The Critique of Practical Reason is the second of Immanuel Kant's three critiques, published in 1788. It follows on from Kant's Critique of Pure Reason and deals with his moral philosophy.
The second Critique exercised a decisive influence over the subsequent development of the field of ethics and moral philosophy, beginning with Johann Gottlieb Fichte's Doctrine of Science and becoming, during the 20th century, the principal reference point for deontological moral philosophy.
Kant sketches out here what is to follow. Most of these two chapters focus on comparing the situation of theoretical and of practical reason and therefore discusses how the Critique of Practical Reason compares to the Critique of Pure Reason.
The first Critique, "of Pure Reason", was a criticism of the pretensions of those who use pure theoretical reason, who claim to attain metaphysical truths beyond the ken of applied reasoning. The conclusion was that pure theoretical reason must be restrained, because it produces confused arguments when applied outside of its appropriate sphere. However, the Critique of Practical Reason is not a critique of pure practical reason, but rather a defense of it as being capable of grounding behavior superior to that grounded by desire-based practical reasoning. It is actually a critique, then, of the pretensions of applied practical reason. Pure practical reason must not be restrained, in fact, but cultivated.
Kant informs us that while the first Critique suggested that God, freedom, and immortality are unknowable, the second Critique will mitigate this claim. Freedom is indeed knowable because it is revealed by God. God and immortality are also knowable, but practical reason now requires belief in these postulates of reason. Kant once again invites his dissatisfied critics to actually provide a proof of God's existence and shows that this is impossible because the various arguments (ontological, cosmological and teleological) for God's existence all depend essentially on the idea that existence is a predicate inherent to the concepts to which it is applied.
Kant insists that the Critique can stand alone from the earlier Groundwork of the Metaphysics of Morals, although it addresses some criticisms leveled at that work. This work will proceed at a higher level of abstraction.
While valid criticisms of the Groundwork are to be addressed, Kant dismisses many criticisms that he finds unhelpful. He suggests that many of the defects that reviewers have found in his arguments are in fact only in their brains, which are too lazy to grasp his ethical system as a whole. As to those who accuse him of writing incomprehensible jargon, he challenges them to find more suitable language for his ideas or to prove that they are really meaningless. He reassures the reader that the second Critique will be more accessible than the first.
