The Critique of Practical Reason is the second of Immanuel Kant's three critiques and it deals with Kant's own moral philosophy and his views on free will. A masterpiece of philosophical writing.
The theoretical use of reason was concerned with objects of the cognitive faculty only, and a critical examination of it with reference to this use applied properly only to the pure faculty of cognition; because this raised the suspicion, which was afterwards confirmed, that it might easily pass beyond its limits, and be lost among unattainable objects, or even contradictory notions. It is quite different with the practical use of reason. In this, reason is concerned with the grounds of determination of the will, which is a faculty either to produce objects corresponding to ideas, or to determine ourselves to the effecting of such objects; that is, to determine our causality. -Immanuel Kant