Kant’s critique of perfectionism in the Groundwork of the Metaphysics of Morals launches lively debate on the limits of coercion and the requisites for free action, foundational for post-Kantian perfectionism. The Critique of Practical Reason reformulates the Leibnizian concept of spontaneity as a ‘true apology for Leibniz’, salvaging what is most vital in his thought. Spontaneous freedom does not externalise a unique content, as in Leibniz, but now conceived as negative liberty, signifies the will’s ability to abstract from external causes or to admit them selectively according to rational criteria. Spontaneity is the condition for an order of right, as the sphere of compatible external actions among juridical subjects. Here Kant effects a second modification of Leibniz, in the idea of mutual causality or reciprocity. The Metaphysics of Morals of 1797 elaborates the distinction between pure and empirical practical reason, freedom and happiness, and delineates the sphere of rightful interaction. Neither happiness nor virtue are subject to constraint, but in the sphere of right coercion or mutual limitation is the condition that assures and generalises freedom.