Inventing the Individual: The Origins of Western Liberalism by Larry Siedentop Allen Lane, 2014, 448 pages, $39.99 Sir Isaiah Berlin made a career out of writing variations on one big theme, in the form of short essays, long essays, and his celebrated public lectures. This is fine: if it is a good theme, and its influence is widely felt, then all the conditions for a successful academic career seem to have been fulfilled. “Two Concepts of Liberty”, the most important of Berlin’s essays, popularised the distinction between “positive” and “negative” liberty, which was so dear to political thought in the…