Kant’s critical turn aims to restrict the use of pure reason to possible experience. While this avoids the dogmatic enthusiasm of rationalism and the skeptical consequences of empiricism, it imposes a distinction between the appearances we know in experience and the thing in itself lying beyond experience. It thereby inspires post-Kantian idealism to prove reason’s absolute capacity for explanation, a capacity unrestricted by an unknowable thing in itself and unthreatened by mechanistic systems like Spinoza’s. Fichte and Hegel thus defend reason’s absolute freedom as a way of perfecting Kant’s critical turn. The critique of post-Kantian idealism that emerges in the work of Schopenhauer, Kierkegaard, Nietzsche, and Marx raises important questions. What sort of insight into reason can we have? Can reason fully explain its own possibility? Can an account of reason be wholly objective? Can reason overcome all presuppositions? Is absolute knowledge sufficient to change the world? After an introduction to the idealist systems of Fichte and Hegel, we will trace the critique of idealism through these questions and evaluate the positive accounts offered by post-idealist critics.