The durable notion we call conscience has outlived political ideas, epochs, and empires for over 2,000 years. It has been embraced first by Romans, then by early Christians, Reformed Protestants, and Catholics. In the 17th and 18th centuries, it re-crossed the religious–secular divide, as the attention moved from religious perfection to ethical and social betterment. Non-religious and religious alike embrace it with equal passion. It appears in theology, devotion, art, and philosophy. Why has conscience endured so well? Is it because it is a fixed notion or rather because it is malleable, and capable of self-modification and adaptation? The Introduction takes a brief tour of conscience through the ages and nations.