1. When investigating the development of theories of time in chronological order, we are exposed to the danger of being completely lost and disoriented, in a mass of the most varied opinions, which are sometimes highly capricious. For this reason we will preface our historical review with a general overview of the literature, which allows us to classify these theories from the logical point of view. But what shall we adopt as a principle for this classification? Evidently, we should use, as the distinctive features of the systems, those characteristics of the concept of time which have played the most important role in the history of this problem. There is no doubt that all of our systems are grouped around two fundamental problems: the reality of time and the origin of the idea of time.