“Freedom is the ratio essendi of the moral law, while the moral law is the ratio cognoscendi of freedom.”Immanuel Kant · Critica rațiunii practice · 1788 · Partea I, Cartea I, §5, nota, AA 5:4
We cannot prove freedom, but the feeling of duty is its only proof.
Kant ties together two things we cannot prove directly. Only a free being can be bound by a moral law, since obligation presupposes that you could have chosen otherwise. Yet freedom is not observable; its only evidence is the felt weight of duty. When you sense you must tell the truth even though a lie would benefit you, that pressure reveals the free agent capable of choosing.
It might look like a vicious circle: freedom explains the law, the law proves freedom. Kant does not see a trap here, but a starting point. He does not begin by proving freedom; he starts from the fact that we feel duty. Everything else follows from there. If we feel obligation, there must be someone capable of being obligated. Think of the moment when you could keep a wallet you found, with no consequences, yet something in you refuses. That something proves nothing from outside, but it is enough for you. Next step: what happens to those who claim they had no choice.
Why it matters The idea helps anyone wondering whether their choices matter: the sense of responsibility you feel before a hard decision is itself proof that you are free, not a pointless burden.