The Metaphysics of Morals by Immanuel Kant Chapter 1 Page 22

another's; I can at most, if my own, approve it; if another's, sometimes even love it; i.e., look on it as favourable to my own interest. It is only what is connected with my will as a principle, by no means as an effect- what does not subserve my inclination, but overpowers it, or at least in case of choice excludes it from its calculation- in other words, simply the law of itself, which can be an object of respect, and hence a command.

Now an action done from duty must wholly exclude the influence of inclination and with it every object of the will, so that nothing remains which can determine the will except objectively the law, and subjectively pure respect for this practical law, and consequently the maxim * that I should follow this law even to the thwarting of all my inclinations.