Philip Dodd and guests discuss the ideas, life and legacy of Sir Isaiah Berlin, widely considered to be the leading liberal philosopher of the 20th century. Most famous for his development of ideas about liberty, Berlin was also admired for his brilliant essays and speeches. Born in Riga in 1909 he witnessed both the February and October revolutions of 1917, before his family moved to Britain following anti-Semitic encounters. Settling in Oxford, Berlin delivered his influential essay Two Concepts of Liberty in 1958. He defined negative liberty as the absence of constraints, which he regarded as the safest and most favourable form of liberty. Positive liberty, however, was for him associated with self-determination, which he saw as potentially leaning towards political totalitarianism. These concepts of liberty are seen to have influenced generations of philosophers and politicians.