In her new book, Patricia Kitcher supports Onora O'Neill's view that the categorical imperative is the highest principle of both practical and theoretical reason. I claim that neither O'Neill's original interpretation nor Kitcher's additional evidence in favour of it are convincing. At its core, this misconception of Kant's position consists in the identification of self-referential critique of reason with the concept of autonomy. It will be shown that the ‘common principle’ (Kant) of both practical and theoretical reason is not the categorical imperative, but the reflective power of judgement, as Kant claims in the Critique of the Power of Judgement.