Aileen Kelly, Emerita Reader at King's College, Cambridge, gave the 2018 annual Isaiah Berlin Lecture at Wolfson College, Oxford. The lecture, which was given on November 8th, was introduced by Sir Tim Hitchens. Although for most of his professional lifetime Isaiah Berlin was commonly classified not under his original label as a philosopher but as a historian of ideas, he is now regarded internationally as a philosopher of continuing importance because of his distinctive contributions to our understanding of the philosophical problems associated with liberty and pluralism. The first aim of the lecture is to show how both points of view can be correct at the same time: without the historical understanding he obtained from his study of thinkers in several countries and centuries and how their orientations depended on period and historical context, he would not have had such a substantial base for the philosophical position that he reached. It will then be argued in detail that the most significant of the various influences on his thought came from a direction - Russia in the nineteenth century - that there has been a regrettable recent tendency to ignore, and that the most characteristic representative of that influence on both his pluralism and his attitude to liberty was the publicist, journalist, publisher, author and thinker Alexander Herzen.