In the opening of The Groundwork of the Metaphysics of Morals Kant starts talking about good will to build grounds for, in so far as I understand, the concept of acting from duty. Could someone provide any explanation of how the transition from good will to duty is done? Because it's not clear to me from the text. He just starts talking about duty without having defined it or having said anything about it previously in fact.
I'm not even necessarily asking how it's done in the text; if anyone could provide an explanation as to how good will leads to (?) acting from duty, I'd appreciate that.