We have had enough of people telling us our conscience, the inner voice is really just a biological instinct like mother-love or winning-instinct, self-preservation etc. They claim there’s no real moral law of nature or Moral law given by God. we aren’t expected to behave in any way: We’re just how we are. If we weren’t like this , we would all die.
It would mean I help my friend or keep promises not because my conscience obligates me but because it is an instinct. Then there will be people who will say that when they fail to, it is because the instinct is not strong in them but strong in others. And if we are unfair to others, we could say, ‘well, that is what I feel from within my conscience to do, we all want to prosper so bad, its a human instinct, sorry about it! We’re hardwired this way, so take caution’.
Peter Kreeft adopting ideas from C.S Lewis explains that
the problem with that explanation is that we believe we ought to disobey an instinct—any instinct—on some occasions. But we do not believe we ought ever to disobey our conscience(that’s why the two are different). You should usually obey instincts like mother love, but not if it means keeping your son back from risking his life to save his country in a just and necessary defensive war, or if it means injustice and lack of charity to other mothers’ sons. There is no instinct that should always be obeyed. The instincts are like the keys on a piano; the moral law is like sheet music. Different notes are right at different times.
Furthermore, instinct fails to account not only for what we ought to do but also for what we do do. We don’t always follow instinct. Sometimes we follow the weaker instinct, as when we go to the aid of a victim even though we fear for our own safety. The instinct to save another who is drowning is weaker than the instinct for self-preservation, but our conscience, like sheet music, tells us to play the weak note here rather than the strong one.
What helped you decide between the two instincts cannot be another instinct. What made you decide between two keys of a piano cannot be another key. This something that helps us decide which instinct to kick in, is the conscience.
Quite simply, conscience tells you that you ought to do or not do something, while instincts simply drive you to do or not do something. Instincts make something attractive or repulsive to your appetites, but conscience makes something obligatory to your choice, no matter how your appetites feel about it.