NO INVESTIGATION INTO THE ORIGINS of German Idealism can ignore the importance of Spinoza. To get an understanding of this, we only have to remember that, as convincing as many contemporaries found Kant's Critique of Pure Reason and Critique of Practical Reason to be, there were certain problems associated with them that begged for a solution. There was, for example, the problem of the Ding an sich, which seemed to be, as Jacobi observed, an essential precondition of Kant's system, but the assumption of which seemed to introduce fatal contradictions into precisely this system. There was the problem of the status of Kant's own philosophy: since Kant strictly limited the possibility of synthetic propositions a priori, it could be argued that his own statements, presumably themselves synthetic a priori, were excluded by his own criteria. There was the problem of how the transcendental principles established in the first critique (space, time, and the twelve categories, and in particular the three analogies of experience) could be used to deduct a priori principles of physics. (Kant tried to give such an account in his Metaphysische Anfangsgründe der Naturwissenschaft, but he himself appears to have grown suspicious of his answer.) And there was the problem that although there were clear connections between the two critiques, they appeared to be haphazard, and as a consequence Kant's system had to be viewed as unfinished insofar as there was no satisfying explanation of the unity of reason in its theoretical and practical modes.