Second question · Morals

What ought I to do?

For Kant the second question is “merely practical”: a moral question, not one of knowledge.

Kant worked it out in the “Groundwork of the Metaphysics of Morals” (1785) and the “Critique of Practical Reason” (1788). He takes the answer neither from religion nor from experience, but from practical reason itself. An action is permissible only if I can will that its rule, the maxim, should become a law for everyone. That is the categorical imperative: not a catalogue of commandments, but a testing procedure. For Kant an action has moral worth only if it is done from duty.

Behind this door you will also find Kant's essay on enlightenment (1784), which encourages us to use our own reason, and his doctrine of right and the state in the “Metaphysics of Morals” (1797). Both carry the question from the single person into living together.

Sources: Kant, Critique of Pure Reason, Ak. 3:523 (“merely practical”; our translation) — Kant corpus (German) · Groundwork, Ak. 4:421 and 4:424 (“can will”) · Wikipedia: Categorical imperative · Wikipedia: Critique of Practical Reason · Wikipedia: Metaphysics of Morals

Which chapters and devices stand behind which door is a division for learning, not Kant's own: many of his ideas belong to several questions.