192 The Rule of Succession. [CHAP. vm. 



less strong than it was at the time. Why are not rules of 

 oblivion inserted in treatises upon Probability ? If a man is. 

 told how firmly he ought to expect the tide to rise again, 

 because it has already risen ten times, might he not also ask 

 for a rule which should tell him how firm should be his belief 

 of an event which rests upon a ten years recollection? 1 The 

 infractions of a rule of this latter kind could scarcely be more 

 numerous and extensive, as we shall see presently, than those 

 of the former confessedly are. The fact is that the agencies, 

 by which the strength of our conviction is modified, are so 

 indefinitely numerous that they cannot all be assembled into 

 one science ; for purposes of definition therefore the quantity 

 of belief had better be omitted from consideration, or at any 

 rate regarded as a mere appendage, and the science, defined 

 from the other or statistical side of the subject, in which, 

 as has been shown, a tolerably clear boundary-line can be 

 traced. 



3. Induction, however, from its importance does merit 

 a separate discussion ; a single example will show its bearing 

 upon this part of our subject. We are considering the pros 

 pect of a given man, A.B. living another year, and we find 

 that nine out of ten men of his age do survive. In forming 

 an opinion about his surviving, however, we shall find that 

 there are in reality two very distinct causes which aid in 

 determining the strength of our conviction ; distinct, but in 

 practice so intimately connected that we are very apt to 

 overlook one, and attribute the effect entirely to the other. 



(I) There is that which strictly belongs to Probability; 



1 John Craig, in his often named problems as: Quando evanescet pro- 

 work, Theologice Christiana Prin- babilitas cujusvis Historic, cujussub- 

 cipia Mathematica (Lond. 1699) at- jectum est transiens, viva tantum 

 tempted something in this direction voce transmissas, determinare. 

 when he proposed to solve such 



