Liberty is a revised and expanded edition of the book that Isaiah Berlin regarded as his most important-Four Essays on Liberty, a standard text of liberalism, constantly in demand and constantly discussed since it was first published in 1969. Writing in Harper"s, Irving Howe described it as "an exhilarating performance-this, one tells oneself, is what the life of the mind can be." Berlin"s editor Henry Hardy has revised the text, incorporating a fifth essay that Berlin himself had wanted to include. He has also added further pieces that bear on the same topic, so that Berlin"s principal statements on liberty are at last available together in one volume. Finally, in an extended preface and in appendices drawn from Berlin"s unpublished writings, he exhibits some of the biographical sources of Berlin"s lifelong preoccupation with liberalism. These additions help us to grasp the nature of Berlin"s "inner citadel," as he called it-the core of personal conviction from which some of his most influential writing sprung.