Living in the 20th century, a period when totalitarianism eclipsed liberalism gradually, Isaiah Berlin, as a liberalist, tried to figure out why western civilization was heading for dangerous situation. In rethinking such a question, Berlin developed his own political philosophy, including three subjects as follows: 1. the criticism of monism, 2. the verification of pluralism, 3. the liberal philosophy which defends passive liberty. By clarifying and explaining these subjects, which are independent with each other on the surface but inextricably intertwined in essence, Berlin not only achieved great academic status but also set the basic tone of liberalism at the present time, infusing much vitality into liberalism.