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 he never clarifies what this principle is, leaving scholars to advance different possibilities. I advance a new response to this problem: I claim that Kant begins to refer to what he calls the ‘autonomy of ideas of reason’ in his final decade, enabling a new approach to finally bridging the theoretical and the (...) practical. This concept of autonomy, however, is no longer equivalent to the moral autonomy attributed to the self-legislating will, nor does it refer to the spontaneity of theoretical reason. Instead, it corresponds to the autonomy ascribed to reflective judgment in representing both theoretical and practical ideas of reason. Thus, this conception furnishes a principle, the principle of purposiveness, which I propose as the unifying principle Kant had been seeking. (shrink)