From 1770 to 1780, he mainly worked on preparing "The Critique of Pure Reason".

             The Critical period lasted from 1781 to 1794. During this period, he wrote "The Critique of Pure Reason" in 1781, and "Foundation for hte metaphysics of Ethics" in 1785. Following the critical works, Kant published "Critique of Practical Reason", "Critique of Judgment", and "Religion within the Limits of Mere Reason". .

             Three main discussions of Kant are Duty, the Formula of the End, and the Kingdom of Ethics. Kant feels that we act morally when we do our duty, however it is important to distinguish between acting according to duty and acting from duty. Acting according to duty is when the duty has been imposed by someone else. This is an example of heteronymous will. An example of this is Adolf Eichmann, a German nazi general of WWII, who formulated the 'final solution'. He said that according to Kant, he acted morally, since he was following orders, as it was his duty to do so. This is wrong because Kant says that we are only acting morally if we act from duty, as dictated by our innate reason. This is an example of autonomous will.

             Along with duty is the difference between the Categorical Imperative and the Hypothetical Imperative. An example of this is two grocers in a town are John and Joe. John wants to keep his trade, so he insists on selling the best goods, giving the best service, being friendly and polite, and offering value for money. Joe does the same, not to keep his trade, but because it is what he should do. According to Kant, even though the actions are the same, John is acting immorally, according to the Hypothetical Imperative, while Joe is acting morally, according to the Categorical Imperative. For Kant, the act is not important. As long as you are acting from duty and the motive is right, the act must be right. However, the Principles of Universalisability puts a twist on this.