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determinate ratio between the chances for its happening . and 

 failing, though he cannot affign that ratio ; for if there were no 

 finite ratio, either the numbey of chances for its happening muft 

 be infinitely greater, or infinitely lefs than the chances for its 

 failing; in the former cafe, the event- would appear certain, in the- 

 latter impoffible, therefore probable in neither. 



It may perhaps be objedcd, that if we cannot determine the 

 a£tual number of chances, all confideratioa of the manner of 

 ©xpreffing mathematically, the probability of events is nugatory. 

 But it is by no means f o ; becaule though we cannot determine 

 the exa£l degree of credit, which we ought to give to each witnefs, 

 yet we can determine according to what law our belief ought to 

 vary in the cafe of concurring witneffes, each of equal credibility. 

 Things that are quite unknown, fays Hartley, have often fixed 

 relations to one another, and. fometimes relations to things known ;• 

 and as, in Algebra, it is impoffible to exprefs the relation of the 

 unknown, quantity to other q-uantities known- or unknown, 'till 

 it has a fymbol affigned to it, of the fame kind with thofe that 

 detiote the others ; fo in-philofophy, we muft give names to un- 

 known quantities, (qualities, caufcs, &c not in order to reft in 

 .theiii, as the Ariftotelians did, but to have a fixed expfeffion, 

 • under wh^ph to Ireafure up all that can be known of the unknown 

 caufe, &c. in the imagination, and memory^ or in writing, for 

 futurff enquirers. Voltl. p. 348. 



■ . We 



