Niels Bohr, who pioneered the quantum theory of the atom, had a broad conception of his obligations as a physicist. They included not only a responsibility for the consequences of his work for the wider society, but also a compulsion to apply the philosophy he deduced from his physics to improving ordinary people's understanding of the moral universe they inhabit. In some of these concerns Bohr resembled Einstein, although Einstein could not accept what he called the 'tranquilizing philosophy' with which Bohr tried to resolve such ancient conundrums as the nature (or possibility) of free will. In this book, John Heilbron draws on sources never before presented in English to cover the life and work of one of the most creative physicists of the 20th century.