Critique of Practical Reason
Reason tells us that there is a God, the supreme good, who arranges a future life in a moral world. If not, moral laws would be idle fantasies. Our happiness in that intelligible world will exactly depend on how we have made ourselves worthy of being happy. The union of speculative and practical reason occurs when we see God's reason and purpose in nature's unity of design or general system of ends. The speculative extension of reason is severely limited in the transcendental dialectics of the "Critique of Pure Reason", which Kant would later fully explore in the "Critique of Practical Reason".