  Despite the fragmentary nature of the Opus postumum, Kant makes it clear that it is designed to fill an important gap in his system. Just as the Metaphysical Foundations had attempted to connect the transcendental principles of the Critique of Pure Reason and the principles that explain how matter is possible, the Opus postumum undertakes the task of effecting a transition from the special metaphysics of nature contained in the Metaphysical Foundations to physics itself. However, Kant does not clarify adequately what systematic principles would guide this transition project, nor is it clear whether he takes the project to yield substantive principles or heuristic guidelines. On the one hand, in a note that stems from a period shortly after the publication of the Metaphysical Foundations, Kant suggests that one could “follow the clue given by the categories and bring into play the moving forces of matter according to their quantity, quality, relation, and modality” (21:311), a procedure that could be similar to that of the Metaphysical Foundations. On the other hand, if the Metaphysical Foundations already presupposes an empirical concept (namely matter), the transition to be carried out in the Opus postumum cannot be understood as moving from something non-empirical to something empirical. As Kant struggles with the problems that result from trying to account for now much more specific features of matter, it is unclear that (or how) the categories are supposed to be of help in structuring Kant's argument.  Thus, the precise argumentative structure of the Opus postumum (i.e., its relationship to Kant's other works and its fundamental presuppositions) remains problematic. Equally problematic is the exact nature of Kant's transition project. Given his stated aim to account for specific features of matter—e.g., particular aggregation states, special forces—one would think that such contingent features are a problem for empirical science not philosophy, since Kant sees the latter as pursuing knowledge that is, in various senses, necessary.