Which C.S. Lewis book argues that moral law points to an objective reality?

Mere Christianity argues that moral law points to an objective reality.

Mere Christianity developed from talks C.S. Lewis gave for the BBC during the Second World War. The broadcasts were later revised and published in book form in stages, with the combined title Mere Christianity becoming standard in 1952.

Lewis begins with what he calls the “law of human nature,” the widespread sense that people ought to behave in certain ways even when they fail to do so. He uses this moral experience as part of an argument for a reality beyond individual preference. The book then discusses Christian belief, morality, and the differences among Christian traditions.

The title does not refer to a new denomination. “Mere Christianity” means the basic beliefs shared by historic Christian traditions. The work is apologetic rather than a conventional autobiography, and its arguments have made it one of Lewis’s most frequently cited religious books.