Abstract: In the Groundwork and all three Critiques, Kant expresses the hope of eventually unifying theoretical and practical reason in one system, with a principle common to both. But Kant never tells us what such a principle is, leaving scholars to advance different possibilities. In this talk, I elaborate a new response to this problem. I claim that, in Kant’s post-critical philosophy, the autonomy of ideas of reason, or pure concepts that elude our cognitive grasp, comes to replace the specifically moral conception of autonomy governing the self-legislating will. This notion ultimately becomes Kant’s candidate for a principle common to both theoretical and practical reason. In order to understand this shift, I focus, first, on alterations in the respective epistemic statuses of moral and theoretical ideas of reason, and second, on Kant’s recognition of the role that a new faculty, the power of reflective judgment, plays in orienting the epistemic normativity at work in grasping ideas of reason. The autonomy attributed to reflective judgment in the third Critique, which can operate in both theoretical and practical dimensions of reasoning, is expanded into a broader conception of the autonomy of ideas, thus enabling Kant to finally bridge the theoretical and practical guises of reason.