomer predicts and all 

 the world knows that his predictions daily come true. 

 His means of prophecy are aided by deduction from 

 certain notions of which, be the cause what it may, we 

 are as certain as of our own existence. From his very 

 distinct (and therefore often called intuitive) perception, 

 that two straight lines cannot inclose a space, and various 

 other axioms of arithmetic and geometry, he is able to 

 make his observations tell him more as to the future 

 motions of our system than his unassisted perceptions 

 of the past could ever have accomplished. He is a 

 dealer in probabilities of a very high order. But be- 

 fore his prediction appears^ it is necessary that he 

 should consider much more doubtful questions of pro- 

 bability. The minute errors of observation, coupled with 

 the various trifling effects which result from yet un- 

 discovered causes, oblige him to have recourse to the 

 principles which we have explained in the preceding 

 chapters. 



Again, there is no prophecy of particular events in 

 the theory of probabilities, of which it is the very es- 

 sence that there should be more or less tendency to 

 falsehood in every one of its assertions. No result is 

 announced, except as having a certain chance in its 

 favour, which implies also a certain chance against it. 



With regard to the second class of assertions, namely, 

 that unless the theory of probabilities enable us to 

 predict, it can be of no use it may be said that, for 

 the purpose contemplated, it is of no use. Theory would 



