Many people think of conscience as a mere feeling or inclination. But conscience is not itself an inclination but rather an arbiter of inclinations, what Adam Smith once called the “impartial spectator” that gives us our highest sense of ourselves. When there is a strong inclination like the instinct for survival and a weak inclination like the instinct to help a stranger, conscience typically intervenes on the side of the weaker instinct. Except in pathological people, its voice is clear and incontrovertible. It uses no compulsion in urging us to follow its edicts, yet it exercises both a critical and even a kind of judicial authority: you are obliged to do this, no matter how you feel about it.

There is no other voice in our experience that speaks to us in this way. Conscience is our perennial guide and personal moral tutor. It seemingly requires nothing besides itself, and it provides individualized instruction tailored to each specific situation. Conscience is unconcerned with convenience or reputation, and it seems to operate most strongly when no one is looking. We can, of course, reject the appeal of conscience, but if we do, we cannot help but pass judgment on ourselves using the very criteria supplied by conscience. Conscience has the ability to impose self-reproach, remorse, and shame, and at the same time to make us feel that such consequences are deserved. It is truly one of the most mysterious and powerful aspects of our humanity.

For Kant, conscience is a kind of noumenal voice that speaks to us directly from within ourselves, giving us a certainty that is unavailable to us from outer or phenomenal experience. This is a philosophical way to describe morality, but Lewis puts it more simply: conscience is nothing other than the voice of God within our souls. It is the bridge that links the creature to the creator. Even the atheist hears this internal clarion call because even the atheist has morality at the core of his being, and while the atheist may have rejected God, God has not rejected him.