 While working on his biography of Marx in the mid 1930s, Berlin came across the works of two Russian thinkers who would be important influences on his political and historical outlook. One of these was Alexander Herzen, who became a hero, and to whom Berlin would sometimes attribute many of his own beliefs about history, politics and ethics. The other was the Russian Marxist publicist and historian of philosophy G. V. Plekhanov. Despite his opposition to Marxism, Berlin admired and praised Plekhanov both as a man and as a historian of ideas. It was initially by reading Plekhanov’s writings that Berlin became interested in the naturalistic, empiricist and materialist thinkers of the Enlightenment, as well as their Idealist and historicist critics. Both Herzen and Plekhanov fuelled Berlin’s absorption in the political debates of nineteenth- and early twentieth-century Russian liberals and radicals of various stripes, which in turn informed his concern with both the philosophy of history and the ethics of political action.