The Moral Law Within

Kant's central claim in the Critique of Practical Reason is that pure reason, without any empirical supplement, can determine the will. Moral obligation does not rest on the contingencies of human psychology, cultural tradition, or divine command — it arises solely from the formal structure of rational agency. Because every rational being as such is subject to reason, the moral law binds all rational agents universally and necessarily.

Unlike the theoretical Critique, which sought a deduction of the categories, Kant holds that the moral law cannot be further justified — it is a 'fact of reason,' a datum of our rational self-consciousness. We are simply aware that we stand under its demand. This is not a failure of argument but a reflection of the law's status: it is the supreme principle from which all moral reasoning proceeds, not a conclusion derived from something prior.

Kant allows one emotion a role in morality: reverence (Achtung) for the law. This is not an inclination that motivates action alongside the law — it is the effect the recognition of the law has on a sensible being. When I grasp the unconditional demand of the moral law, my self-love is humbled and a feeling of respect arises. This reverence is itself produced by reason and so has a uniquely moral character.

The locus classicus is the Conclusion of the Critique of Practical Reason, the 'two things' passage, one of the most quoted in all of philosophy.