Duty! Sublime and mighty name that embraces nothing charming or insinuating but requires submission, and yet does not seek to move the will by threatening anything that would arouse natural aversion or terror in the mind but only holds forth a law that of itself finds entry into the mind and yet gains reluctant reverence (though not always obedience), a law before which all inclinations are dumb, even though they secretly work against it; what origin is there worthy of you, and where is to be found the root of your noble descent which proudly rejects all kinship with the inclinations, descent from which is the indispensable condition of that worth which human beings alone can give themselves?
This quote encapsulates Kant's view of moral motivation. Acting from duty is set apart completely from all other ways of acting, which are taken to spring from mere "inclination." Moral action must spring solely from the motive of duty, not fear of punishment, hope of reward, or any other reason other than pure dutifulness. In Kant's view, non-moral motivation is always driven by self-love. When I am not acting on duty, I am trying to bring about one of my desires, say, a desire to achieve a reward from God, or to avoid a punishment from him. What lies behind my attempt to bring about my desire is an attempt to bring about the satisfaction of my desire, that is, to bring about the pleasure of having satisfied the desire. Only action from dutifulness is not in the end a means of pleasing oneself. This is reflective of how acting from duty has a different origin metaphysically from other ways of acting: acting from duty is action caused from the noumenal realm. That moral actions are not motivated by desire is the negative way of understanding our freedom when we act morally; that they are caused by the noumenal is the positive way of understanding that freedom.