Famously, in the Canon of Pure Reason, Kant present a practical argument to the effect that our beliefs in God and immortality are justified. I argue that reading the Critique of Pure Reason as the doctrine of method of metaphysics provides a straightforward explanation of the systematic place of Kant’s practical argument in the Canon. Customarily, the relevance of the Critique for the practical part of metaphysics is restricted to proving the theoretical undecidability of God and immortality, but the Canon clearly goes beyond establishing that. According to its positive task as the doctrine of method of metaphysics, the Critique must establish that metaphysics can achieve ‘architectonic unity’. I argue that showing that this unity is possible for metaphysics requires showing that there is conceptual space for coherently accommodating our commitment towards the existence of God and immortality within it.