[ 9' ] 



From having obfervecl, that iron has floated ten thoufand times on 

 quitkfilver, there arifes an expe(5tation, that it will likwifc float on 

 it in the next trial ; but this expedlation is not certainty. It does 

 not follow, fays Hartley, that becaufe a thing has happened a thou- 

 fand or ten thoufand times, that it never has failed, nor ever 

 can fail. Vol. II. p. 142. 



The fources therefore of probability are of two fpecies ; the 

 firft comprehends thofe probabilities, which are derived from 

 confidering the number of caufes, which may influence the truth 

 of the propofition : the other is founded folely on experience, 

 from which we conclude, that the future will be like the pafl:; at 

 leaft when we are aflTured, that the fame caufes, which produced 

 the paft, ftill exifl; and are efficient. Thus, firft, let us fuppofe, 

 that 30,000 flips of paper are contained in a wheel, of which 

 io,ooo are black, and 20,000 are white; and that it is required 

 to determine what are the odds, that I fliall at random draw a. 

 white paper. In this cafe, from the nature of things we per- 

 ceive, that the number of chances for drawing a white paper 

 is 20,000, and the whole number of chances is 30,000, therefore 



the probability or drawing a white paper is or — of cer- 

 tainty ; and the probability of drawing a black paper is - of cer- 



talnty. But fuppofe that I am ignorant of the contents of the 

 wheel, and know only in general, that it contains feveral white 



M 2 and 



