Harvard philosopher whose Anarchy, State, and Utopia (1974) is the canonical statement of libertarianism as a rigorous political philosophy. Nozick's book was partly written as an answer to his colleague John Rawls's A Theory of Justice, published three years earlier; the two books have since been read together as the opening salvos of contemporary political philosophy's main debate.
Nozick starts from a broadly Lockean premise: individuals have inviolable natural rights (life, liberty, property) that function as "side constraints" — they may not be sacrificed even for great aggregate gains. From this foundation he derives three striking claims. First, a minimal "night-watchman" state — limited to protection against force, fraud, and breach of contract — can be justified without violating anyone's rights. Second, any more extensive state cannot be so justified, because redistribution violates the rights of those taxed. Third, "liberty upsets patterns": any principle that tries to maintain a particular distribution of holdings (equality, or the difference principle, or anything else) will require constant coercive interference with voluntary exchanges.
His "Wilt Chamberlain" thought experiment illustrates the last point vividly. Start from any distribution you like; let everyone voluntarily pay a star a quarter to watch him play; the resulting distribution is different — but how can it be unjust, if each step was freely chosen?
Nozick later partly distanced himself from these views, and his subsequent books ranged across epistemology, decision theory, and metaphysics. But Anarchy, State, and Utopia remains the libertarian argument serious political philosophy has to answer.