Towards the conclusion of the First Section of the Groundwork of the Metaphysics of Morals, Kant describes a process whereby a subject can undergo a kind of moral corruption. This process, which he calls a “natural dialectic”, can cause one to undermine one’s own or¬dinary grasp of the demands of morality. Kant also claims that this natural dialectic is the basis of the need for moral philosophy itself, since first-order moral reasoning is insufficient to protect against it. I show that this passage is closely related to another in the First Section, one where Kant warns against the threat of “misology”, or the hatred of reason. I argue that both these passages must be read as engaging with specific claims from Rousseau’s writings. Uncovering the historical context and rhetorical function of Kant’s account of moral self-deception can re-orient the reader to his ambitions for the Groundwork itself.