The Metaphysics of Morals by Immanuel Kant Chapter 1 Page 31

discern what I have to do in order that my will may be morally good. Inexperienced in the course of the world, incapable of being prepared for all its contingencies, I only ask myself: Canst thou also will that thy maxim should be a universal law? If not, then it must be rejected, and that not because of a disadvantage accruing from it to myself or even to others, but because it cannot enter as a principle into a possible universal legislation, and reason extorts from me immediate respect for such legislation.

I do not indeed as yet discern on what this respect is based (this the philosopher may inquire), but at least I understand this, that it is an estimation of the worth which far outweighs all worth of what is recommended by inclination, and that the necessity of acting from pure respect for the practical law