Throughout the critical period Kant enigmatically insists that reason is a ‘unity’, thereby suggesting that both our theoretical and practical endeavors are grounded in one and the same rational capacity. How Kant's unity thesis ought to be interpreted and whether it can be substantiated remain sources of controversy in the literature. According to the strong reading of this claim, reason is a ‘unity’ because all our reasoning, including our theoretical reasoning, functions practically. Although several prominent commentators endorse this view, it (...) is widely thought to lack exegetical support. This paper seeks to strengthen the case for this reading by showing how theoretical reason's positive function, as Kant presents it in the Appendix to the Transcendental Dialectic, may be construed as fundamentally practical. I argue that reason's supreme regulative principle ought to be understood as a categorical practical imperative. This interpretation, I suggest, resolves the apparent inconsistencies that blight Kant's account of the principle in the Appendix, while bringing greater overall coherence to his account of theoretical reason's regulative function. (shrink)