No century in philosophy’s long history produced a sharper break with what came before. The twentieth century opened in the shadow of Nietzsche’s declaration that God was dead, Darwin’s upending of the natural order, and a wave of scientific revolutions—relativity, quantum mechanics, formal logic—that made the universe stranger than any philosopher had imagined. Two World Wars shattered Enlightenment confidence in reason and progress. The Holocaust forced an agonizing reckoning with the capacity for evil in supposedly civilized societies. Decolonization challenged the parochialism of Western thought. The Cold War gave political philosophy a new urgency. And by the century’s end, the rise of digital technology and biotechnology had raised questions about human identity that earlier generations could not have foreseen.
The most conspicuous feature of twentieth-century philosophy is a divide that still shapes the discipline today: the split between analytic and continental traditions. Analytic philosophy, rooted in the work of Gottlob Frege, Bertrand Russell, and G. E. Moore in Britain and Germany, emphasized logical rigor, the analysis of language, and close engagement with the natural sciences. Continental philosophy, descending from Edmund Husserl and Martin Heidegger on the European mainland, prioritized human experience, history, culture, and the critique of Enlightenment rationalism. The divide was partly methodological—logical analysis versus phenomenological description—and partly sociological, reflecting the different institutional settings in which philosophy was practiced. It was never absolute: many thinkers resisted it, and by the century’s end some of the most interesting work crossed the line freely. But it organized professional philosophy in ways that still matter.
This article surveys the century’s major movements, figures, and debates. It begins with the analytic tradition and moves through continental philosophy, logical positivism, ordinary language philosophy, poststructuralism, philosophy of science, philosophy of mind, political philosophy, feminist philosophy, philosophy of race, environmental thought, applied ethics, and several other currents that reshaped what philosophy is and who gets to do it. The scope is broad by design: a twentieth-century introduction that left out any of these threads would misrepresent the century.
Analytic Philosophy
Analytic philosophy began as a revolt. In the early 1900s, Bertrand Russell and G. E. Moore rejected the British Idealism of F. H. Bradley and J. M. E. McTaggart—the dominant school in English-language philosophy at the time—in favor of a philosophy grounded in logical analysis and common-sense realism. Their rebellion was fueled by the revolutionary work of Gottlob Frege, a German mathematician whose Begriffsschrift (1879) had formalized logic in a way that transformed the discipline. Frege’s distinction between sense (Sinn) and reference (Bedeutung)—between what an expression means and what it picks out in the world—became a foundational problem for analytic philosophy of language. Though neglected in his own lifetime, Frege is now widely regarded as the father of the analytic tradition.
Russell pursued logic with extraordinary ambition. In Principia Mathematica (1910–13), co-authored with Alfred North Whitehead, he attempted to derive all of mathematics from pure logic—a project known as logicism. Along the way he discovered Russell’s Paradox (does the set of all sets that don’t contain themselves contain itself?), which triggered a crisis in the foundations of mathematics and led him to develop the theory of logical types as a solution. In The Problems of Philosophy (1912), Russell developed his theory of descriptions, showing that sentences like “The present king of France is bald” could be analyzed logically without assuming that a king of France exists. This technique—dissolving philosophical puzzles through careful logical analysis—became the signature method of the analytic tradition. Russell also had an extraordinary public life: he was a Nobel laureate in literature, a vocal pacifist, and a campaigner against nuclear weapons.
Moore made his mark in ethics and epistemology. His Principia Ethica (1903) argued that “good” is a simple, indefinable property—any attempt to define it in terms of natural properties like pleasure commits the naturalistic fallacy. His open question argument drove the point home: for any proposed definition of “good” (say, “pleasure”), it always makes sense to ask “But is pleasure really good?”—which shows that “good” and “pleasure” don’t mean the same thing. Moore’s later A Defence of Common Sense (1925) insisted that ordinary beliefs (“the earth has existed for many years”) are more certain than any philosophical argument against them.
Ludwig Wittgenstein
No single philosopher better illustrates the century’s restlessness than Ludwig Wittgenstein, who produced two radically different philosophies in one lifetime. His early Tractatus Logico-Philosophicus (1921) argued that language mirrors the logical structure of reality. Meaningful propositions are “pictures” of possible states of affairs; anything that cannot be stated in this way—ethics, aesthetics, the mystical—falls outside the limits of language. The book’s final line became one of philosophy’s most quoted sentences: “Whereof one cannot speak, thereof one must be silent” (Wovon man nicht sprechen kann, darüber muss man schweigen).
Wittgenstein’s later Philosophical Investigations (published posthumously in 1953) dismantled much of the Tractatus. Meaning, he now argued, is not a matter of picturing reality but of use—words get their meaning from how they function in what he called language games, the diverse social practices in which language is embedded. He attacked the idea of a private language (a language only its speaker could understand), argued that concepts hang together by family resemblance rather than shared essences, and reconceived philosophy itself as a kind of therapy: the treatment of intellectual confusions that arise when language “goes on holiday.” The shift from the Tractatus to the Investigations is one of the most dramatic reversals in the history of philosophy.
Later Analytic Philosophy
The analytic tradition evolved dramatically in the second half of the century. W. V. O. Quine dismantled one of its founding assumptions in “Two Dogmas of Empiricism” (1951), arguing that the distinction between analytic truths (true by definition) and synthetic truths (true by experience) cannot be maintained. Knowledge, Quine argued, faces experience as a whole—not statement by statement—and even logic and mathematics are in principle revisable. His naturalized epistemology proposed that epistemology should be continuous with empirical psychology rather than a foundational discipline standing above science.
Saul Kripke’s Naming and Necessity (1980) overturned the dominant theory of how names work. Names, Kripke argued, are rigid designators—they refer to the same individual in every possible world, not through associated descriptions but through a causal chain stretching back to an original act of naming. He also showed that the categories a priori/a posteriori and necessary/contingent cut across each other in ways philosophers had missed: some truths are necessarily true yet only knowable through experience (such as “water is H₂O”). David Lewis took the notion of possible worlds in a startling direction, arguing for modal realism: possible worlds are not merely useful fictions but concrete realities as real as our own.
Other major figures expanded the tradition’s range further. Donald Davidson developed a theory of radical interpretation grounded in a principle of charity—that we should interpret others as mostly rational and mostly right. His anomalous monism held that mental events are physical events, but there are no strict laws connecting the mental and the physical. Elizabeth Anscombe’s Intention (1957) founded the philosophy of action, and her essay “Modern Moral Philosophy” helped launch the virtue ethics revival. Hilary Putnam’s Twin Earth thought experiment argued that “meanings ain’t in the head”—what our words mean depends partly on the external environment, not just our internal mental states—establishing semantic externalism as a major position in philosophy of language and mind.
Continental Philosophy
Continental philosophy is less a unified doctrine than a family of approaches united by shared concerns: the nature of human experience, the role of history and culture in shaping thought, the limits of scientific rationality, and the structures of meaning that make human life intelligible. Its roots lie in Kant, Hegel, and Nietzsche, but its twentieth-century form begins with phenomenology.
Phenomenology: Husserl and Heidegger
Edmund Husserl founded phenomenology as a rigorous science of consciousness. His method involved the epoché (εποχή)—a deliberate suspension of all assumptions about whether the external world exists—in order to examine the structures of experience itself. Every conscious act, Husserl insisted, is intentional: it is always about something. His Logical Investigations (1900–01) explored the relationship between acts of consciousness and their objects; Ideas (1913) introduced the phenomenological reduction as a systematic method. His later Crisis of European Sciences (1936) introduced the concept of the Lebenswelt (life-world)—the pre-theoretical world of everyday experience that science presupposes but cannot explain. Husserl also grappled with the problem of intersubjectivity: how can we know other minds if phenomenology begins from the first-person perspective?
Martin Heidegger, Husserl’s most brilliant and controversial student, transformed phenomenology into something its founder barely recognized. In Being and Time (Sein und Zeit, 1927), Heidegger argued that Western philosophy had forgotten the most fundamental question of all: the question of Being (Sein). To approach it, he analyzed human existence—what he called Dasein (literally “being-there”)—as always already embedded in a world of practical concern. We don’t encounter objects as neutral data; we encounter them as tools, obstacles, and possibilities within the projects that define our lives. Heidegger’s concepts of thrownness (Geworfenheit)—the fact that we find ourselves already in a situation we did not choose—care (Sorge), and being-toward-death became central to existentialist and continental thought. Authentic existence, for Heidegger, requires confronting one’s own mortality rather than fleeing into the comforting anonymity of what “they” (das Man) say and do.
Heidegger’s later work took a different direction, a shift he called the turn (Kehre). He moved from analyzing Dasein to meditating on Being itself, on language as “the house of Being,” and on the dangers of modern technology, which he described as Enframing (Gestell)—a way of revealing the world that reduces everything to a standing reserve to be optimized. Heidegger’s philosophical legacy is inseparable from the stain of his involvement with National Socialism in the 1930s, a fact that continues to provoke fierce debate about whether his philosophy is contaminated by his politics or can be separated from them.
Maurice Merleau-Ponty developed a phenomenology centered on the body. His Phenomenology of Perception (1945) argued that perception is not a mental act performed by a disembodied mind but an achievement of the lived body (le corps propre)—a body that is simultaneously physical and experiential, subject and object. Merleau-Ponty’s work dismantled the Cartesian mind-body split from within the phenomenological tradition and influenced cognitive science, psychology, and theories of embodied cognition.
Existentialism: Sartre, de Beauvoir, and Camus
Existentialism became the most publicly visible philosophical movement of the century, thanks largely to Jean-Paul Sartre. In Being and Nothingness (L’Être et le néant, 1943), Sartre drew on Husserl and Heidegger to argue that human consciousness is radically free. Unlike a stone or a table, which simply is what it is (being-in-itself, en-soi), consciousness is always projecting beyond itself, making choices, defining itself through action (being-for-itself, pour-soi). There is no fixed human nature; “existence precedes essence.” To deny this freedom—to pretend we are determined by our roles, our upbringing, or our circumstances—is what Sartre called bad faith (mauvaise foi). Sartre also analyzed the inescapable tension of being-for-others (pour-autrui): the way other people’s gazes threaten to reduce us to objects, fixing our identity from outside. His later Critique of Dialectical Reason (1960) attempted to synthesize existentialism with Marxism, arguing that individual freedom must be understood within material and historical conditions.
Simone de Beauvoir was Sartre’s intellectual partner and, in important respects, a more original thinker. Her The Second Sex (Le Deuxième Sexe, 1949) is the founding text of modern feminist philosophy. Its most famous line—“One is not born, but rather becomes, a woman”—argued that femininity is not a biological destiny but a social construction, the product of a culture that defines women as the Other to a male-defined norm. De Beauvoir applied existentialist categories—freedom, situation, authenticity, bad faith—to women’s oppression with devastating analytical precision. Her The Ethics of Ambiguity (1947) developed an existentialist ethics grounded in the idea that genuine freedom requires the freedom of others. De Beauvoir’s influence on second-wave feminism and gender studies was incalculable.
Albert Camus shared existentialism’s preoccupations but rejected the label. His philosophical starting point was the absurd—the collision between the human desire for meaning and the universe’s indifference to that desire. In The Myth of Sisyphus (Le Mythe de Sisyphe, 1942), Camus argued that the proper response to absurdity is not suicide but revolt: an ongoing refusal to accept meaninglessness, paired with a passionate engagement with life. His novels The Stranger and The Plague explored these themes in fiction. A bitter public break with Sartre over the question of revolutionary violence—Camus opposed it; Sartre defended it in certain contexts—became one of the century’s defining intellectual confrontations. Karl Jaspers, writing in a more academic mode, developed an existentialist philosophy centered on limit situations (Grenzsituationen)—death, guilt, suffering, and struggle—as the moments where human existence confronts its own boundaries and the possibility of transcendence.
Critical Theory and the Frankfurt School
The Frankfurt School brought Marxism into conversation with psychoanalysis, sociology, and aesthetics. Max Horkheimer and Theodor Adorno’s Dialectic of Enlightenment (first circulated 1944; published 1947) argued that the Enlightenment’s project of rational mastery over nature had turned against itself, producing new forms of domination: the culture industry that manufactured consent, and an instrumental reason that reduced everything—nature, people, art—to means for efficient control. Adorno’s Negative Dialectics (1966) pushed further, developing a philosophy that resisted the reduction of the particular to the general—what he called identity thinking. In aesthetics, Adorno argued that modernist art’s difficulty and resistance to easy consumption was its ethical strength: art was the last refuge of non-identity.
Herbert Marcuse’s One-Dimensional Man (1964) became a touchstone for the New Left, arguing that advanced industrial society had absorbed all genuine opposition through consumerism and what he called repressive desublimation—the system’s ability to satisfy desires in ways that neutralize their critical potential. His Eros and Civilization (1955) synthesized Freud and Marx, arguing that a non-repressive civilization was possible. Walter Benjamin brought a literary and messianic sensibility to Critical Theory; his essay “The Work of Art in the Age of Mechanical Reproduction” (1935) explored how mass reproduction strips art of its aura—its unique presence in time and space—transforming both art and politics. His unfinished Arcades Project remains one of the century’s most extraordinary intellectual artifacts.
Jürgen Habermas, the Frankfurt School’s most influential heir, moved in a more constructive direction. His Theory of Communicative Action (1981) distinguished between communicative rationality—the kind of reason aimed at mutual understanding—and the instrumental rationality his predecessors had diagnosed. Where Horkheimer and Adorno saw Enlightenment reason as inherently dominating, Habermas argued that reason also contains emancipatory potential, realized through open, undistorted communication. His discourse ethics and theory of deliberative democracy attempted to ground democratic politics in the norms implicit in rational conversation itself.
Hermeneutics and the Ethics of the Other
Hans-Georg Gadamer’s Truth and Method (Wahrheit und Methode, 1960) developed hermeneutics—the theory of interpretation—into a comprehensive philosophical position. Understanding, Gadamer argued, is not a method we apply but a condition of our existence. We always interpret from within a tradition, carrying prejudices (Vorurteile) that are not obstacles to understanding but its preconditions. Genuine understanding occurs in a fusion of horizons (Horizontverschmelzung)—a meeting between our own perspective and that of the text or person we seek to understand. Gadamer’s rehabilitation of tradition as a source of insight rather than error provoked a famous debate with Habermas, who insisted that tradition could also be a vehicle for domination and that critical reflection must be able to stand outside it.
Paul Ricoeur extended hermeneutics into narrative theory, arguing in Time and Narrative (1983–85) that human identity is constituted by the stories we tell about ourselves. His Oneself as Another (1992) developed a hermeneutics of the self that wove together personal identity, ethics, and the recognition of others. Emmanuel Levinas pushed continental philosophy in an explicitly ethical direction. In Totality and Infinity (1961), he argued that Western philosophy had systematically reduced the Other to the Same—assimilating everything foreign into its own categories. For Levinas, ethics begins not with abstract principles but with the encounter with another person’s face, which makes an infinite demand that precedes all philosophical theorizing.
Structuralism and Poststructuralism
Structuralism emerged from the linguistics of Ferdinand de Saussure, who argued in his Course in General Linguistics (published posthumously in 1916) that language is a system of differences with no positive terms: the sign “cat” means what it means not because of any natural connection to felines but because it differs from “bat,” “hat,” and every other sign in the system. Saussure distinguished between langue (the underlying system of language) and parole (individual speech acts). Claude Lévi-Strauss applied this structural approach to anthropology, analyzing myths and kinship structures as systems of binary oppositions. Roland Barthes turned it on culture at large, showing in Mythologies (1957) how consumer culture naturalizes its ideological messages, and later declaring “the death of the author” —the provocative claim that the meaning of a text belongs to the reader, not its creator. Louis Althusser developed a structural Marxism that analyzed how ideology operates through interpellation: the process by which social institutions “call” individuals into being as subjects.
Poststructuralism radicalized structuralism’s insights while questioning its claim to scientific objectivity. Jacques Derrida’s method of deconstruction argued that Western thought is organized around binary oppositions (speech/writing, presence/absence, nature/culture) in which one term is always privileged. Close reading reveals that these hierarchies are unstable—the subordinate term turns out to be the condition of possibility for the privileged one. His concept of différance (différance)—a neologism combining “difference” and “deferral”—pointed to the way meaning is never fully present but always deferred through an endless chain of signs. Derrida’s influence extended far beyond philosophy into literary theory, legal theory, and architecture.
Michel Foucault rejected the label “poststructuralist” but transformed how philosophy thinks about power and knowledge. Through detailed historical studies of madness, medicine, the prison, and sexuality, Foucault showed that what counts as knowledge in a given era is inseparable from the power relations that produce it. Discipline and Punish (1975) analyzed how modern institutions—prisons, schools, hospitals—produce docile bodies through surveillance, using Jeremy Bentham’s Panopticon as its central image. His concept of biopower—the modern state’s regulation of populations through statistics, medicine, and public health—became indispensable to political philosophy, queer theory, and critical race studies.
Gilles Deleuze, often writing with Félix Guattari, developed a philosophy of difference, immanence, and becoming that rejected the Western metaphysical tradition’s emphasis on identity and transcendence. Their Anti-Oedipus (1972) challenged psychoanalysis’s grip on desire with the concept of desiring machines and the body without organs; A Thousand Plateaus (1980) introduced the rhizome—a non-hierarchical, horizontally spreading model of thought and organization—as an alternative to the tree-like, hierarchical structures of Western metaphysics. Deleuze’s solo works, including Difference and Repetition (1968) and The Logic of Sense (1969), developed a philosophy of pure difference that owes debts to Nietzsche, Bergson, and Spinoza.
Later poststructuralists extended these ideas into new domains. Judith Butler’s Gender Trouble (1990) argued that gender is not a natural fact but a performance—something constituted through repeated, stylized acts rather than expressing a pre-existing identity. Butler drew on Foucault and Derrida to show that even biological sex is partly a discursive construction. Her work became foundational for queer theory and reshaped feminist philosophy. Jean Baudrillard diagnosed a culture of simulacra—copies without originals—in which the distinction between reality and representation has collapsed into what he called the hyperreal. Julia Kristeva developed a theory of the semiotic—a pre-linguistic, bodily dimension of language associated with rhythm, affect, and the maternal—that disrupts the symbolic order of rational discourse.
Logical Positivism and Its Aftermath
The Vienna Circle—a group of philosophers, mathematicians, and scientists meeting in 1920s Vienna—produced one of the century’s most ambitious philosophical programs. Logical positivism held that a statement is meaningful only if it can be verified through sensory experience or is true by definition (a tautology). Metaphysics, theology, and ethics, on this view, were not false but literally meaningless—they expressed emotions or attitudes, not genuine propositions about the world.
Rudolf Carnap developed the logical syntax and semantics of scientific language with extraordinary technical sophistication. His The Logical Structure of the World (1928) attempted to show how all meaningful concepts could be constructed from a base of sensory experience. Moritz Schlick articulated the movement’s core commitments. A. J. Ayer brought logical positivism to the English-speaking world with Language, Truth and Logic (1936), a dazzlingly confident book that declared most traditional philosophy to be nonsense. Otto Neurath championed the unity of science and contributed a vivid metaphor for epistemology: we are like sailors who must rebuild their ship at sea, plank by plank, with no dry dock to retreat to—there is no Archimedean point outside our web of beliefs.
Logical positivism’s verification principle proved self-undermining: the principle itself is neither empirically verifiable nor a tautology, so by its own standard it is meaningless. Combined with Quine’s attack on the analytic–synthetic distinction and the Duhem–Quine thesis (that individual statements cannot be tested in isolation from the larger theoretical framework they belong to), logical positivism lost its foundations by the 1960s. But its influence was enormous: it set the agenda for philosophy of science, shaped the standards of rigor in analytic philosophy, and forced subsequent thinkers to clarify what they meant by meaning.
Ordinary Language Philosophy
If the Vienna Circle looked to formal logic and science, the ordinary language philosophers at Oxford and Cambridge looked to everyday speech. The movement had a “therapeutic” conception of philosophy: many philosophical problems, its practitioners believed, arise not from deep features of reality but from confusions generated by the misuse of ordinary words. Philosophy’s task is to dissolve these confusions, not to build grand theories.
J. L. Austin’s How to Do Things with Words (1962) showed that language does far more than describe the world. When a judge says “I sentence you to ten years,” or a couple says “I do” at a wedding, they are not reporting facts but performing actions—what Austin called speech acts. He distinguished between what a sentence says (its locutionary force), what it does (its illocutionary force), and what effects it produces (its perlocutionary force). Austin’s Sense and Sensibilia dismantled the sense-datum theory that had dominated British epistemology.
Gilbert Ryle’s The Concept of Mind (1949) attacked Cartesian dualism with a memorable phrase: the mind-body problem rests on a category mistake, treating the mind as if it were a “ghost in the machine.” Mental concepts, Ryle argued, refer not to hidden inner events but to publicly observable dispositions and abilities. P. F. Strawson developed a “descriptive metaphysics” of the basic concepts that structure our experience. John Searle extended Austin’s speech act theory and later formulated the famous Chinese Room argument against strong artificial intelligence: a person following rules for manipulating Chinese symbols can produce correct outputs without understanding a word of Chinese, suggesting that computation alone—syntax without semantics—does not produce genuine understanding.
Philosophy of Science
The twentieth century saw philosophy of science emerge as one of the discipline’s most consequential fields. The central question was deceptively simple: what distinguishes science from non-science, and how does scientific knowledge grow?
Karl Popper offered the most influential answer. In The Logic of Scientific Discovery (Logik der Forschung, 1934), he rejected the logical positivists’ verification principle in favor of falsificationism: what makes a theory scientific is not that it can be confirmed but that it makes predictions that could, in principle, be shown false. A theory that cannot be falsified—one that explains everything no matter what happens—is not science but pseudo-science. Popper took Marxism and Freudian psychoanalysis as cautionary examples. Science advances not by accumulating confirmations but by bold conjectures and rigorous attempts to refute them—by elimination of error rather than accumulation of truth. Popper’s influence extended beyond philosophy of science: his The Open Society and Its Enemies (1945) developed a political philosophy of liberalism grounded in critical rationalism and the rejection of utopian planning.
Thomas Kuhn’s The Structure of Scientific Revolutions (1962) challenged Popper’s picture of orderly, incremental progress. Kuhn argued that science operates within paradigms—shared frameworks of theory, method, and exemplary problem-solutions that define a field. Most scientific work is normal science: puzzle-solving within the paradigm. But anomalies accumulate, crises erupt, and eventually a paradigm shift occurs—a revolution that is not purely rational but involves something like a gestalt switch. Competing paradigms, Kuhn claimed, are incommensurable: they don’t just disagree about answers but about what the questions are. Scientists working in different paradigms “live in different worlds.” Kuhn’s book became one of the most cited academic works of the twentieth century and reshaped how historians, sociologists, and philosophers understood science.
Imre Lakatos tried to mediate between Popper and Kuhn with his methodology of scientific research programmes: a programme has a “hard core” of unfalsifiable assumptions protected by a “protective belt” of auxiliary hypotheses. A programme is progressive when its adjustments predict new facts; it is degenerative when they merely patch up failures. Paul Feyerabend took a more radical line: in Against Method (1975), he argued that science has no single method—its greatest advances violated every proposed methodological rule—and that the slogan “anything goes” is the only methodological principle that doesn’t inhibit progress. Later philosophers of science, including Bas van Fraassen with his constructive empiricism and Nelson Goodman with the “grue” paradox, continued to probe the foundations of scientific reasoning.
Philosophy of Mind
The mind-body problem—how mental experience relates to physical processes—became one of the century’s most active fields, driven by advances in neuroscience, psychology, and computer science.
The century opened with behaviorism: the view, developed by Ryle and others, that mental states are nothing more than dispositions to behave in certain ways. Behaviorism had the virtue of avoiding mysterious inner realms but struggled to account for inner experience—what it’s like to feel pain, for instance, even when one suppresses all outward signs. The mind-brain identity theory (J. J. C. Smart, Herbert Feigl) proposed that mental states are simply identical to brain states, but faced the problem of multiple realizability: pain can presumably be realized in organisms with very different neurologies, so pain cannot be identical to one specific brain state.
Functionalism, championed first by Hilary Putnam and developed by Jerry Fodor, offered a more flexible framework. Mental states are defined not by what they are made of but by their causal roles—their relationships to sensory inputs, behavioral outputs, and other mental states. This opened the door to the computational theory of mind: the idea that thinking is information-processing, and that minds are to brains what software is to hardware. Alan Turing’s 1950 paper “Computing Machinery and Intelligence” had already posed the question of whether machines can think, proposing the Turing Test as a criterion. Fodor’s Language of Thought hypothesis went further, arguing that cognitive processes operate on a mental language (“mentalese”) with a syntax and semantics of its own.
At the opposite extreme, eliminative materialism (Paul and Patricia Churchland) argued that our everyday understanding of the mind—what philosophers call folk psychology—is a fundamentally flawed theory that will eventually be replaced by neuroscience, much as alchemy was replaced by chemistry.
But the deepest puzzles proved resistant. Thomas Nagel’s “What Is It Like to Be a Bat?” (1974) argued that consciousness has an irreducibly subjective character that no third-person account can capture. Frank Jackson’s thought experiment about Mary—a scientist who knows everything physical about color but has never seen it—suggested that physical knowledge leaves something out. David Chalmers formalized these intuitions as the “hard problem” of consciousness: even if we explain all the cognitive functions associated with consciousness, we have not explained why there is something it is like to have them. Daniel Dennett pushed back vigorously in Consciousness Explained (1991), arguing that the “hard problem” is an illusion generated by our folk-psychological assumptions and defending a “multiple drafts” model of consciousness that denies the existence of a single, unified stream of experience.
Political Philosophy
The twentieth century renewed political philosophy with an urgency born of catastrophe. Totalitarianism, world war, decolonization, and the Cold War made questions about justice, freedom, and political authority impossible to ignore.
Hannah Arendt responded to totalitarianism with some of the century’s most penetrating political thought. The Origins of Totalitarianism (1951) analyzed how anti-Semitism, imperialism, and the collapse of the nation-state made totalitarian regimes possible—regimes Arendt understood as genuinely new forms of government, not merely extreme versions of tyranny. The Human Condition (1958) distinguished three fundamental human activities—labor (biological survival), work (creating a durable world), and action (initiating something new in concert with others)—and argued that modern societies dangerously privilege labor at the expense of political action. Her report on the trial of Adolf Eichmann introduced the concept of the “banality of evil”: the idea that radical evil can be perpetrated not by monsters but by ordinary, thoughtless functionaries who simply fail to think.
Isaiah Berlin’s “Two Concepts of Liberty” (1958) drew an influential distinction between negative liberty (freedom from interference) and positive liberty (freedom to govern oneself). Berlin warned that positive liberty, however noble in intention, can become coercive when a state claims to know what people “really” want and forces them to be “free.” His broader commitment to value pluralism—the idea that fundamental human values are irreducibly plural and sometimes incompatible—challenged all monistic political ideologies, whether Marxist, utilitarian, or religious.
John Rawls’s A Theory of Justice (1971) is widely regarded as the most important work in twentieth-century political philosophy. Rawls asked: what principles of justice would rational people choose if they didn’t know their place in society—their wealth, talents, race, or gender? Behind this “veil of ignorance”, he argued, they would choose two principles: equal basic liberties for all (the liberty principle), and the arrangement of social and economic inequalities so that they benefit the least advantaged members of society (the difference principle). Rawls’s framework challenged utilitarianism by insisting that justice cannot be sacrificed for aggregate welfare. His later Political Liberalism (1993) responded to critics by developing the idea of public reason: in a society marked by deep disagreements about the good life, political philosophy should appeal to reasons all citizens can accept.
Robert Nozick’s Anarchy, State, and Utopia (1974) offered the most powerful libertarian response to Rawls. Nozick argued that only a minimal state—one limited to protecting individuals against force, theft, and fraud—is morally legitimate. His entitlement theory held that a distribution is just if it arose through just acquisition and voluntary transfer, regardless of the pattern it produces. His Wilt Chamberlain argument illustrated the point: if people freely pay to watch a basketball star play, the resulting inequality is just, and any redistribution to restore a pattern would violate their rights. The Rawls–Nozick debate framed much of late-twentieth-century political philosophy.
The communitarian critique pushed back from a different angle. Alasdair MacIntyre, Michael Sandel, Charles Taylor, and Michael Walzer argued that Rawlsian liberalism presupposes an implausible picture of the self—what Sandel called the “unencumbered self,” a self that exists prior to its social roles and commitments. Community, tradition, and shared conceptions of the good, they insisted, are not obstacles to justice but its preconditions. Taylor’s Sources of the Self (1989) traced the moral sources of modern identity. Walzer’s Spheres of Justice (1983) argued that distributive justice must be sensitive to the social meanings of goods—no single principle of distribution fits all domains of life.
Feminist Philosophy
Feminist philosophy challenged the discipline’s claim to universality. If philosophy is the pursuit of truths that hold for all rational beings, why had it systematically excluded or marginalized women’s perspectives? The answer, feminist philosophers argued, is that supposedly neutral philosophical positions often reflect the experiences and interests of a specific group: educated, Western men.
De Beauvoir’s The Second Sex (discussed above) launched the tradition. In the 1960s through 1980s, second-wave feminist philosophers developed its insights in new directions. Betty Friedan’s The Feminine Mystique (1963) identified what she called “the problem that has no name”—the dissatisfaction of educated women confined to domestic roles—and helped catalyze liberal feminism’s push for equal rights. But feminist philosophy went deeper than demands for legal equality. Carol Gilligan’s In a Different Voice (1982) challenged the dominant model of moral development by arguing that women tend to reason morally through an ethics of care—emphasizing relationships, responsibility, and responsiveness to particular others—rather than through abstract principles of justice. Her work raised a fundamental question for moral philosophy: had the tradition’s emphasis on impartiality and universal rules systematically devalued forms of moral reasoning associated with women? Nel Noddings and Virginia Held built care ethics into a systematic alternative to Kantian and utilitarian approaches, arguing that caring relations, not individual rights or aggregate utility, are morally fundamental.
Feminist epistemology questioned the ideal of a “view from nowhere.” Sandra Harding developed standpoint epistemology: the claim that knowledge is always produced from a particular social location, and that marginalized perspectives can offer a “strong objectivity” unavailable from the center. Donna Haraway’s “Situated Knowledges” argued for an epistemology that acknowledges its own partiality without collapsing into relativism. Martha Nussbaum developed a capabilities approach to justice, arguing that the proper measure of a society is whether it secures for every person a set of core human capabilities—including bodily integrity, education, political participation, and emotional health.
Philosophy of Race
The philosophy of race addresses one of the most consequential questions in modern thought: what is race, and what role does it play in structuring experience, knowledge, and political life?
W. E. B. Du Bois, writing at the start of the century, gave the field its foundational concept. In The Souls of Black Folk (1903), he described double consciousness—the sense of “always looking at one’s self through the eyes of others”—as the defining experience of Black life in America. Du Bois understood race as a sociohistorical reality, not a biological one, and his work anticipated much of twentieth-century thought on the social construction of identity. Influenced by William James’s pragmatism, Du Bois combined philosophical analysis with sociology, history, and political activism in ways that foreshadowed later interdisciplinary approaches.
Frantz Fanon brought existentialism and psychoanalysis to bear on colonialism. Black Skin, White Masks (Peau noire, masques blancs, 1952) analyzed the psychological damage colonialism inflicts on the colonized, who internalize the colonizer’s values and standards. The Wretched of the Earth (Les Damnés de la terre, 1961) argued that decolonization requires not just political independence but a fundamental restructuring of consciousness. Fanon’s work influenced postcolonial theory, critical race studies, and liberation movements worldwide.
Late-twentieth-century philosophers of race continued and deepened this work. Charles Mills’s The Racial Contract (1997) argued that the Western social contract tradition—from Hobbes through Locke to Rawls—rests on an unacknowledged racial contract that excludes non-white peoples from full moral and political standing. Mills developed the concept of an epistemology of ignorance: white ignorance operates not as mere individual prejudice but as a systematic, socially produced failure to know—a structured inability to see racial injustice that is itself maintained by the systems it protects. Cornel West’s Race Matters (1993) combined Du Bois, pragmatism, and Christian prophetic traditions into a distinctive voice addressing race, democracy, and what he called the “nihilistic threat” facing Black America. Kwame Anthony Appiah pushed back against racial essentialism in In My Father’s House (1992), arguing that racial categories are philosophically confused and that a cosmopolitan ethic can honor cultural identity without treating race as a fixed, defining feature of persons.
Environmental Philosophy
Traditional Western ethics had little to say about the natural world. Most moral frameworks assumed that only humans matter morally—an assumption the twentieth-century environmental crisis made increasingly untenable.
Aldo Leopold’s “land ethic,” articulated in A Sand County Almanac (1949), extended moral consideration from individuals to ecosystems: the health of the biotic community, not just individual welfare, became a matter of ethical concern. Leopold is widely regarded as the founding figure of environmental philosophy.
Arne Næss coined the term deep ecology in 1973 to distinguish between environmentalism pursued for human benefit (“shallow” ecology) and the recognition that nature has intrinsic value independent of human interests. Næss developed his own “Ecosophy T,” a personal philosophy of ecological harmony and biocentric equality. Peter Singer’s Animal Liberation (1975) took a different approach, arguing from utilitarian premises that the capacity to suffer, not species membership, is the relevant criterion for moral consideration—making speciesism as arbitrary as racism or sexism. Holmes Rolston III and Paul Taylor developed systematic accounts of intrinsic value in nature and biocentrism respectively, while Val Plumwood and Karen Warren developed ecofeminism, arguing that the domination of nature and the domination of women share a common logic of dualistic thinking—what Plumwood called the “master model” of rationality that elevates reason over nature, male over female, culture over the wild.
Applied Ethics
One of the century’s most significant developments was the explosion of applied ethics—the systematic application of moral philosophy to concrete problems in medicine, technology, politics, and everyday life. Until the 1960s, English-language moral philosophy was dominated by abstract questions about the meaning of moral language (meta-ethics). The social upheavals of the 1960s and 1970s—civil rights, Vietnam, the women’s movement, advances in medical technology—pushed philosophers to engage directly with real-world moral questions.
Bioethics became the most institutionally successful branch, driven by medical advances—organ transplantation, life support, genetic testing—that outpaced existing ethical frameworks. Tom Beauchamp and James Childress’s Principles of Biomedical Ethics (1979) established the dominant framework: four principles—autonomy, beneficence, non-maleficence, and justice—that provide a shared vocabulary for ethical reasoning in medicine and became known as the “Georgetown mantra.” Judith Jarvis Thomson’s “A Defense of Abortion” (1971), with its famous violinist thought experiment, demonstrated that philosophical argumentation could illuminate the most contested public debates. James Rachels challenged the traditional distinction between active and passive euthanasia, asking whether letting someone die is really morally different from killing them when the intention and outcome are the same.
Peter Singer’s utilitarian approach to global poverty, animal welfare, and end-of-life decisions made him perhaps the most publicly influential philosopher of the late twentieth century. His “drowning child” analogy—if you would ruin a new suit to save a drowning child, why not sacrifice comparable wealth to save a child dying of preventable disease abroad?—challenged comfortable assumptions about the limits of moral obligation and helped inspire the effective altruism movement. Michael Walzer’s Just and Unjust Wars (1977) revived just war theory as a framework for evaluating the ethics of armed conflict, distinguishing between the justice of going to war (jus ad bellum) and justice in the conduct of war (jus in bello).
Neo-Pragmatism, Virtue Ethics, and Process Philosophy
Several additional movements reshaped the century’s philosophical landscape. Neo-pragmatism, led by Richard Rorty, revived the American pragmatist tradition with radical implications. Rorty’s Philosophy and the Mirror of Nature (1979) argued that philosophy’s centuries-long project of “mirroring” reality—of providing foundations for knowledge—was bankrupt. The pragmatist alternative was to treat inquiry as an ongoing conversation aimed not at truth-as-correspondence but at expanding human solidarity. His Contingency, Irony, and Solidarity (1989) made the case for the liberal ironist—someone who holds moral and political commitments with conviction while acknowledging that they rest on no deeper metaphysical foundation. Hilary Putnam’s later work took a similar pragmatist turn, developing an internal realism that challenged both naive realism and relativism.
The virtue ethics revival began with Elizabeth Anscombe’s 1958 essay “Modern Moral Philosophy,” which argued that both consequentialism and Kantian deontology rely on an incoherent notion of moral obligation divorced from any account of human flourishing. Philosophy should return to the Aristotelian tradition of asking what it means to live well. Philippa Foot developed an account of natural goodness tied to the life-form of living things, arguing that the is-ought gap is narrower than Hume supposed. Foot also introduced the trolley problem—one of the most discussed thought experiments in contemporary ethics. Alasdair MacIntyre’s After Virtue (1981) was the movement’s most influential work, arguing that modern moral discourse is in disorder because the Enlightenment project of grounding morality without tradition had failed. Only the Aristotelian framework—with its account of telos (τέλος, purpose) and the virtues that lead to human flourishing—could restore coherence to ethical life.
Alfred North Whitehead’s process philosophy offered a radical metaphysical alternative. In Process and Reality (1929), Whitehead argued that reality is not composed of enduring substances but of momentary events he called actual occasions. Everything is in process; creativity is the ultimate metaphysical principle. His concept of the fallacy of misplaced concreteness—the error of treating abstractions as if they were concrete realities—remains a powerful diagnostic tool. Whitehead’s system influenced process theology, ecological philosophy, and speculative metaphysics, and its emphasis on interconnection and becoming resonates with contemporary environmental and systems thinking.
Connections and Legacy
The twentieth century transformed philosophy in ways that are still unfolding. Three shifts stand out as permanently consequential.
First, the linguistic turn. Whether through Frege’s logic, Wittgenstein’s language games, Saussure’s structural linguistics, or Derrida’s deconstruction, philosophy became intensely focused on language—not as a transparent medium for expressing thought but as a structure that shapes, and sometimes distorts, what can be thought at all. This orientation cuts across the analytic-continental divide and remains central to the discipline.
Second, the emergence of philosophy of science as a major field. Popper, Kuhn, Lakatos, and Feyerabend showed that science—the most successful form of human inquiry—raises deep philosophical questions about rationality, progress, and the nature of knowledge. Their work remains essential for anyone trying to understand what science is and what it can tell us about the world.
Third, the expansion of who counts as a philosophical subject. Women, non-white peoples, colonized populations, non-human animals, and the natural world all entered philosophical discourse in the twentieth century not as objects to be theorized about but as perspectives from which to theorize. De Beauvoir, Du Bois, Fanon, Singer, and the environmental philosophers did not merely add topics to the philosophical agenda; they challenged the discipline’s foundational assumptions about universality, neutrality, and the scope of moral concern.
The analytic-continental divide, for all its institutional reality, increasingly looks like a historical artifact—a product of specific personalities, departmental politics, and the Cold War geography of intellectual life rather than a deep philosophical necessity. Many of the century’s most interesting thinkers—Arendt, Putnam, Rorty, Nussbaum, Butler—crossed the line freely. As philosophy moves into the twenty-first century, the boundaries that defined its twentieth-century form are dissolving, and the range of questions it takes seriously has never been wider.
The roots of the twentieth century’s debates run deep into the companion article on 19th Century Philosophy—Nietzsche’s death of God, Darwin’s challenge to teleology, and Hegel’s dialectic all set the agenda that twentieth-century thinkers inherited. The century’s expanding philosophical horizon also brought non-Western traditions into the conversation: the Chinese philosophy article traces the traditions that attracted thinkers from Heidegger to Buber, while Indian philosophy and Japanese philosophy reveal how thinkers outside the Western tradition engaged the same questions—consciousness, language, ethics, political freedom—from entirely different starting points. Searle’s Chinese Room argument, for instance, invites direct comparison with the Chinese philosophical tradition’s own rich debates about language and mind. The twentieth century made philosophy genuinely global; the full scope of that achievement becomes visible only when the Western story is read alongside the others.