This a priori does not consist of a set of constant problems uninterruptedly presented to men's curiosity by concrete phenomena as so many enigmas; nor is it made up of a certain state of acquired knowledge laid down in the course of the preceding ages and providing a ground for the more or less irregular, more or less rapid, progress of rationality; it is doubtless not even determined by what is called the mentality or the 'framework of thought' of any given period, if we are to understand by that the historical outline of the speculative interests, beliefs, or broad theoretical options of the time.