This study discusses in terms of historical genesis whether it is reasonable for Bayes to introduce a prior uniform distribution. In this study, we try to analyze the way he dealt with postulates, lemmas, and propositions in Bayes' essay and to understand its characteristics. The results of the study show that Bayes used random variables for two parameters and that the two random variables were converted to each other through cumulative distribution. Furthermore, it is revealed that the introduction of prior uniform distribution can be justified by this way.