Chapter four shows that pure practical reason, as the source of the moral law, is the most important element in Kant’s ethics. Taking seriously Kant’s claims that reason is a cause, and even a natural cause, reason is identified as an empirical faculty that would be part of the natural ontology provided by a robust empirical psychology. This faculty of reason is also a timeless transcendentally free cause, as Kant stresses in the Third Antinomy, in virtue of its unchanging structure that produces the categorical imperative to systematize free acts, themselves understood as decisions by the empirical power of choice determined within nature by empirical reason. Such a view is transcendentally idealist, since the validity of the moral law is justified as a product of reason, which itself constitutes the very possibility of any moral agent at all, and simultaneously an empirically realist view, since reason and its laws would be valid independent of each and every actual empirical agent. I note that Kant falls short of endorsing this view when he denies that the moral law can be subject to a transcendental deduction and when he limits his claim to the practical point of view.