The stage which Kant had now reached in his philosophical development was one of great significance. The doctrine of knowledge expressed in the _Dissertation_ was the final form which the Wolffian rationalism could assume for him, and, though many of the elements of the _Kritik_ are contained therein, it was not really in advance of the Wolffian theory. The doctrine of space and time as forms of sense-perception, the reference of both space and time and the pure intellectual notions to the laws of the activity of mind itself, the distinction between sense and understanding as one of kind, not of degree, with the correlative distinction between phenomena and noumena,--all of these reappear, though changed and modified, in the _Kritik_. But, despite this resemblance, it seems clear that, so far as the _Dissertation_ is concerned, the way had only been prepared for the true critical inquiry, and that the real import of Hume's sceptical problem had not yet dawned upon Kant. From the manner, however, in which the doctrine of knowledge had been stated in the _Dissertation_, the further inquiry had been rendered inevitable. It had become quite impossible for Kant to remain longer satisfied with the ambiguous position assigned to a fundamental element of his doctrine of knowledge, the so-called pure intellectual notions. Those notions, according to the _Dissertation_, had no function save in relation to things-in-themselves, i.e. to objects which are not directly or immediately brought into relation to our faculty of cognition. They did not serve as the connecting links of formed experience; on the contrary, they were supposed to be absolutely dissevered from all experience which was possible for intelligence like ours. In his previous essays, Kant, while likewise maintaining that such pure, irreducible notions existed, had asserted in general terms that they applied to experience, and that their applicability or justification rested on experience itself, but had not raised the question as to the ground of such justification. Now, from another side, the supreme difficulty was presented--how could such notions have application to any objects whatsoever? For some time the correlative difficulty, how _objects_ of sense-perception were possible, does not seem to have suggested itself to Kant. In the _Dissertation_ sense-perception had been taken as receptivity of representations of objects, and experience as the product of the treatment of such representations by the logical or analytical processes of understanding. Some traces of this confused fashion of regarding sense-perceptions are left even in the _Kritik_, specially perhaps in the _Aesthetik_, and they give rise to much of the ambiguity which unfortunately attaches to the more developed theory of cognition. So soon, however, as the critical question was put, On what rests the reference of representations in us