Now the _Dissertation_ is quite vague as to how the “mind” (_animus_), active in accordance with laws generative of the intuitions space and time, differs from “understanding” (_intellectus_), active in accordance with laws generative of pure concepts. Kant’s reasons, apart from the intuitive character of space and time, for contrasting the former with the latter, as the sensuous with the intellectual, were the existence of the antinomies and his belief that through pure concepts the absolutely real can be known. When, however, that belief was questioned by him, and he had come to regard the categories as no less subjective than the intuitional forms, the antinomies ceased to afford any ground for thus distinguishing between them. The intuitional nature of space and time, while certainly peculiar to them, is in itself no proof that they belong to the sensuous side of the mind.[695] A difficulty which immediately faced Kant, from the new Critical standpoint, was that of distinguishing between space and time, on the one hand, and the categories on the other. This is borne out by the _Reflexionen_ and by the following passage in the _Prolegomena_.[696] “Only after long reflection, expended in the investigation of the pure non-empirical elements of human knowledge, did I at last succeed in distinguishing and separating with certainty the pure elementary concepts of sensibility (space and time) from those of the understanding.” The first stage in the development of the metaphysical deduction would seem to have consisted in the attempt to view the categories as acquired by reflection upon the activities of the understanding in “comparing, combining, or separating”;[697] and among the _notiones rationales_, _notiones intellectus puri_, thus gained, the idea of space is specially noted. The following list is also given: “The concepts of existence (reality), possibility, necessity, ground, unity and plurality, parts, all, none, composite and simple, space, time, change, motion, substance and accident, power and action, and everything that belongs to ontology proper.”[698] In _Reflexionen_, ii. 507 and 509, the fundamental feature of such rational concepts is found in their _relational_ character. They all agree in being concepts of form.[699] Quite early, however, Kant seems to have developed the view, which has created so many more difficulties than it resolves, that space and time are given to consciousness through outer and inner sense. Though still frequently spoken of as concepts, they are definitely referred to the receptive, non-spontaneous, side of the mind. This is at once a return to the _Dissertation_ standpoint, and a decided modification of its teaching. It holds to the point of view of the _Dissertation_ in so far as it regards them as sensuous, and departs from it in tracing them to receptivity.[700]