Bertrand Russell was not only one of the greatest philosophers of the twentieth century; he was also a humanitarian and activist who fought for many moral, social, and political causes. During his lifetime, the general public knew him for his activism and popular works, in which he tackled such diverse topics as sexual ethics, religion, war, and nuclear disarmament.

Besides the great achievements in mathematical logic on which his reputation rests, Russell was a pioneer in moral philosophy, and his work in this area informed and guided his activism. Russell created one of the first...