An Oxford don, intellectual historian, essayist, and Cold War liberal who wrote comparatively little and influenced a great deal. Berlin's two enduring contributions are his distinction between negative and positive liberty (Negative Liberty, Positive Liberty), set out in Two Concepts of Liberty (1958), and his defense of value pluralism — the view that the genuine human goods (liberty, equality, justice, community, truth, beauty) are plural, sometimes incommensurable, and sometimes tragically in conflict. No master value, no formula, can rank them without loss.
This makes Berlin a distinctive kind of liberal. He is not a utilitarian maximizer (there is no common currency) nor a Kantian systematizer (no single rational principle governs political life). He is instead a cautious pluralist: liberalism matters because it keeps open the space for people to make their own tragic choices among irreducibly plural goods, rather than having those choices made for them by anyone claiming access to the One Truth. His historical essays — on the Counter-Enlightenment, Herder, Vico, Tolstoy, Russian thinkers — are a sustained attempt to give voice to traditions that resist the Enlightenment's confidence in a single rational synthesis.
His most famous essay, The Hedgehog and the Fox (1953), divided thinkers between those who know one big thing (hedgehogs — Plato, Hegel, Marx) and those who know many things (foxes — Aristotle, Montaigne, John Stuart Mill). Berlin, unmistakably, was a fox.