Many of us consider conscience one of the most important – if not the fundamental – quality that makes us human, distinguishing us from animals, on the one hand, and machines on the other. But what is conscience, exactly? Is it a product of our biological roots, as Darwin thought, or is it a purely social invention? If the latter, how did it come into the world?

In this biography of that most elusive human element, Martin van Creveld explores conscience throughout history, ranging across numerous subjects, from human rights to health to the environment. Along the way he considers the evolution of conscience in its myriad, occasionally strange and ever-surprising permutations. The Old Testament – erroneously, it turns out – is normally seen as the fountainhead from which the Western idea of conscience has sprung, while Antigone was the first person on record to explicitly speak of conscience. The story of conscience involves the philosophers Zeno, Cicero and Seneca; Christian thinkers such as Paul, Augustine, Aquinas and, above all, Martin Luther; and modern intellectual giants such as Machiavelli, Rousseau, Kant, Hegel, Nietzsche and Freud. Individual chapters are devoted to Japan, China and even the Nazis, as well as the most recent discoveries in robotics and neuroscience and how they have contributed to the ways we think about our own morality. Ultimately, van Creveld shows that conscience remains as elusive as ever, a continuously mysterious voice that guides how we think about right and wrong.