‘What is justice?’ asked Socrates in Plato's Republic. When Thrasymachus defined justice as a trick played by the strong upon the weak, Socrates embarked upon the demanding task of showing that he was mistaken. Over the centuries philosophers have continued to debate the fundamental questions of politics. What is the best form of government? Is obedience to the state grounded in consent, nature, tradition, utility, or God's will? Is there a right of resistance against tyrants? Is law the command of the ruler, or the custom of the community? Is private property legitimate? The books which offer compelling answers make up the canon of texts in the history of political thought. They include those of Plato and Aristotle in the ancient world, Augustine and Thomas Aquinas in the Christian middle ages, Machiavelli and Thomas More in the Renaissance, Thomas Hobbes and John Locke in revolutionary England, Rousseau and Adam Smith during the Enlightenment, and Hegel, Marx and John Stuart Mill in the nineteenth century.

The study of these philosophers is well established in universities across the world. Yet the subject often has an uneasy relationship with the disciplines within which it falls: history, politics and philosophy. The historian seeks to know when and why a book was written; the philosopher asks whether its arguments are coherent.