C. S. Lewis argues strongly for the existence of a universal and objective Moral Law, which he also refers to as the Law of Human Nature or the Rule of Decent Behaviour. A central point is that this Moral Law is distinct from our natural instincts. While we experience various instincts like self-preservation and the herd instinct, the Moral Law acts as a "third thing" that judges between these conflicting impulses, telling us which one we ought to follow. It often sides with the weaker instinct, further proving it's not simply the strongest desire.

Lewis also contends that the Moral Law is not merely a social convention. Although we learn moral rules from our upbringing, this doesn't make them arbitrary inventions, similar to how learning mathematics reveals real truths. He highlights the underlying moral agreement across diverse cultures and our inherent belief in moral progress, which implies a real standard to progress towards. When we judge some moral ideas as better than others, we are implicitly appealing to this objective standard, a "Real Morality."

Furthermore, Lewis explains that differing beliefs about facts can lead to different moral actions without necessarily indicating a fundamental disagreement in moral principles. The example of witch executions illustrates this, where the change in practice stemmed from a change in factual beliefs, not a change in the core moral principle about punishing harm.

In essence, the Moral Law, according to Lewis, is a real, objective standard of right and wrong inherent in human nature. It judges our instincts, transcends cultural variations, underpins our concept of moral improvement, and is distinct from mere social customs. Our understanding and application of this law are also intertwined with our beliefs about the world.

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