 Bernoulli ’ s progress over time can be pursued by means of the Meditationes . Three working periods with respect to his " discovery " can be distinguished by aims and times . The first period , which lasts from 1684 to 1685 , is devoted to the study of the problems regarding the games of chance posed by Christiaan Huygens ; during the second period ( 1685 @-@ 1686 ) the investigations are extended to cover processes where the probabilities are not known a priori , but have to be determined a posteriori . Finally , in the last period ( 1687 @-@ 1689 ) , the problem of measuring the probabilities is solved . 
