62 INDUCTION. 



To be able (it has been said) to pronounce two events equally 

 probable, it is not enough that we should know that one or 

 the other must happen, and should have no grounds for con 

 jecturing which. Experience must have shown that the two 

 events are of equally frequent occurrence. Why, in tossing up 

 a halfpenny, do we reckon it equally probable that we shall 

 throw cross or pile ? Because we know that in any great 

 number of throws, cross and pile are thrown about equally 

 often; and that the more throws we make, the more nearly 

 the equality is perfect. We may know this if we please by 

 actual experiment; or by the daily experience which life affords 

 of events of the same general character ; or deductively, from 

 the effect of mechanical laws on a symmetrical body acted 

 upon by forces varying indefinitely in quantity and direction. 

 We may know it, in short, either by specific experience, or on 

 the evidence of our general knowledge of nature. But, in one 

 way or the other, we must know it, to justify us in calling the 

 two events equally probable ; and if we knew it not, we should 

 proceed as much at haphazard in staking equal sums on the 

 result, as in laying odds. 



This view of the subject was taken in the first edition of 

 the present work : but I have since become convinced, that 

 the theory of chances, as conceived by Laplace and by mathe 

 maticians generally, has not the fundamental fallacy which I 

 had ascribed to it. 



We must remember that the probability of an event is not 

 a quality of the event itself, but a mere name for the degree 

 of ground which we, or some one else, have for expecting it. 

 The probability of an event to one person is a different thing 

 from the probability of the same event to another, or to the 

 same person after he has acquired additional evidence. The 

 probability to me, that an individual of whom I know nothing 

 but his name, will die within the year, is totally altered by my 

 being told, the next minute, that he is in the last stage of a 

 consumption. Yet this makes no difference in the event 

 itself, nor in any of the causes on which it depends. Every 

 event is in itself certain, not probable : if we knew all, we 

 should either know positively that it will happen, or positively 



