Many people, quite a few philosophers among them, are described as radicals. But what does it mean to be radical? What have radicals traditionally been radical about? In what form might radicalism be defended today? These are the three basic questions that the author attempts to answer in this book. Thus, the book constitutes an attempt to reach a definition of radicalism, outline the history of Western radical political thought in the modern period, and defend radicalism in a historically progressive and politically humanistic form.