vi Preface to the First Edition. 



them, if I may so express it, an unreality about the whole 

 treatment of the subject. To many persons the mention of 

 Probability suggests little else than the notion of a set of 

 rules, very ingenious and profound rules no doubt, with which 

 mathematicians amuse themselves by setting and solving 

 puzzles. 



It must be admitted that some ground has been given 

 for such an opinion. The examples commonly selected by 

 writers on the subject, though very well adapted to illustrate 

 its rules, are for the most part of a special and peculiar 

 character, such as those relating to dice and cards. When 

 they have searched for illustrations drawn from the practical 

 business of life, they have very generally, but unfortunately, 

 hit upon just the sort of instances which, as I shall endea 

 vour to show hereafter, are among the very worst that could 

 be chosen for the purpose. It is scarcely possible for any 

 unprejudiced person to read what has been written about the 

 credibility of witnesses by eminent writers, without his ex 

 periencing an invincible distrust of the principles which they 

 adopt. To say that the rules of evidence sometimes given 

 by such writers are broken in practice, would scarcely be 

 correct ; for the rules are of such a kind as generally to defy 

 any attempt to appeal to them in practice. 



This supposed want of harmony between Probability and 

 other branches of Philosophy is perfectly erroneous. It 

 arises from the belief that Probability is a branch of mathe 

 matics trying to intrude itself on to ground which does not 

 altogether belong to it. I shall endeavour to show that .this 

 belief is unfounded. To answer correctly the sort of questions 

 to which the science introduces us does generally demand 

 some knowledge of mathematics, often a great knowledge, 

 but the discussion of the fundamental principles on which 

 the rules are based does not necessarily require any such 



