Philosophy

Ancient Philosophy

Pre-Socratics

• Thales of Miletus — often called the first Western philosopher; proposed water as the fundamental substance (arche) of all things; predicted a solar eclipse (585 BCE).
• Anaximander — student of Thales; posited the apeiron (“the indefinite”) as the primordial substance; earliest known prose cosmological writing.
• Anaximenes — held that air is the fundamental substance; introduced condensation and rarefaction to explain change.
• Heraclitus — known for “everything flows” (panta rhei); the Logos unifies opposites; “you cannot step into the same river twice”; fire as a central symbol.
• Parmenides — founder of Eleatic school; argued that Being is one, eternal, and unchanging; change and plurality are illusions; influenced Plato’s theory of Forms.
• Zeno of Elea — Parmenides’s student; famous paradoxes (Achilles and the Tortoise; the Arrow) argued against the reality of motion and multiplicity.
• Empedocles — proposed four roots (earth, water, fire, air) combined by Love and separated by Strife; anticipated evolutionary ideas.
• Anaxagoras — introduced nous (mind) as the cause of cosmic order; first to explain lunar eclipses correctly; exiled from Athens for impiety.
• Leucippus — credited as the originator of atomism alongside his student Democritus; posited that reality consists of indivisible atoms and void; almost no writings survive and his biography is poorly attested.
• Democritus — along with Leucippus, developed atomism: reality consists of indivisible atoms moving through void; no divine intervention required.
• Protagoras — the most prominent of the Sophists (paid teachers of rhetoric and argument in 5th-century Athens); famous dictum: “Man is the measure of all things”; argued that truth and morality are relative to the individual or community; Plato critiqued Sophistic relativism in the Theaetetus and Protagoras.
• Gorgias — Sophist and rhetorician; argued (in On Nature, or the Non-Existent) that nothing exists, that if it did it could not be known, and if known it could not be communicated; represents the most extreme Sophistic skepticism.
• Pythagoras — held that numbers are the fundamental nature of reality; the Pythagorean brotherhood; transmigration of souls.

Socrates, Plato, and Aristotle

• Socrates (470–399 BCE) — left no writings; known through Plato’s dialogues; the Socratic method (elenchus) uses questioning to expose contradictions and approach truth; executed for impiety and corrupting youth; “the unexamined life is not worth living.”
• Plato (428–348 BCE) — student of Socrates; founded the Academy in Athens; wrote dialogues (Republic, Meno, Phaedo, Symposium, Theaetetus, Timaeus); Theory of Forms: particular things in the world are imperfect copies of eternal, abstract Forms (e.g., the Form of the Good).
• Allegory of the Cave — from Republic Book VII: prisoners in a cave mistake shadows for reality; the philosopher escapes to see the sun (the Good); illustrates the ascent from opinion to knowledge.
• Platonic epistemology — knowledge (episteme) is of the eternal Forms; mere belief (doxa) is of the changing world; the divided line ranks images, visible objects, mathematical objects, and Forms.
• Plato on the soul — tripartite soul: rational, spirited, appetitive; justice in the soul (and city) is each part performing its proper function; Phaedo argues for the soul’s immortality.
• Aristotle (384–322 BCE) — student of Plato; founded the Lyceum; empiricist bent; major works include Nicomachean Ethics, Politics, Metaphysics, De Anima, Poetics, and the logical Organon (Categories, Prior Analytics, Posterior Analytics, etc.).
• Aristotle’s four causes — material (what it’s made of), formal (its shape/structure), efficient (the maker or mover), final (its purpose/telos).
• Substance and form — hylomorphism: every physical thing is matter (hyle) informed by form (morphe); rejects Platonic Forms as separate existences.
• Eudaimonia — “flourishing” or well-being; the highest good for humans; achieved through virtuous activity in accordance with reason; not merely pleasure.
• Syllogistic logic — Aristotle’s system for deductive reasoning: all A are B, all B are C, therefore all A are C; the dominant formal logic until Frege.
• Aristotle on politics — humans are “political animals” (zoon politikon); the polis is natural; classified constitutions in Politics by who rules and in whose interest.
• Aristotle on tragedy — Poetics: tragedy achieves catharsis through pity and fear; defined plot, character, and the unities.
• Plato’s Republic — ten books; argues for philosopher-kings; presents the theory of Forms and the allegory of the cave; tripartite soul parallels the just city; critiques democracy and tyranny; concludes with the Myth of Er.
• Plato’s Phaedo — set on the day of Socrates’s death; presents four arguments for the soul’s immortality (Affinity, Recollection, Opposites, Final Cause); Theory of Forms central; Socrates calmly drinks hemlock.
• Plato’s Symposium — series of speeches on eros (love); Aristophanes’s myth of the original split humans; Diotima’s ladder of love (from beautiful bodies to the Form of Beauty itself); Alcibiades’s encomium of Socrates.
• Plato’s Meno — asks whether virtue can be taught; introduces the paradox of inquiry (how can you search for what you don’t know?); anamnesis (recollection): learning is recalling knowledge the soul already has; Socrates demonstrates with a slave boy and geometry.
• Plato’s Phaedrus — on rhetoric and love; the charioteer allegory (soul as charioteer driving two horses, noble and base); argues for a philosophical rhetoric based on truth; also discusses writing as inferior to living dialogue.
• Plato’s Theaetetus — attempts to define knowledge; rejects knowledge as perception (refuting Protagoras), as true belief, and as true belief plus an account; ends in aporia; key source on Sophistic relativism.
• Aristotle’s Nicomachean Ethics — ten books on ethics dedicated to Aristotle’s son Nicomachus; eudaimonia as the highest good; doctrine of the mean; intellectual virtues (phronesis, practical wisdom); the happy life is one of virtuous activity; Book X on contemplation as the highest life.
• Aristotle’s Metaphysics — investigates “first philosophy” and being qua being; the four causes; substance, form, and matter; the unmoved mover as the ultimate cause of motion; critique of Platonic Forms.
• Aristotle’s Politics — humans as political animals; analysis of constitutions (monarchy, aristocracy, polity, and their corruptions tyranny, oligarchy, democracy); defense of the polis; critique of Plato’s ideal city; education and leisure.
• Aristotle’s Poetics — defines tragedy as imitation (mimesis) of serious action; six elements (plot, character, diction, thought, spectacle, song); hamartia (tragic flaw); the unities of action (and later interpreters added time and place).

Hellenistic Schools

• Stoicism — founded by Zeno of Citium (~300 BCE); virtue (reason-conforming life) is the only good; externals are “indifferent”; logos governs the cosmos; key figures: Chrysippus, Epictetus (Discourses), Marcus Aurelius (Meditations), Seneca.
• Epicureanism — founded by Epicurus; the good is ataraxia (tranquility) and aponia (freedom from pain); pleasure is the highest good but primarily means absence of pain, not indulgence; atoms and void; death is nothing to fear (“where death is, I am not”).
• Pyrrhonian Skepticism — Pyrrho of Elis; suspend judgment (epoche) on all matters beyond appearances to achieve tranquility; later systematized by Sextus Empiricus in Outlines of Pyrrhonism.
• Academic Skepticism — Arcesilaus and Carneades at Plato’s Academy argued for withholding assent; Cicero transmitted these views to Rome.
• Neoplatonism — Plotinus (3rd c. CE): the One is the ultimate reality; below it emanate Nous (Intellect) and the Soul; influenced early Christian theology.
• Plotinus (204–270 CE) — founder of Neoplatonism; his writings collected by Porphyry as the Enneads (six groups of nine treatises); the soul returns to the One through contemplation and ascent; mystical union with the One as the highest goal; deeply influenced Augustine and later Christian mysticism.
• Philo Judaeus (Philo of Alexandria) — (c. 20 BCE–50 CE) Jewish philosopher of Alexandria who synthesized Hebrew scripture with Greek (chiefly Platonic and Stoic) philosophy; developed the concept of the Logos as an intermediary between God and creation; a major influence on early Christian theology.
• Porphyry — (c. 234–305 CE) Neoplatonist and student of Plotinus; edited and published the Enneads; his Isagoge (introduction to Aristotle’s Categories) became a standard logic text in the Latin West and launched the medieval debate over universals.

Eastern Philosophy

Confucianism

• Confucius (Kongzi) (551–479 BCE) — the most influential thinker of the Chinese tradition, active in the Spring and Autumn period; taught a program of moral and social cultivation centered on ren (humaneness, proper behavior toward others), li (ritual propriety), and filial piety; the ideal product of this cultivation is the junzi (the gentleman or “superior man”); emphasized the five relationships and rule by moral example rather than coercion; left no writings of his own, his teachings being collected by disciples in the Analects.
• The Analects (Lunyu) — the central text of Confucianism; a collection of sayings of Confucius and his pupils, compiled by his followers in the generations after his death (during the Warring States period); opens with the line that “to learn and at times to practice what one has learned” is a pleasure; states that the junzi practices ren in accordance with li; describes serving parents according to ritual in life, death, and sacrifice; the Lu version has twenty chapters and a final section preserving words attributed to the sage-king Yao; one of the Four Books (alongside the Great Learning, the Doctrine of the Mean, and the Mencius) canonized by Zhu Xi.
• Mencius (Mengzi) (c. 372–289 BCE) — the “second sage” of Confucianism and its most important early follower; held that human nature is innately good, possessing four sprouts (the feelings of commiseration, shame, courtesy, and the sense of right and wrong) that develop into the cardinal virtues; argued an unjust ruler forfeits the Mandate of Heaven and may be overthrown; his namesake book records conversations with kings and became one of the Four Books; tradition holds his mother moved three times to secure a proper environment for his upbringing.
• Xunzi (Hsün Tzu) (c. 310–235 BCE) — Confucian philosopher who, against Mencius, argued that human nature is inherently bad (or unrefined) and must be corrected through ritual, education, and deliberate effort; teacher of the Legalist thinkers Li Si and Han Feizi.
• Neo-Confucianism — a Song-dynasty revival and metaphysical reformulation of Confucian thought; Cheng Yi and Zhu Xi (the latter the author of Family Rituals and a commentary on the Four Books) were its central figures; expanded on the doctrines of Mencius and dominated later Chinese intellectual and examination culture.

Daoism

• Daoism (Taoism) — the Chinese philosophical and religious tradition centered on following the Dao (the “Way”), the ineffable source and pattern of the cosmos; its defining principle is wu wei (“non-action” or action through inaction, knowing when to act and when to refrain), associated with ziran (naturalness, spontaneity); founded according to legend by Laozi, with Zhuangzi as its other great thinker; emphasizes the three treasures (compassion, frugality/moderation, humility); its symbols include the yin-yang taijitu; in its religious form it venerates figures such as the Jade Emperor and the Eight Immortals; the trio of Confucius, the Buddha, and Laozi appears in the painting The Vinegar Tasters.
• Laozi (Lao Tzu, “the Old Master”) — the legendary founder of Daoism, traditionally a court archivist of the Zhou dynasty and an older contemporary of Confucius; credited with the Tao Te Ching; surrounded by legend (said to have been conceived when his mother gazed at a falling star and to have been born already aged, and in some versions to have given birth to himself); his historicity is disputed.
• The Tao Te Ching (Daodejing) — the foundational text of Daoism, attributed to Laozi; its title is rendered “the Classic of the Way and Virtue (Integrity)”; opens with the paradox that “the Dao that can be spoken is not the eternal Dao”; expounds wu wei and the three treasures (compassion, moderation, humility); its terse, poetic verses were read as subverting Confucian formalism.
• Zhuangzi (Chuang Tzu) — Daoist philosopher of the Warring States (“Hundred Schools”) period and author of the eponymous Zhuangzi; best known for the butterfly dream, in which he wakes unsure whether he is a man who dreamt he was a butterfly or a butterfly dreaming it is a man; his text opens with a parable of a small bird failing to comprehend a great one, and includes the story of Cook Ding, whose blade glides effortlessly through an ox by following its natural structure; the analogy that humans live in the Dao as fish live in water is associated with this tradition.

Philosophical Traditions in Conversation

Compare arguments about knowledge, personhood, and obligation across traditions; do not treat a shared English label as proof that the underlying concepts are identical.

• Nyāya and knowledge — classical Nyāya recognizes perception, inference, comparison, and testimony as means of knowledge (pramāṇas). This makes testimony and the reliability of a speaker central epistemological questions, alongside the individual knower’s evidence. Compare the epistemology section below with the Indian schools in World Religions. Stanford Encyclopedia of Philosophy.
• Mohism — the school associated with Mozi argues for inclusive care and opposes aggressive warfare. Its concern with standards, practical benefit, and impartial concern gives a different account of moral and political order from the Confucian emphasis on differentiated relationships. Stanford Encyclopedia of Philosophy.
• African sage philosophy — Henry Odera Oruka’s project records and examines arguments by individual African thinkers, including people working in oral traditions. Its distinction between shared custom and a person’s critical reflection challenges the assumption that oral traditions cannot contain philosophy. Read Sage Philosophy: Indigenous Thinkers and Modern Debate on African Philosophy (1990). Stanford Encyclopedia of Philosophy.
• Ubuntu — a family of southern African ideas about personhood and ethical life through relationships with others. Treat it as a topic of philosophical interpretation and debate rather than a single doctrine shared by all African societies. Compare Mogobe Ramose’s African Philosophy Through Ubuntu with the discussions of virtue, rights, and duties below. OpenStax introduction.
• Sophie B. Olúwọlé — Socrates and Ọ̀rúnmìlà — a comparison of Greek and Yoruba thought that asks how oral transmission affects whose ideas are recognized as philosophy. Read the comparisons as Olúwọlé’s arguments, rather than as an uncontested account of every Greek or Yoruba tradition. OpenStax discussion.

Reading connection: the Philosophy catalog includes South Asian, East Asian, and global study units. The Religion catalog offers the wider textual and ritual settings.

Medieval Philosophy

• John Scotus Eriugena (c. 815–877) — Irish scholar working at the Carolingian court; translated Pseudo-Dionysius the Areopagite into Latin; his Periphyseon (On the Division of Nature) presented a Christian Neoplatonist system that verged on pantheism; an isolated but influential figure in early medieval philosophy.
• Augustine of Hippo (354–430) — bishop, theologian, philosopher; Confessions and City of God; addressed the problem of evil; reconciled Neoplatonism with Christianity; original sin; just war theory; time exists only as present memory/present present/present expectation.
• Boethius (c. 480–524) — The Consolation of Philosophy written in prison; reconciled fortune’s wheel with providence; transmitted Aristotle’s logic to the Latin West; defined “person” for medieval Trinitarian debates.
• Anselm of Canterbury (1033–1109) — ontological argument for God’s existence: God is that than which nothing greater can be conceived; if existing only in the mind, something greater (which also exists) could be conceived; therefore God must exist in reality.
• Roscelin of Compiègne (c. 1050–1125) — early medieval nominalist; argued that universals are merely vocal utterances (voces); his position was condemned for allegedly implying tritheism when applied to the Trinity; a key provocateur in the debate over universals that Abelard and others responded to.
• Solomon ibn Gabirol (Avicebron) (c. 1021–1058) — Andalusian Jewish philosopher and poet; his Fons Vitae (Fountain of Life), written in Arabic and translated into Latin, influenced Scholastic debates on matter and form; proposed universal hylomorphism (all created beings, including intellects, composed of matter and form).
• Peter Abelard (1079–1142) — conceptualism on universals: universals are concepts in the mind, not independent entities; famous for Sic et Non (yes and no method of dialectic).
• Albertus Magnus (c. 1200–1280) — Dominican friar and teacher of Thomas Aquinas; produced extensive commentaries on Aristotle that helped introduce Aristotelian natural philosophy into the Latin university curriculum; also wrote on theology and natural science; canonized and declared a Doctor of the Church.
• Roger Bacon (c. 1214/1220–1292) — Franciscan friar at Oxford; advocated empirical methods and mathematical reasoning in natural philosophy; wrote Opus Majus outlining a reformed curriculum for the sciences; sometimes anachronistically styled a forerunner of the scientific method.
• Thomas Aquinas (1225–1274) — Five Ways (quinque viae): cosmological arguments for God’s existence from motion, causation, contingency, degrees of perfection, and teleology; reconciled Aristotle with Catholic theology; Summa Theologica; natural law ethics.
• John Duns Scotus (c. 1266–1308) — haecceity (thisness): the principle of individuation distinct from matter; univocal predication of being; “the subtle doctor.”
• William of Ockham (c. 1287–1347) — Ockham’s razor: entities should not be multiplied beyond necessity; nominalism: universals are names, not real entities; separated theology from natural philosophy.
• Islamic philosophy — Al-Kindi (9th c.): first major Islamic philosopher, reconciled Greek thought with Islam; Al-Farabi: political philosophy, emanationist cosmology; Avicenna (Ibn Sina) (980–1037): The Book of Healing, floating-man argument for the soul’s self-awareness; Averroes (Ibn Rushd) (1126–1198): influential Aristotle commentaries; monopsychism controversy; Al-Ghazali: Incoherence of the Philosophers, critique of Aristotelian metaphysics.
• Maimonides (1135–1204) — Guide for the Perplexed; reconciled Aristotle with Jewish theology; negative theology (God’s attributes can only be stated negatively).
• Ibn Khaldun (1332–1406) — North African historian and political thinker; the Muqaddimah analyzes social organization, labor, taxation, knowledge, and dynastic cycles through asabiyyah (group solidarity). His work belongs to the medieval Islamic intellectual world, even though later disciplines have claimed him as a precursor.
• Avicenna (Ibn Sina) (980–1037) — Persian philosopher and physician; floating man argument: a person deprived of all sensory input would still be aware of their own existence, an argument for immediate self-awareness that is sometimes compared with Descartes’s cogito, without establishing direct historical influence; The Book of Healing (encyclopedia of philosophy and science) and The Canon of Medicine; influential on Scholastic debates about essence and existence. Stanford discussion of the floating man.
• Averroes (Ibn Rushd) (1126–1198) — Andalusian philosopher; wrote extensive commentaries on Aristotle that were essential for Scholastic philosophy (called “the Commentator” by Aquinas); monopsychism: argued the active intellect is a single, shared entity separate from individual humans; this was condemned by the church; The Incoherence of the Incoherence defended philosophy against Al-Ghazali.
• Boethius’s Consolation of Philosophy — written while awaiting execution under Theodoric; Lady Philosophy argues that true goods (virtue, God) are not subject to Fortune’s wheel; mixes Neoplatonism with Stoicism; widely read throughout the Middle Ages as a meditation on providence and fate.
• Anselm’s ontological argument — presented in the Proslogion (1077–78); Gaunilo objected with the “perfect island” parody; Aquinas rejected it; Descartes revived it; Kant argued existence is not a predicate; contemporary versions by Plantinga use modal logic.
• Duns Scotus’s haecceity — each individual thing has an individuating principle (“thisness”) distinct from its common nature; debates with Aquinas’s view that matter individuates; Scotus also argued for the Immaculate Conception and univocal predication of being across God and creatures.
• William of Ockham’s razor — the principle of parsimony: “Numquam ponenda est pluralitas sine necessitate” (plurality is never to be posited without necessity); Ockham used it to eliminate Scotistic formal distinctions and argue that only particular things exist.

Early Modern Philosophy

• Giordano Bruno (1548–1600) — Italian Dominican friar and philosopher; extended the Copernican heliocentric model to propose an infinite universe with innumerable worlds; held a form of pantheism and identified God with infinite nature; burned at the stake by the Roman Inquisition; a symbol of conflict between free inquiry and religious authority.
• Francis Bacon (1561–1626) — Novum Organum (1620); argued for an inductive method based on systematic observation and experimentation as the basis for natural philosophy; critiqued the “idols” (false notions) that impede inquiry; influential in shaping the idea of organized scientific research; Lord Chancellor of England.
• Michel de Montaigne (1533–1592) — inventor of the essay as a literary form; Essais (1580–88) explore the self through personal observation and skeptical reflection; “Que sais-je?” (“What do I know?”) as his motto; influenced later humanist and skeptical traditions; the essay on cannibals relativized European cultural norms.
• Giambattista Vico (1668–1744) — The New Science (1725): humans can only truly know what they have made (verum factum principle); proposed a cyclical theory of history (corso e ricorso); critique of Cartesian rationalism applied to history; precursor of hermeneutics and philosophy of history.

Rationalists

• René Descartes (1596–1650) — Meditations on First Philosophy (1641); methodological doubt: doubt everything until finding something indubitable; cogito ergo sum (“I think, therefore I am”) as the foundation; mind-body substance dualism; res cogitans (thinking substance) vs res extensa (extended substance); Cartesian circle (critics allege); Discourse on the Method.
• Descartes’s Meditations on First Philosophy — six meditations: (1) grounds for doubt; (2) the cogito; (3) God’s existence from the idea of a perfect being; (4) error and free will; (5) God’s existence from the ontological argument; (6) the real distinction of mind and body; the evil demon (malin génie) is introduced in Meditation I as the ultimate skeptical hypothesis.
• Baruch Spinoza (1632–1677) — Ethics (written in geometric proof form); pantheism / panentheism: God and Nature are one substance (Deus sive Natura); everything follows necessarily from God’s nature; free will is an illusion caused by ignorance of causes; conatus (striving to persist in being); Tractatus Theologico-Politicus (1670): one of the foundational texts of biblical criticism and liberal political philosophy, arguing that the Bible must be interpreted historically rather than dogmatically and that freedom of thought is compatible with political stability; helped launch the Enlightenment critique of revealed religion.
• Spinoza’s Ethics — written more geometrico (in the geometric manner) with definitions, axioms, propositions, and proofs; five parts: God, the nature of the mind, the origin of the emotions, human bondage (to passions), and the power of the intellect (freedom); excommunicated (cherem) from the Amsterdam Jewish community in 1656.
• Leibniz’s Monadology — 90-section work (1714) summarizing Leibniz’s metaphysics; monads are simple, unextended, “windowless” substances that perceive the universe from their own perspective; pre-established harmony explains apparent interaction; God chose the best of all possible worlds (satirized by Voltaire in Candide).
• Gottfried Wilhelm Leibniz (1646–1716) — monadology: reality consists of windowless, simple substances called monads; pre-established harmony accounts for mind-body interaction; principle of sufficient reason: nothing happens without a reason; calculus (independently of Newton); principle of identity of indiscernibles.
• Nicolas Malebranche (1638–1715) — occasionalism: God is the only true cause; apparent causal relations between finite things are occasions for God to act.

Empiricists

• John Locke (1632–1704) — An Essay Concerning Human Understanding; mind at birth is a tabula rasa (blank slate); all knowledge derives from sensation and reflection; primary qualities (shape, solidity) vs secondary qualities (color, taste) that exist only in perceivers; Two Treatises of Government: natural rights (life, liberty, property); social contract; right of revolution.
• Locke’s Essay Concerning Human Understanding (1689) — four books: critique of innate ideas, theory of ideas (sensation + reflection), words and language, and knowledge; defines real vs nominal essence; personal identity through memory continuity; a founding text of British empiricism.
• Locke’s Two Treatises of Government (1689) — First Treatise refutes Filmer’s divine right of kings (Patriarcha); Second Treatise argues for consent of the governed, natural rights (life, liberty, estate), separation of powers, and the right to revolt against tyranny; profoundly influenced the American Declaration of Independence.
• Thomas Reid (1710–1796) — Scottish common sense philosophy: rejected Hume’s skeptical conclusions by appealing to self-evident first principles that all sane humans accept; An Inquiry into the Human Mind on the Principles of Common Sense (1764); direct realism in perception; founder of the Scottish School that influenced 19th-century American philosophy.
• George Berkeley (1685–1753) — idealism / immaterialism: esse est percipi (“to be is to be perceived”); material objects exist only as ideas in minds; God continuously perceives all things; A Treatise Concerning the Principles of Human Knowledge and Three Dialogues Between Hylas and Philonous.
• David Hume (1711–1776) — A Treatise of Human Nature; Enquiries; radical empiricism: ideas are faint copies of impressions (sensory experiences); bundle theory of the self: no persistent self, just a bundle of perceptions; problem of induction: no logical justification for inferring general laws from observed cases; is-ought gap (Hume’s guillotine): descriptive facts cannot logically entail normative claims; critique of miracles and natural religion; causation is constant conjunction plus habit, not necessary connection; Dialogues Concerning Natural Religion.
• Hume’s Treatise of Human Nature (1739–40) — three books (Understanding, Passions, Morals); Hume’s most systematic work, presenting the “experimental method of reasoning” applied to human nature; famously “fell dead-born from the press”; laid out bundle theory, induction problem, and the passions’ role in motivation.
• Hume’s Enquiry Concerning Human Understanding (1748) — a more accessible restatement of Treatise Book I; Section X (“Of Miracles”) argues that no human testimony can establish a miracle; Section XII distinguishes Pyrrhonian from mitigated skepticism; the is-ought passage is more explicit in the Treatise but the Enquiry is the more commonly cited text.
• George Berkeley’s idealism — esse est percipi; attacked Locke’s distinction between primary and secondary qualities (both are mind-dependent); the three dialogues: Philonous defeats Hylas’s materialism; God is the mind that sustains the existence of the physical world when no finite mind perceives it.

Kant and German Idealism

• Immanuel Kant (1724–1804) — “Copernican revolution” in philosophy: the mind imposes structure on experience; synthetic a priori knowledge (mathematics, causation) is possible because we supply the forms; space and time are forms of intuition; categories of understanding (e.g., causality, substance) structure experience; phenomenon (thing as experienced) vs noumenon (thing-in-itself, unknowable); Critique of Pure Reason (1781/1787).
• Kant’s Critique of Pure Reason — the “First Critique” (1781, revised 1787); the Transcendental Aesthetic (space and time), Transcendental Analytic (categories and schematism), and Transcendental Dialectic (antinomies and paralogisms showing where pure reason overreaches); refutation of idealism; the paralogisms target rational psychology; the antinomies show reason generates contradictions about the world as a whole.
• Kant’s Critique of Practical Reason — the “Second Critique” (1788); grounds morality in pure practical reason independent of inclination; the moral law is the categorical imperative; the postulates of practical reason (God, freedom, immortality) are required for moral coherence even if theoretically unknowable; highest good (summum bonum).
• Kant’s Critique of Judgment — the “Third Critique” (1790); aesthetic judgment: the beautiful engages free play of imagination and understanding; sublimity: the mathematically and dynamically sublime; teleological judgment: organisms are understood as if purposive; the judgment of taste claims universal assent without a determining concept.
• Categorical imperative — Kant’s supreme moral principle from Groundwork for the Metaphysics of Morals: (1) Formula of Universal Law: “Act only according to that maxim whereby you can at the same time will that it should become a universal law”; (2) Formula of Humanity: treat persons always as ends in themselves, never merely as means; (3) Formula of Autonomy / Kingdom of Ends.
• Kant on aesthetics — Critique of Judgment (1790): judgments of taste claim universal validity without being based on concepts; the sublime overwhelms sensory limits; teleological judgment in biology.
• Johann Gottlieb Fichte (1762–1814) — subjective idealism; in the Wissenschaftslehre (Science of Knowledge) posited the self-positing Ego as the ground of all reality; argued that the non-Ego (the world) is posited by the Ego as its own limit; influential on German Idealism and German nationalism (Addresses to the German Nation, 1808).
• Friedrich Wilhelm Joseph Schelling (1775–1854) — moved through several phases: early philosophy of nature (Naturphilosophie) treating nature as unconscious spirit; identity philosophy (Absolute as the indifference of subject and object); later positive philosophy critiquing Hegelian rationalism; influenced Coleridge, Heidegger, and later existentialists.
• G. W. F. Hegel (1770–1831) — dialectic: thesis-antithesis-synthesis (though Hegel’s own terms were Aufhebung, sublation); Absolute Idealism: reality is the unfolding of Geist (Spirit/Mind); Phenomenology of Spirit (1807): the journey of consciousness to Absolute knowledge; Philosophy of Right: the state as the actualization of ethical life; Lectures on the Philosophy of History: history as the progress of freedom.
• Arthur Schopenhauer (1788–1860) — The World as Will and Representation: behind phenomena is blind, striving Will; pessimism; aesthetic contemplation and asceticism as escape from suffering; influenced Nietzsche and Freud.
• Schopenhauer’s The World as Will and Representation (Die Welt als Wille und Vorstellung, 1818, expanded 1844) — two books: the world as representation (Kantian framework) and the world as will; the will-to-live as the underlying metaphysical reality; music as the direct expression of the will; compassion as the basis of ethics; the four books are tied together by the transition from epistemology to metaphysics to aesthetics to ethics.
• Hegel’s Phenomenology of Spirit (Phänomenologie des Geistes, 1807) — traces the development of consciousness from sense-certainty through self-consciousness, reason, spirit, religion, and absolute knowing; master-slave dialectic: the struggle for recognition creates an unstable power relation that generates history; unhappy consciousness; Marx inverted the dialectic for material conditions.
• Hegel’s dialectic — the movement of thought (and history) through contradiction: a position (thesis) generates its negation (antithesis), and both are aufgehoben (sublated, simultaneously cancelled and preserved) in a higher synthesis; Hegel himself rarely used the terms thesis-antithesis-synthesis; they were popularized by later commentators.

Enlightenment Philosophy

The Enlightenment was a family of overlapping seventeenth- and eighteenth-century projects rather than one doctrine. Its thinkers disputed reason, religion, nature, commerce, sovereignty, education, and human difference across British, French, German, Scottish, Atlantic, and other settings.¹

• Public reason and critique — Enlightenment authors subjected inherited authority to argument, historical inquiry, natural philosophy, and public criticism. Kant later defined enlightenment through intellectual self-direction, but access to education and print remained sharply unequal.
• Republic of letters and public sphere — correspondence, salons, academies, coffeehouses, periodicals, pamphlets, and clandestine manuscripts connected readers across borders. These networks enabled debate while depending on patronage, censorship regimes, imperial trade, and social privilege.
• Empiricism and rationalism — Locke and Hume examined experience, ideas, causation, and knowledge; Descartes, Spinoza, and Leibniz developed systematic accounts of reason and reality. Enlightenment thought grew from their conflicts rather than replacing one school with the other.
• Natural religion and toleration — deists argued that reason and nature could ground religion without all claims of revelation, while skeptics and materialists pressed further. Locke, Bayle, Voltaire, Hume, and Mendelssohn defended different forms of toleration with different limits.
• Natural law and rights — theories of natural rights, social contract, consent, and resistance challenged divine-right monarchy and informed constitutional revolutions. Their supposedly universal subjects were often defined in ways that excluded women, enslaved people, colonized peoples, religious minorities, and the poor.
• Montesquieu — The Spirit of the Laws compares legal and political systems in relation to institutions, commerce, custom, and environment. His account of divided powers influenced constitutional thought, although the familiar three-branch formula simplifies his comparative argument.²
• Voltaire — philosophical tales, histories, letters, and polemics attacked fanaticism, intolerance, and abuses of authority. Candide satirizes providential optimism; Voltaire’s reformism and personal investments also expose contradictions between critique and participation in elite imperial economies.
• Diderot, d’Alembert, and the Encyclopédie — the multivolume project organized arts, sciences, and trades while embedding cross-references and criticism within a reference work. Thousands of contributors and artisans make it more than the product of two editors.
• Rousseau’s challenge — Rousseau argued that social inequality and dependence could corrupt freedom and that legitimate law required collective self-rule. His critique of civilization places him within the Enlightenment while challenging its confidence in progress and polite society.
• Scottish Enlightenment — Hume, Adam Smith, Adam Ferguson, and others studied moral sentiments, commerce, history, institutions, and unintended social order. Their “science of man” joined philosophical analysis to conjectural history and emerging political economy.
• Mandeville and commercial society — The Fable of the Bees provocatively linked private vice to public economic benefit. Its paradox forced later thinkers to explain how self-interest, norms, institutions, and sympathy might produce social order without moralizing every exchange.
• Mary Wollstonecraft — A Vindication of the Rights of Woman argues that women’s apparent dependence reflects deficient education and social institutions rather than natural incapacity. Her intervention tests whether universal reason and rights actually apply universally.
• Kant and autonomy — Kant connected enlightenment to freedom in the public use of reason and grounded morality in autonomy and universal law. His philosophy is also examined critically for writings on race and hierarchy that conflict with universalist claims.
• Empire, race, slavery, and exclusion — classification of peoples, colonial administration, plantation wealth, and Atlantic slavery were part of the Enlightenment world. Abolitionist argument, the Haitian Revolution, Indigenous diplomacy, and critiques by excluded writers reveal both the reach and the failures of universalism.
• Enlightenment legacies — secular government, human rights, disciplinary science, liberalism, socialism, nationalism, and revolutionary politics all drew selectively from Enlightenment debates. There is no single inheritance that settles their later conflicts.

19th-Century Philosophy

• Jeremy Bentham (1748–1832) — founder of utilitarianism: the right act maximizes aggregate happiness; hedonic calculus to measure pleasure and pain; Introduction to the Principles of Morals and Legislation; the panopticon prison design.
• John Stuart Mill (1806–1873) — revised utilitarianism: higher and lower pleasures (“better to be Socrates dissatisfied than a fool satisfied”); Utilitarianism; On Liberty: the harm principle limits state interference; The Subjection of Women: early feminist argument; A System of Logic.
• Karl Marx (1818–1883) — with Engels, Communist Manifesto (1848); Capital (Das Kapital, 1867); historical materialism: history is driven by class struggle over material conditions; base and superstructure; alienation of labor under capitalism; commodity fetishism; critique of ideology.
• Søren Kierkegaard (1813–1855) — father of existentialism; three stages of existence: aesthetic, ethical, religious; leap of faith required for religious stage; Either/Or, Fear and Trembling, Concluding Unscientific Postscript; pseudonymous authorship; subjectivity and the individual against Hegel’s system.
• Kierkegaard’s Fear and Trembling — under pseudonym Johannes de Silentio; examines Abraham’s willingness to sacrifice Isaac; the teleological suspension of the ethical: faith may require transgressing universal ethical norms; the “knight of faith” vs the “knight of infinite resignation”; stages: problematic, dialectical, aesthetic.
• Kierkegaard’s Either/Or — the aesthetic life (Don Juan, immediacy, pleasure) vs the ethical life (the Judge, commitment, duty); the seducer’s diary in volume one; does not resolve the choice between them; the reader must choose.
• Friedrich Nietzsche (1844–1900) — “God is dead” (The Gay Science, 1882); nihilism as the crisis of values; the Ubermensch (Overman) as an ideal of self-overcoming; will to power as the basic drive; eternal recurrence as the greatest affirmation of life; master and slave morality; critique of Christianity; Thus Spoke Zarathustra, Beyond Good and Evil, On the Genealogy of Morality, The Birth of Tragedy.
• Nietzsche’s The Birth of Tragedy (1872) — contrasts Apollonian (order, reason, individuation) and Dionysian (chaos, intoxication, dissolution of the self) impulses in Greek culture; argues tragedy united both; blamed Socratic rationalism for the death of tragedy; Socratic optimism as the villain; later Nietzsche partially disowned the book’s Schopenhauerian framework.
• Nietzsche’s Thus Spoke Zarathustra (1883–85) — the prophet Zarathustra descends from the mountain to teach the Ubermensch and eternal recurrence; written in biblical/prophetic style; “the last man” as the nihilistic alternative to the Overman; declares the death of God and the need for a revaluation of all values.
• Nietzsche’s Beyond Good and Evil (1886) — critique of dogmatic philosophy (especially Kant and Schopenhauer); perspectivism: there are no facts, only interpretations; will to power as the fundamental drive; the “noble” vs “herd” morality; the free spirit; the philosopher of the future must be a legislator of new values.
• Nietzsche’s On the Genealogy of Morality (1887) — three essays: (1) master/slave morality and ressentiment; (2) guilt, conscience, and the creditor-debtor relation; (3) the ascetic ideal and its meaning for science, art, and religion; one of the most influential texts in continental philosophy and moral psychology.
• Nietzsche’s eternal recurrence — the thought experiment: if you had to live your life over and over in exactly the same way, infinitely many times, would you affirm it? Intended not as a metaphysical claim but as a test of one’s attitude toward life; introduced in The Gay Science sec. 341 and central to Zarathustra.
• Ralph Waldo Emerson (1803–1882) — central figure of American Transcendentalism; Nature (1836) and essays such as “Self-Reliance” and “The Over-Soul”; argued for the immanence of the divine in nature and the individual; emphasis on intuition over formal doctrine; influenced Thoreau, Whitman, and William James.
• Henry David Thoreau (1817–1862) — Transcendentalist associated with Emerson; Walden (1854): two years of deliberate simple living at Walden Pond as an experiment in self-reliance; Civil Disobedience (1849): moral duty to resist unjust laws; influenced Gandhi and the American civil rights movement.
• F. H. Bradley (1846–1924) — the leading British Idealist; Appearance and Reality (1893): argued that relational thought generates contradictions and that the ultimate reality is a non-relational Absolute; attacked empiricism’s notion of bare sense-data; Russell and Moore developed analytic philosophy partly in reaction to Bradley.
• Auguste Comte (1798–1857) — founder of positivism: knowledge passes through theological, metaphysical, and positive (scientific) stages; coined “sociology.”
• John Stuart Mill’s On Liberty (1859) — the harm principle: the only justification for restricting individual liberty is to prevent harm to others; attacks social tyranny and the tyranny of prevailing opinion; argues for free expression on the grounds that suppression risks silencing truth and prevents the “collision” of ideas.
• John Stuart Mill’s Utilitarianism (1863) — distinguishes higher (intellectual) from lower (bodily) pleasures; argues for the proof of utility (though critics find it fallacious); addresses justice as a strong form of utility; defends aggregate welfare against egoism and intuitionism.
• Alasdair MacIntyre (1929–) — After Virtue (1981): modernity’s moral discourse is in disorder because it abandoned Aristotelian teleology; emotivist ethics (the view that moral claims merely express attitudes) is the dominant but unacknowledged position; recovering virtue ethics requires recovering the concept of a social practice and a narrative view of the self.
• Herbert Spencer (1820–1903) — social Darwinism: applied evolutionary concepts to society; coined “survival of the fittest” (before Darwin used it).

Analytic Philosophy

• Henri Bergson (1859–1941) — French philosopher; Time and Free Will (1889) and Creative Evolution (1907); argued that lived time (durée, duration) is irreducible to the spatialized, measurable time of science; élan vital (vital impulse) drives evolution creatively rather than mechanically; Nobel Prize in Literature (1927); influential on Proust, William James, and Merleau-Ponty.
• George Santayana (1863–1952) — Spanish-American philosopher; naturalist and critic of American idealism; The Life of Reason (5 vols., 1905–06) and Realms of Being; coined the phrase “those who cannot remember the past are condemned to repeat it”; combined materialism with aesthetic sensitivity and literary style; not easily classified as analytic or continental.
• G. E. Moore (1873–1958) — with Russell, a founding figure of British analytic philosophy; Principia Ethica (1903): the naturalistic fallacy (defining “good” in natural terms commits an error); “good” is a simple, indefinable, non-natural property known by intuition; “A Defence of Common Sense”: philosophical common sense as a starting point against idealist skepticism.
• Alfred North Whitehead (1861–1947) — co-authored Principia Mathematica with Russell; later developed process philosophy in Process and Reality (1929): reality consists of “actual occasions” of experience rather than static substances; philosophy of organism; God is not omnipotent but is the lure for novelty; influenced process theology.
• Gottlob Frege (1848–1925) — Begriffsschrift (1879): first modern formal logic system; predicate logic superseding Aristotelian syllogistic; sense (Sinn) vs reference (Bedeutung): “the morning star” and “the evening star” have the same reference (Venus) but different senses; the concept of a function applied to logic.
• Edmund Husserl’s phenomenology — the transcendental reduction (epoché): bracketing the natural attitude to study pure consciousness; intentionality: every mental act is directed toward an object; noema (the intentional content) and noesis (the intentional act); Crisis of European Sciences (1936): the life-world (Lebenswelt) as the forgotten foundation of scientific abstraction.
• Bertrand Russell (1872–1970) — with Whitehead, Principia Mathematica (1910–13): grounding mathematics in logic (logicism); Russell’s paradox: the set of all sets that do not contain themselves; theory of descriptions: “the present king of France is bald” analyzed as an existential claim; The Problems of Philosophy; logical atomism.
• Russell’s logical atomism — the world consists of atomic facts; language pictures the world; the logical form of a proposition must match the logical form of the fact; developed in Our Knowledge of the External World (1914) and the 1918 Philosophy of Logical Atomism lectures; influenced the early Wittgenstein.
• Ludwig Wittgenstein (1889–1951) — early: Tractatus Logico-Philosophicus (1921): the world is all that is the case; propositions picture facts; whereof one cannot speak, thereof one must be silent; later: Philosophical Investigations (1953): meaning is use; language games and forms of life; private language argument; family resemblance (concepts lack sharp defining features); rule-following paradox.
• Wittgenstein’s Tractatus Logico-Philosophicus (1921) — 7 numbered propositions and sub-propositions; the picture theory of meaning: propositions share logical form with the facts they depict; the limits of language are the limits of my world; what can only be shown cannot be said; the ladder metaphor (throw it away after climbing); the mystical: “What we cannot speak about we must pass over in silence.”
• Wittgenstein’s Philosophical Investigations (1953, posthumous) — a series of numbered remarks without systematic argument; attacks the Augustinian picture of language (every word names an object); language games: diverse uses of language embedded in forms of life; the private language argument: a language only the speaker could understand is impossible; family resemblance: concepts like “game” have no single common feature but overlapping similarities.
• Logical positivism’s verification principle — a statement is cognitively meaningful only if it is either (a) empirically verifiable in principle or (b) analytically true; protocol sentences as the observational base; the principle itself is neither empirically verifiable nor analytic, which critics used against it; Popper’s falsificationism was partly a response.
• Rudolf Carnap (1891–1970) — Vienna Circle; The Logical Structure of the World (Aufbau, 1928): reconstructing empirical knowledge from immediate experience; logical syntax vs semantics; attacked metaphysics as meaningless; later inductive logic and confirmation theory; “Empiricism, Semantics, and Ontology”: internal vs external questions.
• W. V. O. Quine’s “Two Dogmas of Empiricism”* (1951) — the two dogmas: (1) the analytic-synthetic distinction and (2) reductionism (each statement has its own empirical content); Quine argued our beliefs form a web (“web of belief”) facing experience as a corporate body; ontological relativity: what exists is relative to a theory; Word and Object (1960) argues for the indeterminacy of translation.
• Thomas Kuhn’s The Structure of Scientific Revolutions (1962) — distinguishes normal science (puzzle-solving within a paradigm) from revolutionary science; a paradigm is the entire constellation of beliefs, techniques, and examples shared by a community; anomalies accumulate until a crisis triggers a revolution; paradigms are incommensurable (no neutral language to compare them); coined the term paradigm shift.
• Saul Kripke’s Naming and Necessity (1980, based on 1970 lectures) — rigid designators: proper names and natural kind terms refer to the same object in all possible worlds; a posteriori necessities: “Hesperus is Phosphorus” is necessarily true but known empirically; Kripke semantics for modal logic; critique of the description theory of names (Frege, Russell); identity theory of mind faces a modal objection.
• Derek Parfit (1942–2017) — Reasons and Persons (1984): reductionism about personal identity: persons are not further facts beyond physical and psychological facts; fission cases show identity is not what matters, what matters is psychological continuity; implications for rationality and ethics; On What Matters (2011) aims to reconcile Kantian, consequentialist, and contractualist ethics.
• Thomas Nagel (1937–) — “What Is It Like to Be a Bat?” (1974): subjective experience cannot be captured by objective physical description; the subjective character of experience is irreducible; The View from Nowhere (1986): the tension between subjective and objective perspectives; critique of reductionism about consciousness.
• J. L. Austin (1911–1960) — ordinary language philosophy at Oxford; How to Do Things with Words (1962): the theory of speech acts; performative utterances (saying is doing); illocutionary force; Sense and Sensibilia: critique of sense-data theory; “philosophical bedrock”: attending carefully to how words are actually used dissolves philosophical puzzles.
• Logical positivism / Vienna Circle — Schlick, Carnap, Neurath; verification principle: a statement is meaningful only if it is empirically verifiable or logically true; rejected metaphysics; A. J. Ayer’s Language, Truth and Logic (1936) popularized it in English.
• Karl Popper (1902–1994) — falsificationism: science is distinguished by falsifiable hypotheses, not verifiability; critique of Marxism and psychoanalysis as unfalsifiable; The Logic of Scientific Discovery; verisimilitude (truth-likeness); The Open Society and Its Enemies.
• W. V. O. Quine (1908–2000) — “Two Dogmas of Empiricism” (1951): attacked the analytic-synthetic distinction and reductionism; ontological relativity; Word and Object: the indeterminacy of radical translation; naturalized epistemology.
• Thomas Kuhn (1922–1996) — The Structure of Scientific Revolutions (1962): science proceeds through paradigms and paradigm shifts (revolutions); incommensurability between paradigms; “normal science” vs revolutionary science; popularized “paradigm shift.”
• Donald Davidson (1917–2003) — anomalous monism: mental events are identical to physical events but mental properties are not reducible to physical ones; principle of charity in radical interpretation; causal theory of action.
• Saul Kripke (1940–2022) — Naming and Necessity (1980): rigid designators (names pick out the same object in all possible worlds); a posteriori necessities (e.g., water is H2O is necessarily true but known empirically); critique of description theories of names; possible world semantics for modal logic.
• Gilbert Ryle (1900–1976) — The Concept of Mind (1949): the “ghost in the machine” criticism of Cartesian dualism; argued Descartes committed a category mistake in treating mind as a separate substance; championed ordinary-language analysis of mental concepts; editor of Mind for over two decades.
• Hilary Putnam (1926–2016) — twin earth thought experiment: “water” on twin earth is XYZ, not H2O; semantic externalism: meaning ain’t in the head; multiple realizability of mental states; internal realism.

Neo-Kantianism and Philosophy of Culture

Neo-Kantianism was an influential movement in German academic philosophy from the 1860s until the First World War. Its schools returned to Kant to explain the validity of scientific knowledge, the methods of the historical and cultural sciences, and the place of values; Cassirer later widened that project into a philosophy of culture.³

• “Back to Kant” — Otto Liebmann’s Kant und die Epigonen (1865) supplied the movement’s rallying cry. Friedrich Albert Lange’s History of Materialism (1866) and Hermann von Helmholtz’s studies of perception and geometry also turned to Kant during debates over materialism and the authority of the natural sciences.
• Validity against psychologism — neo-Kantians separated the validity (Geltung) of knowledge and norms from the psychological processes by which people actually think. Hermann Lotze’s Logic (1874) had distinguished laws that thought does obey from laws it ought to obey; Helmholtz and Lange did not share the later schools’ anti-psychologism.
• Marburg School — Hermann Cohen, beginning with Kants Theorie der Erfahrung (1871), and Paul Natorp read Kant’s transcendental method as starting from “the fact of science” and asking which conditions make its objective validity possible. Cohen’s student Ernst Cassirer traced that problem historically in Das Erkenntnisproblem (1906–1907).
• Cohen beyond epistemology — Cohen carried the Marburg program into ethics in Ethik des reinen Willens (1904) and into Jewish philosophy of religion in Religion der Vernunft aus den Quellen des Judentums, published in 1919 after his death.
• Southwest (Baden) School — Wilhelm Windelband and Heinrich Rickert made values and validity the central subject of philosophy. Windelband’s 1894 essay “History and Natural Science” contrasted nomothetic inquiry into general laws with idiographic inquiry into particular historical facts; Rickert’s Die Grenzen der naturwissenschaftlichen Begriffsbildung examined why natural-scientific concept formation cannot capture historical individuality. Emil Lask extended the school’s analysis of validity and judgment.
• Method versus subject matter — Windelband classified sciences by their aims rather than their objects. He therefore rejected Wilhelm Dilthey’s division of the human sciences (Geisteswissenschaften) from the natural sciences and placed psychology among the law-seeking sciences. The dispute over law, individuality, explanation, and understanding shaped later methodology in history and the social sciences.
• Value relation and Max Weber — Rickert argued that historians select objects through their relation to values (Wertbeziehung) while still aiming at objectively valid knowledge. Max Weber adopted and reworked these distinctions in his methodology, but he doubted Rickert’s assumption that cultural values could be universally valid.⁴
• Ernst Cassirer — a philosopher formed in Marburg neo-Kantianism who extended the study of knowledge into a philosophy of culture. The Philosophy of Symbolic Forms (three volumes, 1923–1929) examines language, myth, and knowledge as ways humans organize experience through symbols. An Essay on Man (1944) offers a shorter introduction to this framework. His 1929 Davos debate with Heidegger concerned Kant, human finitude, and the possibility of objectively valid knowledge.⁵
• Cassirer’s route to culture — Substance and Function (1910) treated scientific concepts as systems of relations, and Zur Einsteinschen Relativitätstheorie (1921) presented relativity as consistent with a revised critical epistemology. Later works addressed the Enlightenment (1932), the logic of the cultural sciences (1942), and political myth in The Myth of the State, published in 1946 after his death.⁶
• Decline, exile, and reassessment — after the First World War, neo-Kantians were identified with the old academic order and became targets for philosophers, including their own students, who wanted a fresh start. Hitler’s rise to power in 1933, the year Cassirer left Germany, nearly erased the movement’s institutional memory. Its students nonetheless included Rudolf Carnap, Hans Reichenbach, Martin Heidegger, and Hans-Georg Gadamer, and José Ortega y Gasset studied in Marburg with Cohen and Natorp before replacing Cohen’s primacy of pure thought with “human life.” Historians have since recovered the movement’s systematic arguments.⁷

Phenomenology and Existentialism

• Edmund Husserl (1859–1938) — founder of phenomenology: the philosophical study of structures of experience and consciousness; intentionality: all consciousness is consciousness of something; epoché (bracketing): suspending assumptions about the external world; Logical Investigations, Ideas I, Cartesian Meditations.
• Martin Heidegger (1889–1976) — Being and Time (1927): the question of Being; Dasein (“being-there”): the human mode of existence characterized by care, thrownness, and being-toward-death; authenticity vs falling into the “they-self” (das Man); ready-to-hand vs present-at-hand; later work on technology and language as “the house of Being.”
• Heidegger’s Being and Time (Sein und Zeit, 1927) — Division One: the analytic of Dasein’s everyday structure (world, care, the they-self); Division Two: temporality as the meaning of care; being-toward-death as the ownmost possibility that individualizes Dasein; resoluteness as authentic existence; the question of Being (Seinsfrage) as the orienting problem; Heidegger’s Nazi party membership has shadowed the reception of his work.
• Sartre’s Being and Nothingness (L’Être et le Néant, 1943) — distinguishes being-in-itself (opaque, self-identical things) from being-for-itself (consciousness, defined by lack and negativity); facticity (what we are thrown into) vs transcendence (our freedom to project beyond it); bad faith is the attempt to deny one’s freedom; the look of the Other objectifies us; “existence precedes essence.”
• Jean-Paul Sartre (1905–1980) — existentialism: “existence precedes essence” — humans have no fixed nature; radical freedom and responsibility; bad faith (mauvaise foi): self-deception about one’s freedom; Being and Nothingness (1943); Existentialism Is a Humanism (1945); Nausea (novel); committed Marxist; Nobel Prize declined.
• Simone de Beauvoir (1908–1986) — The Second Sex (1949): women are defined as the “Other” relative to men; “one is not born, but rather becomes, a woman”; existentialist feminism; situated freedom.
• Maurice Merleau-Ponty (1908–1961) — embodied cognition: perception and knowledge are rooted in bodily experience; Phenomenology of Perception; critique of Cartesian dualism and behaviorism.
• Karl Jaspers (1883–1969) — German-Swiss philosopher and psychiatrist; one of the founders of existentialism; concept of Existenz (authentic self-being reached through boundary situations: death, guilt, suffering, struggle); boundary situations reveal what cannot be mastered by science; Philosophy (3 vols., 1932); also contributed foundational work in psychopathology.
• José Ortega y Gasset (1883–1955) — Spanish philosopher; The Revolt of the Masses (1930): diagnosis of mass society’s leveling of culture; perspectivism: each life is an irreducible perspective on reality; “I am myself and my circumstance” (yo soy yo y mi circunstancia); major figure in Spanish-language philosophy.
• Albert Camus (1913–1960) — philosophy of the absurd: conflict between human need for meaning and the universe’s silence; The Myth of Sisyphus (1942); rejected both suicide and leap of faith; revolt as the proper response; Nobel Prize 1957.
• Hannah Arendt (1906–1975) — The Origins of Totalitarianism (1951); The Human Condition: distinction of labor, work, and action; banality of evil (Eichmann in Jerusalem): evil can be performed without malice by those who fail to think; vita activa vs vita contemplativa.
• Arendt’s The Human Condition (1958) — the vita activa has three modes: labor (biological necessity), work (durable artifacts that create a common world), and action (political speech and deed among equals); the public realm as the space of appearance; critique of modernity’s reduction of politics to economics and behavior; the concept of natality (new beginnings) as the political counterpart to mortality.
• Jacques Derrida (1930–2004) — founder of deconstruction; Of Grammatology (1967): Western philosophy privileges speech over writing (logocentrism); différance: meaning is always deferred and differentiated; no text has a fixed, determinate meaning; texts harbor contradictions their authors do not intend; supplement logic; major works: Writing and Difference, Margins of Philosophy, Specters of Marx.
• Michel Foucault’s genealogy — adapted from Nietzsche; reveals that what appears as timeless truth is historically contingent and shaped by power; Discipline and Punish (1975): the panopticon as a model for modern disciplinary society; the great confinement of the mad; the clinical gaze; biopower: modern states govern populations by managing life (birth, death, health, sexuality).
• Gilles Deleuze (1925–1995) — Difference and Repetition (1968): difference is primary, identity is derived; with Guattari, Anti-Oedipus (1972) critiques Freudian psychoanalysis and capitalism; rhizome vs tree as models of thought; assemblage theory; bodies without organs; A Thousand Plateaus (1980).
• Jürgen Habermas — The Theory of Communicative Action (1981): distinguishes instrumental reason (strategic, means-ends) from communicative rationality (oriented toward mutual understanding); the ideal speech situation (no coercion, only force of better argument); discourse ethics: norms are valid if all affected could agree under ideal conditions; critique of postmodernism.
• Max Horkheimer (1895–1973) — director of the Frankfurt Institute for Social Research and leading figure of the Frankfurt School of critical theory; co-authored Dialectic of Enlightenment (1944) with Theodor Adorno, arguing that the Enlightenment’s drive to dominate nature through instrumental reason leads to new forms of domination and the culture industry’s manipulation of mass consciousness; his Eclipse of Reason (1947) develops the critique of instrumental vs. substantive reason.

20th-Century Religious and Theological Philosophy

• Martin Buber (1878–1965) — Austrian-Israeli Jewish philosopher; I and Thou (Ich und Du, 1923): distinguished I-Thou relations (genuine, direct meeting between persons or between person and God) from I-It relations (instrumental, subject-object); dialogical philosophy; also wrote extensively on Hasidism and Zionist thought.
• Karl Barth (1886–1968) — Swiss Reformed theologian; founder of Neo-Orthodoxy (dialectical theology); Church Dogmatics (13 volumes): the massive systematic theology asserting the radical transcendence of God and the centrality of Christ; opposed liberal theology’s accommodation to culture; drafted the Barmen Declaration (1934) against Nazi influence on the German church.
• Reinhold Niebuhr (1892–1971) — American Protestant theologian; The Nature and Destiny of Man (1941–43); Christian Realism: human sinfulness (especially pride and will-to-power) must temper political idealism; critiqued both pacifism and liberal utopianism; influential on Cold War-era U.S. foreign policy thought.
• Paul Tillich (1886–1965) — German-American theologian and philosopher of religion; method of correlation: theology answers the existential questions philosophy poses; God as the Ground of Being rather than a being among beings; Systematic Theology (3 vols.) and The Courage to Be (1952); bridged existentialism and Christian theology.
• Jacques Maritain (1882–1973) — French Catholic philosopher; leading figure of Thomistic personalism (20th-century revival of Aquinas); Integral Humanism (1936): a Christian democratic political vision; contributed to the drafting of the Universal Declaration of Human Rights; French Ambassador to the Holy See.

Political and Social Philosophy

• Sun Tzu’s The Art of War (c. 5th century BCE) — attributed to the Chinese military strategist Sun Tzu; thirteen chapters on strategy, deception, and the philosophy of conflict; deeply Taoist in sensibility: the ideal general wins without fighting, adapts to circumstances rather than forcing them; canonical in East Asian philosophy and strategy; widely read in Western philosophy of war and business ethics.
• Thomas Hobbes (1588–1679) — Leviathan (1651): state of nature is “solitary, poor, nasty, brutish, and short”; social contract creates the sovereign; subjects surrender rights in exchange for security; political absolutism; psychological egoism: all human action is motivated by self-interest; the sovereign cannot be bound by the contract.
• Jean-Jacques Rousseau (1712–1778) — The Social Contract (1762): “man is born free and everywhere he is in chains”; general will (volonté générale) is the source of legitimate authority; civil religion; natural goodness of humans corrupted by society; Discourse on Inequality.
• John Rawls (1921–2002) — A Theory of Justice (1971): original position and veil of ignorance (people choose principles of justice without knowing their place in society); two principles: (1) equal basic liberties; (2) social and economic inequalities must satisfy fair equality of opportunity and the difference principle, which requires the greatest benefit to the least advantaged; equal basic liberties take priority, and fair opportunity takes priority over the difference principle; Political Liberalism (1993).⁸
• Rawls’s A Theory of Justice — revived normative political philosophy against utilitarian dominance; the original position is a hypothetical contractual situation designed to model fair conditions for choosing principles; reflective equilibrium: testing principles against considered judgments; justice as fairness as an alternative to both utilitarian and libertarian theories.
• Nozick’s Anarchy, State, and Utopia (1974) — libertarian response to Rawls; the minimal state (protection only) is justified but no more; entitlement theory: just distribution is whatever results from free exchanges of justly acquired holdings; the Wilt Chamberlain argument shows any patterned distribution will be upset by voluntary transactions; utopia as a framework: the minimal state enables diverse communities.
• Peter Singer’s Animal Liberation (1975) — extends utilitarian logic to non-human animals; speciesism is an arbitrary prejudice analogous to racism; capacity for suffering, not species membership, determines moral consideration; factory farming is morally unjustifiable; Famine, Affluence, and Morality (1972) argues affluent individuals are obligated to give to the point of marginal utility.
• Philippa Foot (1920–2010) — Natural Goodness (2001): revived Aristotelian naturalism in ethics; natural normativity: what counts as good for a living thing is determined by its natural kind; originator of the trolley problem (1967, “The Problem of Abortion and the Doctrine of Double Effect”).
• G. E. M. Anscombe (1919–2001) — Modern Moral Philosophy (1958): coined the term consequentialism; argued the concept of “moral obligation” without God is incoherent; urged return to virtue ethics; Intention (1957): a foundational text in action theory; student and literary executor of Wittgenstein.
• Robert Nozick (1938–2002) — Anarchy, State, and Utopia (1974): libertarian response to Rawls; entitlement theory of justice: holdings are just if acquired and transferred fairly; minimal state justified; Wilt Chamberlain argument against patterned principles; experience machine thought experiment.
• Jurgen Habermas (1929–) — communicative action: rationality is embedded in the pragmatics of speech; ideal speech situation; discourse ethics; The Theory of Communicative Action; Between Facts and Norms: deliberative democracy.
• Michel Foucault (1926–1984) — genealogy (after Nietzsche): historical analysis of power-knowledge regimes; Discipline and Punish: the panopticon as a model of modern disciplinary power; The History of Sexuality; Madness and Civilization; biopolitics: the state’s regulation of life and bodies.

Ethics: Key Frameworks and Problems

• Deontology — actions are right or wrong in themselves, regardless of consequences; Kant’s categorical imperative is the paradigm; rights-based theories; prima facie duties (W. D. Ross): a list of duties (fidelity, reparation, gratitude, justice, beneficence, non-maleficence) that can conflict.
• Consequentialism / Utilitarianism — rightness determined solely by outcomes; maximize aggregate welfare; act vs rule utilitarianism; preference utilitarianism (Peter Singer).
• Virtue ethics — Aristotle (Nicomachean Ethics); virtues are stable character traits lying between extremes (doctrine of the mean — e.g., courage as the mean between cowardice and rashness); contemporary revival by Elizabeth Anscombe (Modern Moral Philosophy, 1958), Alasdair MacIntyre (After Virtue, 1981), Philippa Foot.
• Trolley problem — Philippa Foot (1967): is it permissible to divert a trolley to kill one person to save five? The footbridge variant (Judith Jarvis Thomson): permissible to push a large man off a bridge to stop the trolley? Illustrates doctrine of double effect and the distinction between doing and allowing.
• Doctrine of double effect — an action with both good and bad effects is permissible if the good is intended and the bad is merely foreseen, not intended, and not disproportionate.
• Peter Singer (1946–) — Animal Liberation (1975): speciesism is unjustifiable; Famine, Affluence, and Morality (1972): we are obligated to give substantially to prevent suffering; effective altruism.
• Judith Jarvis Thomson (1929–2020) — violinist argument for abortion rights; trolley problem variants; self-defense and rights-forfeiture.

Epistemology: Key Problems and Concepts

• JTB account of knowledge — classical definition: knowledge is justified true belief; Gettier cases (Edmund Gettier, 1963): counterexamples showing JTB is insufficient.
• Gettier problem — Edmund Gettier’s 1963 three-page paper “Is Justified True Belief Knowledge?” presents cases where an agent has justified true belief but lacks knowledge (e.g., Smith believes Jones will get a job on good evidence; Jones does not but Smith coincidentally does; Smith’s belief “the man who will get the job has ten coins in his pocket” is justified and true but not knowledge); launched decades of responses including reliabilism, safety, and sensitivity conditions.
• The brain-in-a-vat — contemporary version of Cartesian skepticism: if you were a disembodied brain stimulated by a computer to have all your current experiences, you could not tell; externalist responses (Putnam): a brain-in-a-vat cannot refer to “brains” or “vats” in the normal sense, so the hypothesis is self-refuting; used by Nozick’s sensitivity condition and Williamson’s knowledge-first epistemology.
• The ship of Theseus — ancient puzzle (attributed to Plutarch): if a ship’s planks are replaced one by one until no original plank remains, is it still the same ship? If the old planks are reassembled, which is the original? Illustrates the problem of identity over time and material constitution; applied to personal identity debates.
• Buridan’s ass — a puzzle about rational choice: a donkey placed between equally attractive food sources cannot choose and starves. The familiar donkey example does not occur in Buridan’s surviving writings; it belongs to the later reception of debates about will and choice, with earlier versions of the underlying problem in Aristotle. Stanford Encyclopedia of Philosophy.
• The experience machine — Nozick (Anarchy, State, and Utopia, 1974): a machine that provides any experiences you desire (achievements, relationships, pleasures); would you plug in permanently? Most say no, suggesting people value actual states of affairs beyond subjective experience; a standard objection to hedonistic utilitarianism.
• The problem of universals — do properties like “redness” or “humanity” exist independently of particular things? Platonic realism: universals exist separately; Aristotelian realism: universals exist only in particulars; nominalism: only particulars exist, universals are names; trope theory: only particular instances of properties exist; central to medieval philosophy (Abelard, Ockham) and contemporary metaphysics.
• The is-ought problem (Hume’s guillotine) — from Treatise Book III: in every moral argument Hume observed authors moving from “is” statements to “ought” statements without explanation; this transition requires justification that naturalistic ethics cannot provide; foundation of non-cognitivism and emotivism; Moore’s naturalistic fallacy is related.
• The knowledge argument (Mary’s Room) — Frank Jackson (1982): Mary is a scientist who knows all physical facts about color perception but has lived her whole life in a black-and-white room; when released and she sees red for the first time, does she learn something new? If yes, physicalism is false (there are non-physical facts about qualia); epiphenomenalism is Jackson’s initial conclusion; he later recanted.
• Qualia and the hard problem — David Chalmers (1995): the “easy problems” of consciousness (explaining cognitive functions) are tractable in principle; the hard problem is explaining why there is subjective experience at all; qualia are the intrinsic, subjective properties of experience; property dualism and panpsychism are proposed solutions.
• The Chinese Room — John Searle, “Minds, Brains, and Programs” (1980): a person in a room follows rules to manipulate Chinese symbols, producing correct responses in Chinese without understanding any Chinese; challenges strong AI (the claim that running the right program is sufficient for understanding); systems reply: the whole system understands, even if the person does not; Searle’s counter: internalize the whole system, still no understanding.
• Zeno’s paradoxes — Achilles and the Tortoise: Achilles can never overtake the tortoise because he must always first reach where the tortoise was; the Arrow: at each instant an arrow in flight is at rest, so motion is impossible; the Dichotomy: to cross a room you must first cross half, then half of the remainder, ad infinitum; mathematical responses use convergent infinite series; philosophical debate continues about whether they are resolved by mathematics or whether something deeper is at stake.

• Pascal’s wager and objections — the many gods objection: the argument does not specify which God to believe in; belief is not voluntary: you cannot simply choose to believe; pragmatic inconsistency: basing belief on expected utility rather than evidence; Bayesian reformulations have tried to rescue the argument; the wager appears in Pensées fragment 418 (Lafuma numbering).

• Skepticism — Cartesian demon; brain-in-a-vat; contextualism (DeRose, Cohen): what counts as knowledge shifts with context; relevant alternatives theory.
• Foundationalism vs coherentism — foundationalism: knowledge rests on basic, non-inferential beliefs; coherentism: beliefs are justified by their coherence with a web of beliefs; reliabilism (Alvin Goldman): beliefs are justified if produced by reliable processes.
• A priori vs a posteriori — a priori: knowable independent of experience (logic, math); a posteriori: knowable only through experience.
• Analytic vs synthetic — analytic truths are true in virtue of meaning (“bachelors are unmarried”); synthetic truths depend on the world; Quine challenged this distinction.
• Epistemic injustice — Miranda Fricker: testimonial injustice (deflated credibility due to identity prejudice); hermeneutical injustice (gaps in collective interpretive resources harm certain groups); Epistemic Injustice (2007).

Philosophy of Mind and Language

• Mind-body problem — how does consciousness relate to the physical brain? Dualism (Descartes); identity theory: mental states are brain states; functionalism (Putnam): mental states are defined by causal roles, not physical substrate; eliminative materialism (Churchland): folk psychology will be replaced.
• Mary’s Room — see the detailed knowledge argument entry above under Epistemology; its central question is whether complete physical knowledge includes knowledge of experience.
• Philosophical zombies — David Chalmers: conceivable beings physically identical to us but lacking conscious experience; argues for the hard problem of consciousness: explaining why there is subjective experience.
• Chinese Room — see the detailed Chinese Room entry above under Epistemology for Searle’s argument, the systems reply, and his response.
• Speech act theory — J. L. Austin: How to Do Things with Words; locutionary, illocutionary, perlocutionary acts; performatives (utterances that do things, e.g., “I promise”); developed by John Searle.
• Descriptivism vs externalism — names refer via associated descriptions (Frege, Russell) vs via causal-historical chains (Kripke, Putnam).

Pragmatism

• Charles Sanders Peirce (1839–1914) — founder of pragmatism; pragmatic maxim: the meaning of a concept is its practical effects; semiotics: signs, objects, interpretants; fallibilism; founder of modern logic of relatives.
• William James (1842–1910) — Pragmatism (1907): truth is what it is useful to believe; radical empiricism; The Varieties of Religious Experience; will to believe: it is rational to believe when evidence is insufficient but the stakes are high.
• John Dewey (1859–1952) — instrumentalism: ideas are tools for problem-solving; progressive education: learning by doing; Democracy and Education; inquiry as the method of philosophy; transactionalism.

• Richard Rorty (1931–2007) — neo-pragmatism: rejected the idea of philosophy as a mirror of nature; Philosophy and the Mirror of Nature (1979); solidarity over objectivity; irony and the liberal society.

• Douglas Hofstadter’s Gödel, Escher, Bach: An Eternal Golden Braid (1979) — Pulitzer Prize-winning work exploring how self-reference and formal systems give rise to meaning and consciousness; weaves together Gödel’s incompleteness theorems, Escher’s visual paradoxes, and Bach’s musical structures as illustrations of strange loops; argues that the self and consciousness emerge from self-referential patterns in complex formal systems; canonical in philosophy of mind, cognitive science, and popular philosophy of mathematics.

Logic and Metaphysics

• Arguments, validity, and soundness — an argument offers premises as reasons for a conclusion. A deductively valid argument cannot have true premises and a false conclusion; a sound argument is valid and has true premises. Validity concerns the inference, not whether the premises happen to be true. Inductive arguments instead support conclusions with some degree of probability, so new evidence can change their strength. These distinctions apply when comparing arguments across philosophical traditions.⁹

• Modal logic — formal logic of possibility and necessity; possible worlds semantics (Kripke); de re vs de dicto modality.
• Free will — hard determinism: determinism is true and free will is impossible; compatibilism (Hume, Mill, Frankfurt): free will is compatible with determinism (acting from one’s own desires without coercion); libertarianism (in the free will sense): agent causation or indeterminism grounds free will; Frankfurt cases (Harry Frankfurt): an agent who could not have done otherwise can still be morally responsible.
• Harry Frankfurt (1929–2023) — moral philosopher at Princeton; On Bullshit (2005): a philosophical analysis distinguishing the bullshitter (who is indifferent to truth) from the liar (who knows the truth and subverts it), arguing bullshit is more corrosive to discourse than lying; higher-order desires (Freedom of the Will and the Concept of a Person, 1971): free will consists in having second-order volitions that endorse one’s first-order desires, grounding his compatibilism and Frankfurt cases about moral responsibility.
• atheism — the philosophical position that there is no god or gods; distinguished from agnosticism (the epistemic claim that god’s existence is unknowable); major philosophical arguments include the problem of evil (Hume, Mackie), the argument from divine hiddenness (Schellenberg: a loving God would not permit sincere nonbelief), and critiques of cosmological and ontological arguments; J. L. Mackie’s The Miracle of Theism (1982) is the canonical analytic treatment; positive atheism asserts nonexistence; negative atheism merely withholds assent.
• Daniel Dennett (1942–2024) — philosopher of mind and cognitive science at Tufts; Consciousness Explained (1991): proposes the multiple drafts model, arguing consciousness has no single “Cartesian theater” where experience is unified; eliminativist about qualia as traditionally conceived; Darwin’s Dangerous Idea (1995): defends universal Darwinism; Intuition Pumps and Other Tools for Thinking (2013); a leading figure in naturalistic philosophy of mind.
• Personal identity — Locke: psychological continuity (memory); fission cases and teleporter problems; Parfit (Reasons and Persons, 1984): personal identity is not what matters; reductionism about persons.
• Universals — the problem of universals: do properties (redness, roundness) exist independently? Realism (Plato, Armstrong): universals are real; nominalism: only particulars exist; conceptualism: universals are concepts.
• The Problem of Evil — if God is omnipotent, omniscient, and omnibenevolent, why does evil exist? Theodicy attempts a defense; free will defense (Alvin Plantinga): God allows evil to permit genuine freedom; skeptical theism: we cannot assess God’s reasons.
• Pascal’s Wager — Blaise Pascal (Pensées): even if the probability of God’s existence is small, the expected utility of belief is infinite; rational to believe; critics note the “many gods” objection.