290 The Relativity of Probability. [CHAP. xil. 



found in the chapter on Fallacies, and in that on the Credi 

 bility of Extraordinary Stories. 



13. In connection with the subject at present under 

 discussion we will now take notice of a distinction which we 

 shall often find insisted on in works on Probability, but to 

 which apparently needless importance has been attached. 

 It is frequently said that probability is relative, in the sense 

 that it has a different value to different persons according 

 to their respective information upon the subject in ques 

 tion. For example, two persons, A and B, are going to draw 

 a ball from a bag containing 4 balls : A knows that the 

 balls are black and white, but does not know more; 

 B knows that three are black and one white. It would 

 be said that the probability of a white ball to A is J, and 

 toi. 



When however we regard the subject from the material 

 standing point, there really does not seem to me much more 

 in this than the principle, equally true in every other science, 

 that our inferences will vary according to the data we as 

 sume. We might on logical grounds with almost equal 

 propriety speak of the area of a field or the height of a 

 mountain being relative, and therefore having one value to 

 one person and another to another. The real meaning of the 

 example cited above is this : A supposes that he is choosing 

 white at random out of a series which in the long run would 

 give white and black equally often ; B supposes that he 

 is choosing white out of a series which in the long run would 

 give three black to one white. By the application, there 

 fore, of a precisely similar rule they draw different conclu 

 sions; but so they would under the same circumstances in 

 any other science. If two men are measuring the height of 

 a mountain, and one supposes his base to be 1000 feet, 

 whilst the other takes it to be 1001, they would of course 



