First, there are doubters who see the Moral Law as merely herd instinct that causes to have a strong desire to act in a certain manner -- like the love a mother has for her child which occurs the moment the child is born.
Other instincts include self protection, finding food, seeking fame and playing to win. These are global, across genders, races, nationalities.
If we see people in trouble, we want to help.
If we hear a baby crying, we want to bring comfort.
If we hear someone screaming for help, we want to save the person.
What happens when two instincts conflict? You hear the plea from a drowning woman and with it comes the choice of placing one's self in danger. The Moral Law tells us which of the two instincts we should act on. The instinct for self-preservation is stronger than helping someone in danger, yet we go to the aid and put ourselves to risk. The “right” action overcomes the instinct.
We also are capable of ignoring the Moral Law. Recent news accounts validate that crowds have actually stood around a woman being beaten without lifting a hand to help her. Everyone in the crowd knew it was the wrong thing to do, but did it anyway.
When such a strong set of instinctual options exist, it is the drumbeat of the Moral Law that tells us what is right or wrong. We may rationalize not doing the right thing, but we still know what the right thing is and we still would feel offended if we were the person in need of help that went unanswered. The instincts are present but they do not replace the reality of what is right or wrong.
There seems to be no instinct to always do the right thing.
A piano has not two kinds of notes on it, the “right” notes and the “wrong” notes. Every single note is right at one time and wrong at another. The Moral Law is not any one instinct or set of instincts. It is something which makes a kind of tune by directing the instincts.