CHAPTER X. 



THE THEORY OF PROBABILITY. 



THE subject upon which we now enter must not be 

 regarded as an isolated and curious branch of speculation. 

 It is the necessary basis of the judgments we make in the 

 prosecution of science, or the decisions we come to in the 

 conduct of ordinary affairs. As Butler truly said, " Pro- 

 bability is the very guide of life." Had the science of 

 numbers been studied for no other purpose, it must have 

 been developed for the calculation of probabilities. All 

 our inferences concerning the future are merely probable, 

 and a due appreciation of the degree of probability depends 

 upon a comprehension of the principles of the subject. I 

 am convinced that it is impossible to expound the methods 

 of induction in a sound manner, without resting them upon 

 the theory of probability. Perfect knowledge alone can 

 give certainty, and in nature perfect knowledge would be 

 infinite knowledge, which is clearly beyond our capacities. 

 We have, therefore, to content ourselves with partial 

 knowledge knowledge mingled with ignorance, producing 

 doubt. 



A great difficulty in this subject consists in acquiring a 

 precise notion of the matter treated. What is it that we 

 number, and measiire, and calculate in the theory of pro- 

 babilities ? Is it belief, or opinion, or doubt, or knowledge, 

 or chance, or necessity, or want of art ? Does probability 

 exist in the things which are probable, or in the mind which 

 regards them as such ? The etymology of the name lends 

 us no assistance : for, curiously enough, probable is ultimately 

 the same word as provable, a good instance of one word 

 becoming differentiated to two opposite meanings. 



