"A good will is good not because of what it performs or effects, not by its aptness for the attainment of some proposed end, but simply by virtue of the volition. . ."
The good will chooses or wills to act in accordance with obligation according to its own nature, not in accordance with what it receives; the neutral will acts in accordance with whatever is beneficial, sometimes doing right sometimes wrong; the bad will acts against obligations. So how does the good will know to ignore benefit and act on its own nature? It is guided by reason.
P2. If happiness were Nature's objective it would have made the will instinctively guided by the desire for happiness;
P3. Nature has, in fact, equipped us with the practical (i.e., will ordering) factual of reason which does not always choose happiness.
C. It must be the case that Nature chose Reason to guide the will for some purpose other than securing happiness.
We can designate an action a 'duty' only if it is an action performed because we understand (via reason) we ought to do it (i.e., we understand that it is a law). Morality thus consists in recognizing and obeying laws, which is only possible for a rational creature.