Human reason demands not merely cognition that, but cognition why—it demands to cognize things from their grounds. In both rationalism’s heyday in the seventeenth century and its contemporary renaissance, rationalist philosophers (like Spinoza and Leibniz) purported to satisfy reason’s demand through the principle of sufficient reason and other metaphysical principles. Yet the eighteenth-century German rationalist tradition faces a foundational crisis concerning the very intelligibility of grounds: under what conditions can reason cognize something from its ground? I first argue that (i) this question tears apart Leibniz’s German rationalist successors—including Christian Wolff and Christian Crusius—and (ii) the early Kant’s discernment of inadequacies in their answers helps to awaken him from dogmatic slumber. And against the longstanding trend of taking the Critique of Pure Reason’s account of experience as its cornerstone (by P.F. Strawson and many others), I then propose that its titular project revolves around saving the possibility of rational cognition. On this basis, I offer novel reconstructions of Kant’s radical arguments for idealism and the restriction of rational cognition to the bounds of sense. In short: to be saved, rational cognition must be restrained.