74 INDUCTION. 



north wind on any given day will be one-tenth, even 

 though of the remaining possibilities a west wind 

 should be greatly the most probable. If we know 

 that half the trees in a particular forest are oaks, 

 though we may be quite ignorant how many other 

 kinds of trees it contains, the chance that a tree indis- 

 criminately selected will be an oak is an even chance, 

 or, in mathematical language, one-half. So that the 

 condition which Laplace omitted is not merely one of 

 the requisites for the possibility of a calculation of 

 chances ; it is the only requisite. 



In saying that he has omitted this condition, I am 

 far from meaning to assert, that he does not frequently 

 take it into consideration in particular instances ; nor 

 indeed could he fail to do so, since whenever any 

 experience bearing upon the case really exists, he 

 would naturally consult that experience to assure 

 himself of the fulfilment cf his second condition, that 

 there be no reason for expecting one event rather than 

 another. When experience is to be had, he takes 

 that experience as the measure of the probability : his 

 error is only in imagining that there can be a measure- 

 ment of probability where there is no experience. 

 The consequence of this error has been his adoption of 

 conclusions not indeed contrary to, but unsupported 

 by, experience. He has been led to push the theory 

 and its applications beyond the bounds which confine 

 all legitimate inferences of the human mind; by ex- 

 tending them to subjects on which the absence of any 

 ground for determining between two suppositions, does 

 not arise from our having equal grounds for presuming 

 both, but from our having an equal absence of 

 grounds for presuming either. 



According to his views, indeed, the calculation of 

 chances should be much more universally applicable 



