Critique of Practical Reason
Although all of Kant's work develops his ethical theory, it is most clearly defined in "Groundwork of the Metaphysics of Morals", "Critique of Practical Reason", and "Metaphysics of Morals". As part of the Enlightenment tradition, Kant based his ethical theory on the belief that reason should be used to determine how people ought to act. He did not attempt to prescribe specific action, but instructed that reason should be used to determine how to behave.