We have already felt the force of claims about liberty and freedom (terms I will use interchangeably) at various points in our discussion so far. In chapter 4, we saw how Rousseauan and Rawlsian contractualists seek to justify political arrangements by asking whether agents motivated to maintain their autonomy would freely accept them under appropriate conditions. The possible impact of various forms of economic regulation on personal freedom was a persistent theme in chapters 5 and 6, and in the previous chapter we worried about the compatibility of freedom and authority. But these issues have come up occasionally and unsystematically, much as they do in ordinary political debate. Can we move beyond these rather informal claims about freedom and develop more precise and systematic accounts of the various different forms of human liberty and of their political implications?

For good or ill, recent philosophical efforts in this direction have been profoundly shaped by Isaiah Berlin's seminal essay Two Concepts of Liberty, originally his inaugural lecture as Chichele Professor of Social and Political Theory at the University of Oxford. When Berlin delivered it in 1958, the world was divided into two ideologically opposed blocs – the liberal democratic West and the communist East (from which Berlin himself was an émigré). His lecture was an effort to understand how, despite their bitter enmity, both sides of this Cold War division could nonetheless claim to be crusading for liberty.