Robert Nozick

For Anarchy, State, and Utopia (1974) Nozick received a National Book Award in the category Philosophy and Religion.[8] There, Nozick argues that only a minimal state limited to the narrow functions of protection against "force, fraud, theft, and administering courts of law"[9] could be justified, as any more extensive state would violate people's rights. For Nozick, a distribution of goods is just if brought about by free exchange among consenting adults from a just starting position, even if large inequalities subsequently emerge from the process.

Nozick challenged the partial conclusion of John Rawls's Second Principle of Justice of his A Theory of Justice, that "social and economic inequalities are to be arranged so that they are to be of greatest benefit to the least-advantaged members of society." Anarchy, State, and Utopia claims a heritage from John Locke's Second Treatise on Government and seeks to ground itself upon a natural law doctrine, but reaches some importantly different conclusions from Locke himself in several ways. Nozick appealed also to the Kantian idea that people should be treated as end in themselves (what he termed 'separatedness of persons'), not merely as a means to an end.

Most controversially, and unlike Locke and Kant, Nozick argued that consistent application of self-ownership and non-aggression principle[10] would allow and regard as valid consensual or non-coercive enslavement contracts between adults. He rejected the notion of inalienable rights advanced by Locke and most contemporary capitalist-oriented libertarian academics, writing in Anarchy, State, and Utopia that the typical notion of a "free system" would allow adults to voluntarily enter into non-coercive slave contracts.[11][12][13][14][15]

In Philosophical Explanations (1981), which received the Phi Beta Kappa Society's Ralph Waldo Emerson Award, Nozick provided novel accounts of knowledge, free will, personal identity, the nature of value, and the meaning of life. He also put forward an epistemological system which attempted to deal with both the Gettier problem and those posed by skepticism. This highly influential argument eschewed justification as a necessary requirement for knowledge.[16]^{: ch. 7}

Nozick's four conditions for S's knowing that P were (S=Subject / P=Proposition):

1. P is true
2. S believes that P
3. If it were the case that (not-P), S would not believe that P
4. If it were the case that P, S would believe that P

Nozick's third and fourth conditions are counterfactuals. He called this the "tracking theory" of knowledge. Nozick believed the counterfactual conditionals bring out an important aspect of our intuitive grasp of knowledge: For any given fact, the believer's method must reliably track the truth despite varying relevant conditions. In this way, Nozick's theory is similar to reliabilism. Due to certain counterexamples that could otherwise be raised against these counterfactual conditions, Nozick specified that:

3. If P weren't the case and S were to use M to arrive at a belief whether or not P, then S wouldn't believe, via M, that P.
4. If P were the case and S were to use M to arrive at a belief whether or not P, then S would believe, via M, that P.
[17]

Where M stands for the method by which S came to arrive at a belief whether or not P.

A major criticism of Nozick's theory of knowledge is his rejection of the principle of deductive closure. This principle states that if S knows X and S knows that X implies Y, then S knows Y. Nozick's truth tracking conditions do not allow for the principle of deductive closure. Nozick believes that the truth tracking conditions are more fundamental to human intuition than the principle of deductive closure.

The Examined Life (1989), pitched to a broader public, explores love, death, faith, reality, and the meaning of life. According to Stephen Metcalf, Nozick expresses serious misgivings about capitalist libertarianism, going so far as to reject much of the foundations of the theory on the grounds that personal freedom can sometimes only be fully actualized via a collectivist politics and that wealth is at times justly redistributed via taxation to protect the freedom of the many from the potential tyranny of an overly selfish and powerful few.[18] Nozick suggests that citizens who are opposed to wealth redistribution which fund programs they object to, should be able to opt out by supporting alternative government approved charities with an added 5% surcharge.[19]

However, Jeff Riggenbach has noted that in an interview conducted in July 2001, he stated that he had never stopped self-identifying as a libertarian. Roderick Long reported that in his last book, Invariances, "[Nozick] identified voluntary cooperation as the 'core principle' of ethics, maintaining that the duty not to interfere with another person's 'domain of choice' is '[a]ll that any society should (coercively) demand'; higher levels of ethics, involving positive benevolence, represent instead a 'personal ideal' that should be left to 'a person's own individual choice and development.' And that certainly sounds like an attempt to embrace libertarianism all over again. My own view is that Nozick's thinking about these matters evolved over time and that what he wrote at any given time was an accurate reflection of what he was thinking at that time."[20] Furthermore, Julian Sanchez reported that "Nozick always thought of himself as a libertarian in a broad sense, right up to his final days, even as his views became somewhat less 'hardcore.'"[21]

The Nature of Rationality (1993) presents a theory of practical reason that attempts to embellish notoriously spartan classical decision theory.

Socratic Puzzles (1997) is a collection of papers that range in topic from Ayn Rand and Austrian economics to animal rights. A thesis claims that "social ties are deeply interconnected with vital parts of Nozick's later philosophy", citing these two works as a development of The Examined Life.[22]

His last production, Invariances (2001), applies insights from physics and biology to questions of objectivity in such areas as the nature of necessity and moral value.

Nozick created the thought experiment of the "utility monster" to show that average utilitarianism could lead to a situation where the needs of the vast majority were sacrificed for one individual. He also wrote a version of what was essentially a previously known thought experiment, the experience machine, in an attempt to show that ethical hedonism was false. Nozick asked us to imagine that "superduper neuropsychologists" have figured out a way to stimulate a person's brain to induce pleasurable experiences.[16]^: 210–11 We would not be able to tell that these experiences were not real. He asks us, if we were given the choice, would we choose a machine-induced experience of a wonderful life over real life? Nozick says no, then asks whether we have reasons not to plug into the machine and concludes that since we desire to be really impressed by things and not just feel something pleasurable, it does not seem to be rational to plug in, ethical hedonism must be false.

Nozick was notable for the exploratory style of his philosophizing and for his methodological ecumenism. Often content to raise tantalizing philosophical possibilities and then leave judgment to the reader, Nozick was also notable for drawing from literature outside of philosophy (e.g., economics, physics, evolutionary biology).[23]

In his 2001 work, Invariances, Nozick introduces his theory of truth, in which he leans towards a deflationary theory of truth, but argues that objectivity arises through being invariant under various transformations. For instance, space-time is a significant objective fact because an interval involving both temporal and spatial separation is invariant, whereas no simpler interval involving only temporal or only spatial separation is invariant under Lorentz transformations. Nozick argues that invariances, and hence objectivity itself, emerged through a theory of evolutionary cosmology across possible worlds.[24]