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Science & History

History of Western Philosophy

From asking what water has to do with everything to arguing about what justice would look like behind a veil of ignorance

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Western philosophy begins with a claim that sounds almost too simple: a Greek trader in Miletus decided that everything is, at bottom, water. From that opening bid, thinkers built rival methods for finding the truth, arguing through Socrates' relentless questions, Plato's eternal Forms, and Aristotle's logic and categories, before Rome's Stoics and Epicureans turned philosophy into a guide for living. Christian scholars folded Greek logic into theology, then Renaissance humanists broke from that synthesis to recover the classical texts directly. Descartes and Bacon staked out rival routes to certain knowledge, Hobbes and Locke argued about what government owes the people who consent to it, and Kant tried to reconcile the two camps in one difficult book. The 19th century turned philosophy toward history, economics, and psychology, and the 20th century split it again, between philosophers who treated language and logic as the discipline's proper subject and others who asked what it means to exist as a free, mortal, and social being. This timeline traces the arguments, the texts, and the honest disputes over what a Presocratic fragment or a Socratic dialogue can actually prove.

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  1. c. 585 BCE
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    Best source: Thales of Miletus
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    Thales Declares That Everything Is Water

    Thales of Miletus, active in the early 6th century BCE, was the first thinker on record to ask what the natural world is fundamentally made of and to answer with a single material principle rather than a myth. Aristotle later credited him with declaring water to be the arche, the originating substance behind change, nutrition, and growth. Thales left no writings; everything attributed to him comes through later writers, chiefly Aristotle, more than two centuries afterward. Ancient tradition also credits him with predicting a solar eclipse, usually dated to 585 BCE, though modern scholars who have examined the claim conclude the specific mechanism ancient sources describe almost certainly could not have worked, even if Thales did have some general basis for anticipating the event.

    Why it matters: Aristotle treated Thales as the first person to inquire into nature's causes without invoking gods, making him the traditional starting point for Western philosophy as a distinct activity from myth-making. The three Milesian thinkers who followed him, Thales, Anaximander, and Anaximenes, gave later philosophy its first sense that inquiry into nature could be cumulative, with each answering and revising the one before.

    How we know: Nothing Thales wrote survives, if he wrote anything at all; his views reach us secondhand through Aristotle and later doxographers writing centuries after his death, so specific claims about his reasoning are reconstructions rather than direct quotations.

    Approximate dates: c. 624/620 to c. 546 BCE · Claimed first principle: Water (arche) · Associated eclipse: 585 BCE (traditional, disputed mechanism) · Earliest surviving testimony: Aristotle, writing over two centuries later

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    • Ancient Greece · See the Ancient Greece timeline for the wider Archaic-period world Thales and the Milesian thinkers worked in.
  2. c. early-to-mid 5th century BCE
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    Best source: Heraclitus
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    Heraclitus and Parmenides Split Philosophy Over Change

    Heraclitus of Ephesus, active around 500 BCE, argued that the world is defined by constant change, a doctrine later summarized as panta rhei, everything flows. His surviving fragment on rivers puts it directly: stepping into the same river, one encounters different waters flowing past each time. He also described a rational structure underlying that flux, the logos, which most people fail to grasp even though it is common to all. Parmenides of Elea, active in the early 5th century BCE, argued the opposite in his poem On Nature: what fully is cannot come from what is not, and what is cannot cease to be, so genuine change and motion are illusions produced by the senses rather than features of reality itself. Neither philosopher's work survives complete; both are known only through fragments quoted by later writers.

    Why it matters: Heraclitus and Parmenides set the two poles that later Greek philosophy had to answer to: is reality fundamentally in motion, or fundamentally fixed and unified. Plato's theory of Forms and Aristotle's account of substance and change both work, in different ways, to reconcile the two positions, making this quarrel a hinge point for everything that followed in ancient metaphysics.

    How we know: Both philosophers survive only in fragments, sentences and short passages quoted or paraphrased by later ancient authors, so reconstructing their arguments depends on how those quoting authors framed and preserved the material, a genuine limitation scholars are explicit about.

    Heraclitus, active: c. 500 BCE, Ephesus · Heraclitus's doctrine: Universal flux (panta rhei) and the logos · Parmenides's doctrine: Being is one, unchanging, and eternal · Parmenides's surviving work: Fragments of the poem On Nature

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    • Ancient Greece · See the Ancient Greece timeline for the broader Presocratic world these two philosophers argued within.
  3. c. mid-5th century BCE onward
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    Best source: Socrates
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    Socrates Practices the Elenchus in the Athenian Agora

    Socrates of Athens, born around 469 BCE, spent his adult life questioning fellow citizens in the Agora about virtue, justice, and knowledge, using a method now called the elenchus: cross-examining a stated position or definition until its holder's own commitments contradict it. Socrates wrote nothing himself. Everything known about his views comes secondhand, chiefly through the dialogues of his student Plato and the memoirs of Xenophon, and the two portraits do not always agree, a problem scholars call the Socratic problem. His method aimed less at teaching answers than at exposing unrecognized ignorance in people who assumed they already understood concepts like courage or piety.

    Why it matters: Socrates is credited with turning Greek philosophy from questions about nature toward questions about how to live, and the elenchus became the model for the question-and-answer form of Plato's dialogues and, more broadly, for treating unexamined belief as a target for scrutiny rather than something to accept on authority.

    How we know: Socrates left no writings, so his views are reconstructed entirely from later sources, Plato's dialogues and Xenophon's Socratic writings above all, with real disagreement among scholars about how much of Plato's Socrates reflects the historical man versus Plato's own philosophy voiced through him.

    Approximate birth year: 469 BCE (some sources: 470 BCE) · Wrote: Nothing; no texts by Socrates survive · Primary sources on him: Plato's dialogues, Xenophon's memoirs · Method: Elenchus (cross-examination/refutation)

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    • Ancient Greece · See the Ancient Greece timeline for classical Athens, the city where Socrates spent his entire life.
  4. 399 BCE
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    Best source: Socrates
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    Athens Tries and Executes Socrates

    In 399 BCE an Athenian jury tried Socrates on formal charges that he failed to recognize the gods the city recognized, introduced new divinities of his own, and corrupted the young through his teaching and questioning. He was convicted and sentenced to death. Rather than accept exile or escape, an option his friends offered him, Socrates accepted the sentence and was executed by drinking hemlock, choosing to honor what he took to be his obligations to the laws of Athens over his own survival.

    Why it matters: The trial turned Socrates's death into a founding scene for Western philosophy: a thinker executed by his own city for the practice of relentless questioning became, through Plato's account, the model of philosophical integrity under political pressure, and the episode has been read ever since as a test case for the tension between free inquiry and civic order.

    How we know: The trial and execution are described in detail in Plato's Apology, Crito, and Phaedo, and independently in Xenophon's Apology and Memorabilia, giving two separate ancient sources for the core facts, though the two authors present different versions of Socrates's courtroom speech.

    Year of trial and death: 399 BCE · Formal charges: Impiety and corrupting the youth · Method of execution: Drinking hemlock · Primary accounts: Plato's Apology/Crito/Phaedo; Xenophon's Apology

  5. c. 387 BCE
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    Best source: Plato
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    Plato Founds the Academy

    Plato, born around 428/427 BCE, gathered a circle of fellow philosophers in Athens that gradually developed into the institution known as the Academy, named for the sacred grove of the hero Academus where it met, just outside the city walls. Scholars place the founding anywhere from 387 to 383 BCE, depending on how they date Plato's return from his first trip to Syracuse, and the school appears to have grown gradually rather than opening on a fixed date. The Academy remained Plato's base for the rest of his life and continued after his death in 347 BCE, surviving as an institution until the Roman general Sulla destroyed its grove and gymnasium in 86 BCE, though thinkers identifying as Platonists carried on in Athens for centuries after that.

    Why it matters: The Academy is often treated as the first institution in the Western world organized specifically around sustained philosophical inquiry rather than religious or civic function, and it trained Aristotle, among many others, giving it an outsized role in shaping the following century of Greek thought.

    How we know: The Academy's location, general founding period, and eventual destruction under Sulla are documented across multiple ancient sources describing Plato's life and the site's later history, though the exact founding year is not fixed in any single ancient text and remains a matter of scholarly inference.

    Plato's dates: c. 428/427 to 348/347 BCE · Academy founded: c. 387-383 BCE (exact year disputed) · Location: Grove of Academus, outside Athens's walls · Academy destroyed: 86 BCE, by the Roman general Sulla

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    • Ancient Greece · See the Ancient Greece timeline for the political and military background of 4th-century BCE Athens.
  6. c. 375 BCE (Republic)
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    Best source: Plato
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    Plato Sets Out the Theory of Forms

    In dialogues including the Republic, Plato argued that the physical world perceived by the senses is defective and changeable, while a separate, more real realm of Forms, eternal and unchanging entities such as Equality, Beauty, and the Good, provides the true objects of knowledge and the standard the visible world only imperfectly resembles. In the Republic, Plato used this framework to argue that the ideally just city would be ruled by philosopher-kings, people equipped by education and temperament to grasp the Forms directly, and that justice in an individual soul consists of each part of the soul, reason, spirit, and appetite, performing its proper role rather than overriding the others.

    Why it matters: The theory of Forms gave Western philosophy its first developed account of abstract, unchanging objects of knowledge distinct from the physical world, a move that shaped centuries of debate over universals, mathematics, and the relationship between appearance and reality, and it made the Republic a foundational text for political philosophy's claim that justice can be defined independently of what any particular city happens to reward.

    How we know: The theory of Forms is set out across Plato's dialogues, especially the Republic, Phaedo, and Phaedrus, all of which survive complete in the original Greek and have been continuously read and commented on since antiquity, though scholars still debate how literally to read some of the theory's more mythic presentations.

    Key text: The Republic · Forms include: The Good, Beauty, Equality, Justice · Political claim: Rule by philosopher-kings · Justice defined as: Each part of the soul doing its own task

  7. 335 BCE
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    Best source: Aristotle
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    Aristotle Founds the Lyceum

    Aristotle arrived in Athens at age seventeen and studied at Plato's Academy for twenty years, remaining until Plato's death in 347 BCE. After years away, including time tutoring the young Alexander of Macedon, Aristotle returned to Athens in 335 BCE and founded his own school in a public exercise ground dedicated to Apollo Lykeios, which gave the school its name, the Lyceum. Aristotle rejected Plato's theory of Forms as a separate realm, arguing instead that forms exist embedded in particular things, and he built the first formal system of logic, an account of four kinds of causes (material, formal, efficient, and final), and an empirical program that extended into biology, drawing on direct observation rather than argument from first principles alone.

    Why it matters: Aristotle's split from Plato set up a rivalry between rationalist and empiricist approaches to knowledge that recurs throughout the history of philosophy, and his systematic logic, his biology, and his ethics of habituated virtue became, through later translation and commentary, one of the two central inheritances, alongside Plato's, that later philosophy in the Islamic world, the medieval Christian West, and the Renaissance kept returning to.

    How we know: Aristotle's works, more of which survive than for any other ancient philosopher except perhaps Plato, are extant as continuous texts likely based on his own lecture notes, and the founding of the Lyceum in 335 BCE is corroborated across independent ancient biographical sources.

    Aristotle's dates: 384-322 BCE · Years at Plato's Academy: 20 (age 17 until Plato's death, 347 BCE) · Lyceum founded: 335 BCE, Athens · Key contribution: First formal system of logic; four causes

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    • Ancient Greece · See the Ancient Greece timeline for Alexander of Macedon, whom Aristotle tutored before founding the Lyceum.
  8. c. 306 BCE
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    Best source: Epicurus
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    Epicurus Founds the Garden

    Epicurus, born in 341 BCE, founded philosophical schools in Mytilene and Lampsacus before moving to Athens around 306 BCE, where he purchased a property that became known as the Garden, a name that later applied to his school as a whole. Epicurus taught that pleasure is the highest good, but defined the goal of life not as active sensory indulgence but as ataraxia, freedom from mental disturbance, paired with freedom from bodily pain. Achieving this state, in his view, required banishing needless fears, particularly fear of the gods and of death, through a materialist physics that explained the world without divine intervention in human affairs.

    Why it matters: Epicureanism became one of the two great rival schools of Hellenistic ethics alongside Stoicism, and its redefinition of pleasure as tranquil freedom from disturbance rather than sensual excess shaped later hedonist philosophy, including its influence, via the Roman poet Lucretius, on early modern materialist and empiricist thought.

    How we know: Only a small portion of Epicurus's own writing survives, chiefly letters preserved by the biographer Diogenes Laertius; the fuller Epicurean system is reconstructed from these fragments together with the Roman poet Lucretius's later exposition, On the Nature of Things.

    Epicurus's dates: 341-270 BCE · Garden founded: c. 306 BCE, Athens · Highest good: Ataraxia (freedom from mental disturbance) · Later exponent: Lucretius, De Rerum Natura

  9. c. 300 BCE
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    Best source: Stoicism
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    Zeno of Citium Founds Stoicism at the Painted Porch

    Zeno of Citium, a voracious reader of Socratic dialogues who had studied under the Cynic philosopher Crates and been influenced by Plato's Academy and the Megarian school, founded the Stoic school in Athens around 300 BCE. It took its name from the Stoa Poikile, the painted porch in the Athenian agora where Zeno and his followers met and lectured. Stoic ethics held that virtue, prudence, justice, courage, and moderation, is the only true good, and that external things like health, wealth, and reputation are properly indifferent to a person's happiness. The Stoic aim, later summarized by the successor Cleanthes, was living in agreement with nature: aligning human reason with the rational order the Stoics believed structured the cosmos.

    Why it matters: Stoicism became one of the two dominant schools of Hellenistic ethics, alongside Epicureanism, and its emphasis on virtue as sufficient for a good life, indifference to external fortune, and universal reason shared across all people influenced Roman philosophy through Seneca, Epictetus, and Marcus Aurelius, and continues to circulate today as a practical ethical framework.

    How we know: Zeno's own writings do not survive; his teachings are known through later Stoic authors and through ancient biographical and doxographical sources, chiefly Diogenes Laertius, who preserved fragments and summaries of the early Stoa's doctrines generations after Zeno's death.

    Founder: Zeno of Citium · Founded: c. 300 BCE, Athens · Named for: The Stoa Poikile (painted porch) · Core ethical claim: Virtue is the only true good

  10. c. 397-400 CE
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    Best source: Augustine
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    Augustine Writes the Confessions

    Augustine, born in 354 CE in Roman North Africa and later bishop of Hippo, wrote the Confessions around 397 to 400 CE, a work that scholars describe as philosophy conducted in the form of autobiography rather than autobiography in the modern sense. In it Augustine wove Christian theology together with ideas drawn from Platonism, examining memory, time, the nature of evil, and the will, which he defined in relation to sin as the will to pursue or keep something unjustly. Augustine's broader philosophical position held that everything that exists is good insofar as God created it, with evil understood not as a positive substance but as a privation, a lack or corruption of a good that should be present.

    Why it matters: The Confessions fused Greek philosophical method with Christian doctrine at a formative moment for Western thought, giving later medieval philosophy a model for treating the examined inner life, memory, time, and the will, as legitimate philosophical subject matter, and its privation theory of evil shaped Christian philosophical responses to the problem of evil for centuries afterward.

    How we know: The Confessions survives complete in the original Latin and has been continuously copied, read, and commented on since antiquity; Augustine's dates and biography are independently corroborated by his own extensive surviving correspondence and other writings.

    Augustine's dates: 354-430 CE · Confessions written: c. 397-400 CE · Definition of sin: The will to keep or pursue something unjustly · Theory of evil: Privation (lack of a due good), not a substance

  11. 12th-13th centuries CE
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    Best source: Medieval Philosophy
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    The Latin West Recovers Aristotle Through Arabic Translation

    Through the 12th and 13th centuries, scholars working especially in Toledo and Sicily translated a large body of Aristotle's works into Latin, in many cases by way of Arabic versions rather than directly from the original Greek, giving philosophers in the Christian West access to far more of Aristotle's corpus than had been available since antiquity. The Andalusian philosopher Averroes (Ibn Rushd, 1126-1198) wrote extensive commentaries on Aristotle that were themselves translated into Latin and became indispensable guides for Christian scholars trying to understand the newly available texts. By around 1270 nearly all of Aristotle's genuine works, along with some spurious ones attributed to him, were available for study, and the recovery transformed the university curricula that were then taking shape across Europe.

    Why it matters: This translation movement supplied the philosophical material that made scholasticism possible: without a nearly complete Aristotle, and without Averroes's commentaries explaining how to read him, the syntheses attempted by Albert the Great and Thomas Aquinas in the following century would not have had the raw texts to work with, making the 12th and 13th century translators as consequential to medieval philosophy as any single philosopher of the period.

    How we know: The translation movement is documented through the surviving Latin manuscripts themselves, which can be dated and traced to specific translators and translation centers, and through university curriculum records from the period showing which Aristotelian texts entered the standard course of study and when.

    Period: 12th-13th centuries CE · Key centers: Toledo, Sicily · Key commentator translated: Averroes (Ibn Rushd), 1126-1198 · Near-complete corpus available by: c. 1270 CE

  12. 1265-1274 CE
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    Best source: Aquinas
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    Aquinas Synthesizes Aristotle and Christian Theology in the Summa Theologiae

    Thomas Aquinas, born around 1225 and a Dominican friar, composed the Summa Theologiae between 1265 and his death in 1274, a comprehensive work that draws on the newly recovered Aristotle to address nearly the whole of Aquinas's philosophical and theological concerns. Aquinas held that sound theology presupposes sound philosophical argument, and in the Summa's first part he offered five arguments, the Five Ways, for God's existence, three of them cosmological arguments resting on the claim that an infinite regress of causes is impossible. Each of the five ends with a version of the same formula: this is what everyone calls God.

    Why it matters: Aquinas's synthesis gave Aristotelian philosophy a permanent home inside Christian theology, and the Summa became the central reference work of scholastic philosophy for the rest of the medieval period and beyond, shaping Catholic philosophy so thoroughly that Thomism remains a live philosophical school today.

    How we know: The Summa Theologiae survives complete and has been continuously copied, printed, and studied since the 13th century; Aquinas's authorship and the work's composition dates are corroborated by contemporary biographical accounts from his Dominican order and by his other datable writings.

    Aquinas's dates: c. 1225-1274 · Summa Theologiae composed: 1265-1274 (unfinished at death) · Key argument: The Five Ways (proofs of God's existence) · Aquinas died: March 7, 1274

  13. c. 1440s CE
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    Best source: Lorenzo Valla
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    Lorenzo Valla and the Humanists Break With Scholasticism

    Lorenzo Valla, one of the most important humanists of the 15th century, attacked scholastic-Aristotelian philosophy from a linguistic standpoint in his Repastinatio dialectice et philosophie, arguing that scholastic logic had grown barren and needed to be re-cultivated with the tools of rhetoric and grammar instead. Around the same period Valla used philological analysis, comparing the Latin text against known Latin usage of different eras, to demonstrate that the Donation of Constantine, a document long used to justify papal claims to temporal rule, was a medieval forgery rather than a genuine 4th-century imperial grant. He also compared Jerome's Latin Vulgate translation of the New Testament against the original Greek text, laying groundwork for critical biblical scholarship. Civic humanists more broadly, including Leonardo Bruni, who produced fresh Latin translations of Aristotle and other Greek texts directly from Greek rather than through medieval intermediary translations, championed a return to classical sources and rhetoric, the studia humanitatis, over the technical logic of the scholastic universities.

    Why it matters: Valla's philology showed that careful attention to language and historical usage could overturn claims scholastic argument alone had left unchallenged for centuries, and the wider humanist turn toward classical texts and rhetorical education broke scholasticism's hold on European intellectual life, opening space for the more text-critical, historically minded philosophy that would follow in the Renaissance and early modern period.

    How we know: Valla's works, including the Repastinatio and his treatise on the Donation of Constantine, survive and have been studied continuously since the Renaissance; his philological arguments against the Donation's authenticity have been independently confirmed by later historians examining the document's language and content.

    Valla's dates: c. 1406-1457 · Key humanist slogan: Studia humanitatis (the humanities) · Valla's philological proof: Exposed the Donation of Constantine as a forgery · Fellow humanist named: Leonardo Bruni (translated Aristotle from Greek)

  14. 1620
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    Best source: Francis Bacon
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    Bacon Argues for Induction in the Novum Organum

    Francis Bacon, born in 1561, published the Novum Organum in October 1620 as the second part of his larger Instauratio Magna, a planned six-part program for reforming human knowledge. Against what he considered the sterile deductive logic and reliance on syllogisms inherited from Aristotle and the medieval schoolmen, Bacon argued for building knowledge inductively, moving step by step from particular observations toward general axioms rather than starting from the most general principles and reasoning downward. He diagnosed four systematic sources of error that distort human reasoning, which he called idols of the mind: the Idols of the Tribe, the Cave, the Marketplace, and the Theatre, covering biases built into human nature, individual temperament, language, and inherited philosophical systems respectively.

    Why it matters: Bacon's case for induction and controlled observation, and his diagnosis of the biases that corrupt untrained reasoning, helped set the methodological terms for the Scientific Revolution that unfolded across the following decades, giving experimental science a philosophical rationale independent of scholastic Aristotelianism.

    How we know: The Novum Organum survives complete in its original 1620 Latin printing and has been continuously translated, studied, and reprinted since; Bacon's biography and the work's composition and publication history are corroborated by his extensive surviving correspondence and other writings from the period.

    Bacon's dates: 1561-1626 · Novum Organum published: October 1620 · Part of: The Instauratio Magna (Part II) · Four idols: Tribe, Cave, Marketplace, Theatre

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    • The Scientific Revolution · See the Scientific Revolution timeline for how Bacon's inductive method fed directly into the experimental science of the following decades.
  15. 1641
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    Best source: Rene Descartes
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    Descartes Publishes the Meditations and the Cogito

    Rene Descartes, born in 1596, published the Meditations on First Philosophy in Latin in 1641, with six sets of objections from other thinkers and his own replies included in the same volume. Descartes's method was to treat as false any belief that admitted even the slightest doubt, stripping away sensory belief, mathematical belief, and even the belief that he had a body, until he reached one claim that survived the doubt itself: that the very act of thinking proved his own existence, summarized in the phrase cogito, ergo sum, I think, therefore I am. From that foundation Descartes argued that mind and body are really distinct substances, the mind essentially a thinking thing and the body essentially an extended, divisible thing, a position that created what is still called the mind-body problem.

    Why it matters: The Meditations relocated the starting point of philosophy from external authority, whether Aristotle or scripture, to the individual thinker's own certainty, making Descartes a founding figure of modern rationalism and setting the mind-body relationship as a central problem that later philosophy of mind has never fully closed.

    How we know: The Meditations survives complete in its original 1641 Latin edition alongside the objections and replies exchanged with contemporary critics including Hobbes and Arnauld, giving historians a documented record of how Descartes's contemporaries responded to the argument as it was published.

    Descartes's dates: 1596-1650 · Meditations published: 1641 (Latin, first edition) · Central claim: Cogito, ergo sum (I think, therefore I am) · Metaphysical position: Mind-body dualism

  16. 1651
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    Best source: Hobbes's Moral and Political Philosophy
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    Hobbes Argues the State of Nature Is a War of All Against All

    Thomas Hobbes, born in 1588, published Leviathan in English in 1651, arguing that without a common power to keep people in order, human life would exist in a state of war, in his words a war of every man against every man, in which nothing could count as unjust because no shared authority defined justice. Hobbes concluded that the only way out of this condition was for individuals to covenant together and transfer their rights to an absolute sovereign, an arrangement he called sovereignty by institution, in exchange for the peace and security that only such an authority could guarantee. Without that authority, Hobbes argued, life would be, in his famous phrase, solitary, poor, nasty, brutish, and short.

    Why it matters: Leviathan gave political philosophy its most influential statement of social contract theory grounded not in natural sociability but in fear and self-interest, and its argument that legitimate political authority derives from the consent of the governed, even while justifying an absolute sovereign, set the terms that Locke and later contract theorists would argue against directly.

    How we know: Leviathan survives complete in its original 1651 English printing, along with a later Latin edition Hobbes prepared in 1668, and the work's arguments and reception among contemporary readers and critics are documented in surviving correspondence and published responses from the period.

    Hobbes's dates: 1588-1679 · Leviathan published: 1651 (English); Latin edition, 1668 · State of nature: "War of every man against every man" · Solution proposed: Absolute sovereign by social contract

  17. 1739-1740
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    Best source: David Hume
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    Hume Argues Reason Cannot Justify Induction

    David Hume, born in 1711, published the first two books of A Treatise of Human Nature anonymously in 1739, with the third book following in 1740, before he was thirty years old. In it Hume argued that reasoning from past experience to future events, induction, has no rational foundation: we cannot prove that the future will resemble the past without already assuming the very principle we are trying to establish. Hume concluded that our confidence in cause and effect rests not on reasoned demonstration but on custom or habit, a psychological tendency to expect repetition based on past regularity. He also drew a sharp division, now called Hume's Fork, between relations of ideas, truths knowable through pure thought alone such as mathematics, and matters of fact, claims about the world that depend on experience and can always be coherently denied.

    Why it matters: Hume's argument against a rational foundation for induction became one of philosophy's most durable problems, still called the problem of induction, and it directly provoked Kant's attempt to rescue the possibility of necessary knowledge about experience, making Hume's Treatise a hinge between early modern empiricism and Kant's critical philosophy.

    How we know: The Treatise survives complete in its original 1739-1740 printing; Hume later judged the work a commercial failure and reworked its arguments into the more widely read Enquiry Concerning Human Understanding, giving scholars two datable versions of the same core arguments to compare.

    Hume's dates: 1711-1776 · Treatise published: Books 1-2, 1739; Book 3, 1740 · Problem raised: The problem of induction · Key distinction: Hume's Fork (relations of ideas vs. matters of fact)

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    • The Enlightenment · See the Enlightenment timeline for Hume's later restatement of this argument in the 1748 Enquiry Concerning Human Understanding.
  18. 1781
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    Best source: Immanuel Kant
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    Kant Attempts a Copernican Revolution in Philosophy

    Immanuel Kant, born in 1724, published the Critique of Pure Reason in 1781, revising it for a second edition in 1787, attempting to answer Hume's skepticism about induction and causation while preserving the possibility of necessary, universal knowledge. Kant proposed treating the problem the way Copernicus had treated planetary motion: rather than assuming all cognition must conform to objects as they are in themselves, he proposed that objects, as we experience them, must conform to the basic structures of the human mind. This produced Kant's distinction between phenomena, things as they appear to us, structured by space, time, and the categories of the understanding, and noumena, things as they are in themselves, which Kant held are permanently outside the reach of human knowledge.

    Why it matters: The Critique of Pure Reason attempted to synthesize the rationalist and empiricist traditions that had run in parallel since Descartes and Locke, and its influence set the terms for most philosophy that followed it in the 19th and 20th centuries, from German idealism through phenomenology, even among philosophers who rejected Kant's specific conclusions outright.

    How we know: The Critique of Pure Reason survives in both its 1781 and revised 1787 editions, allowing scholars to compare Kant's own changes to the argument directly; his correspondence and other published works from the period corroborate the text's composition and reception among contemporaries.

    Kant's dates: 1724-1804 · Critique of Pure Reason published: 1781 (1st edition), 1787 (2nd edition) · Kant's framing: A "Copernican revolution" in philosophy · Key distinction: Phenomena (appearances) vs. noumena (things-in-themselves)

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    • The Enlightenment · See the Enlightenment timeline for Kant's 1784 essay answering the question "What Is Enlightenment?", written three years after this Critique.
  19. 1789-1861
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    Best source: Jeremy Bentham
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    Bentham and Mill Build the Case for Utilitarianism

    Jeremy Bentham, born in 1748, set out classical utilitarian theory in An Introduction to the Principles of Morals and Legislation, printed in 1780 and published with additions in 1789, arguing that the principle of utility approves or disapproves of every action according to whether it tends to increase or decrease the happiness of those affected, a standard he summarized as the greatest happiness of the greatest number. Bentham proposed a felicific calculus for measuring pleasure and pain by dimensions including intensity, duration, and certainty. John Stuart Mill, born in 1806, revised and defended the theory in his 1861 essay Utilitarianism, arguing against Bentham's purely quantitative approach that pleasures differ in quality as well as amount, famously concluding that it is better to be a human being dissatisfied than a pig satisfied, since intellectual and moral pleasures rank higher than merely bodily ones. In On Liberty, published in 1859, Mill separately argued for the harm principle, that the only justification for restricting a person's liberty is to prevent harm to others, not to protect the person from themselves.

    Why it matters: Utilitarianism became one of the most influential ethical theories in the modern world, providing the philosophical basis for cost-benefit reasoning in law and public policy, while Mill's revision opened a durable internal debate over whether pleasure can really be ranked by quality and not just quantity, a question utilitarian philosophers still argue over today.

    How we know: Both Bentham's Introduction to the Principles of Morals and Legislation and Mill's Utilitarianism and On Liberty survive in their original 18th and 19th century printings and have been in continuous print and scholarly use since publication.

    Bentham's dates: 1748-1832 · Mill's dates: 1806-1873 · Key texts: Introduction to Morals and Legislation (1789); Utilitarianism (1861); On Liberty (1859) · Mill's revision: Higher vs. lower quality pleasures

  20. 1807
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    Best source: Hegel's Social and Political Philosophy
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    Hegel Traces the Development of Spirit Through History

    Georg Wilhelm Friedrich Hegel, born in 1770, published the Phenomenology of Spirit in 1807, tracing how consciousness develops through successive stages of experience, each one exposed as incomplete and superseded, or in Hegel's term aufgehoben, negated yet preserved, by the next. Hegel's account, contrary to the popular shorthand of thesis-antithesis-synthesis, which is not his own terminology, works through what he called determinate negation. Applied to world history, Hegel argued that history is the necessary development, arising out of the concept of freedom, of successive stages of reason realizing itself through the actions of peoples, states, and individual historical actors, a teleological account of history later adapted, with the idealism stripped out, by Karl Marx.

    Why it matters: Hegel's claim that history has a rational, developmental structure, moving toward greater freedom and self-consciousness through conflict and resolution, gave 19th-century philosophy a new way to think about historical change as meaningful rather than merely sequential, and his framework, reworked in materialist terms, became the direct model for Marx's theory of history.

    How we know: The Phenomenology of Spirit survives complete in its original 1807 German printing and has been continuously translated and studied since; Hegel's biography and the work's composition are corroborated by his university lecture records and surviving correspondence from Jena, where he finished the book.

    Hegel's dates: 1770-1831 · Phenomenology of Spirit published: 1807 · Method: Determinate negation (not "thesis-antithesis-synthesis") · Later adapted by: Karl Marx (materialist reworking)

  21. 1848-1867
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    Best source: Karl Marx
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    Marx and Engels Recast Philosophy as the Critique of Capital

    Karl Marx, born in 1818, and Friedrich Engels published The Communist Manifesto in 1848, and Marx published the first volume of Capital (Das Kapital) in 1867, laying out a theory of historical materialism in which the primary direction of social explanation runs from a society's material conditions of production to its social forms and, in turn, to its forms of consciousness, reversing Hegel's idealist account of history as driven by the development of Spirit. Marx's analysis in Capital begins from the commodity and builds an account of capitalism in which workers experience alienated labor, work that could in principle be creative and fulfilling but is instead organized in ways that estrange workers from what they produce.

    Why it matters: Marx's materialist reworking of Hegel's account of history, combined with his critique of capitalism as inherently exploitative, became one of the most consequential bodies of thought in modern history, directly shaping 20th-century political movements and revolutions on a scale matched by few other philosophers.

    How we know: The Communist Manifesto and Capital survive in their original 19th-century editions and have been continuously translated, printed, and studied since publication; Marx and Engels's collaboration and the works' composition history are corroborated by their extensive surviving correspondence.

    Marx's dates: 1818-1883 · Communist Manifesto published: 1848 (with Friedrich Engels) · Capital, Volume 1 published: 1867 · Core theory: Historical materialism

  22. 1879-1922
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    Best source: Gottlob Frege
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    Frege, Russell, and Wittgenstein Turn Philosophy Toward Logic and Language

    Gottlob Frege, born in 1848, published the Begriffsschrift in 1879, constructing the first fully axiomatic system of what would become modern predicate logic and developing an influential distinction between the sense and reference of linguistic expressions. Bertrand Russell, born in 1872, built on this foundation with Alfred North Whitehead in the three-volume Principia Mathematica, published between 1910 and 1913, attempting to derive mathematics from logic, and developed a theory of logical atomism holding that the world consists of a complex of simple logical facts that an ideal language could describe directly. Ludwig Wittgenstein, born in 1889, studied under Russell and published the Tractatus Logico-Philosophicus in German in 1921 and English in 1922, arguing that language works by picturing facts about the world and that many traditional philosophical problems arise from language misused beyond the limits where it can picture anything meaningful.

    Why it matters: This sequence of work founded analytic philosophy as a distinct tradition centered on logical and linguistic analysis rather than system-building metaphysics, and Frege's influence on both Russell and Wittgenstein directly shaped a style of philosophy that would dominate English-language academic philosophy through most of the 20th century.

    How we know: The Begriffsschrift, Principia Mathematica, and the Tractatus all survive in their original early-20th-century editions and have been continuously studied since publication; the direct lines of influence between the three thinkers, Frege on Russell, both on Wittgenstein, are documented in their own published acknowledgments and surviving correspondence.

    Frege's dates: 1848-1925 · Russell's dates: 1872-1970 · Wittgenstein's dates: 1889-1951 · Key texts: Begriffsschrift (1879); Principia Mathematica (1910-13); Tractatus (1921/22)

  23. 1883-1887
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    Best source: Friedrich Nietzsche
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    Nietzsche Declares the Death of God

    Friedrich Nietzsche, born in 1844, wrote Thus Spoke Zarathustra in four parts between 1883 and 1885 and developed his critique of morality further in works including On the Genealogy of Morals in 1887. Nietzsche's pronouncement that God is dead argued that belief in the Christian God had become unbelievable in modern European culture, and that the whole of European morality built on that faith faced collapse as a result, whether or not people recognized it yet. In the Genealogy, Nietzsche traced what he called a slave revolt in morality, arguing that the priestly class, resentful of the power held by a noble warrior class, inverted the noble evaluation of good and bad into a new opposition between good and evil that redefined weakness as virtue. Nietzsche opposed both moral systems with his concept of the will to power, describing it in one late formulation as what is good: everything that heightens the feeling of power in man.

    Why it matters: Nietzsche's argument that European morality had lost its foundational belief without losing its moral demands became one of the most influential diagnoses of modern secularization, and his genealogical method, tracing moral concepts to the historical power struggles that produced them rather than treating them as timeless truths, shaped 20th-century philosophy from existentialism through poststructuralism.

    How we know: Nietzsche's major works survive in their original German editions from the 1880s and have been continuously translated and studied since; his biography, including his final years of mental and physical decline until his death in 1900, is corroborated by extensive surviving correspondence and contemporary accounts.

    Nietzsche's dates: 1844-1900 · Thus Spoke Zarathustra: Published in parts, 1883-1885 · On the Genealogy of Morals: 1887 · Key concepts: Death of God; slave revolt in morality; will to power

  24. c. 1900-1913
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    Best source: Edmund Husserl
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    Husserl Founds Phenomenology

    Edmund Husserl, born in 1859, developed phenomenology as a rigorous method for describing the structures of conscious experience directly, rather than beginning from inherited metaphysical or scientific assumptions about the mind. Central to his method was intentionality, the claim that consciousness is always consciousness of something, directed toward objects, so that an experience cannot be fully described without describing what it is an experience of. Husserl summarized his method's demand with the phrase back to the things themselves, insisting that philosophy could not rest content with inherited concepts or vague, secondhand intuitions but had to return to direct experience as its evidence base.

    Why it matters: Husserl's method gave 20th-century continental philosophy its foundational technique, and his students and readers, including Heidegger, Sartre, and de Beauvoir, each adapted phenomenological description to different ends, making Husserl's program the shared starting point for existentialism as it developed over the following decades.

    How we know: Husserl's major works survive in their original German editions and in the Husserl Archives, which preserve a large body of his research manuscripts alongside the works he published during his lifetime, allowing scholars to trace the development of his method in detail.

    Husserl's dates: 1859-1938 · Core method: Phenomenology (description of conscious experience) · Key concept: Intentionality (consciousness of something) · Later influenced: Heidegger, Sartre, de Beauvoir

  25. 1927
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    Best source: Martin Heidegger
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    Heidegger Reopens the Question of Being, Then Joins the Nazi Party

    Martin Heidegger, born in 1889, published Being and Time in 1927, an unfinished but hugely influential treatise whose declared aim was the concrete working out of the question of the meaning of Being, a question Heidegger argued Western philosophy had asked at its outset and then largely forgotten. Heidegger's central analysis focused on Dasein, his term for the distinctively human way of existing, whose basic structure he described as being-in-the-world, always already involved with a surrounding world and with other people rather than standing apart from it as a detached observer. In May 1933, several years after the book's publication, Heidegger joined the Nazi Party, a decision one biographer describes as a matter of conviction rather than opportunism, and one that has permanently complicated how his philosophical legacy is read.

    Why it matters: Being and Time reshaped 20th-century continental philosophy's approach to human existence, influencing existentialism, hermeneutics, and later poststructuralist thought, while Heidegger's Nazi Party membership has made him a central case study in the unresolved question of whether a philosopher's political commitments can or should be separated from the philosophical work itself.

    How we know: Being and Time survives in its original 1927 German edition and has been continuously translated and studied since; Heidegger's Nazi Party membership is documented in party records and in his own wartime rectoral address at the University of Freiburg, independently corroborated by postwar historical research into his personal and institutional conduct.

    Heidegger's dates: 1889-1976 · Being and Time published: 1927 · Central concept: Dasein, being-in-the-world · Joined Nazi Party: May 1933

  26. 1943-1949
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    Best source: Jean-Paul Sartre
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    Sartre and de Beauvoir Build an Existentialist Ethics of Freedom

    Jean-Paul Sartre, born in 1905, published Being and Nothingness in 1943, arguing that for human beings existence precedes essence, meaning people are not born with a fixed nature to fulfill but must create their own meaning through free choices, a freedom Sartre described as inescapable: people are condemned to be free, bearing full responsibility and anguish for their situation. Sartre illustrated the alternative, bad faith, through the example of a waiter who plays at being a waiter as though it were a fixed identity rather than a role he has chosen. Simone de Beauvoir, born in 1908, published The Second Sex in 1949, applying and extending existentialist method to argue that one is not born, but rather becomes, a woman, meaning femininity is a social and historical construction rather than a biological destiny, and developing an ethics in which willing one's own freedom requires willing the freedom of others as well.

    Why it matters: Sartre's existentialism and de Beauvoir's feminist extension of it gave mid-20th-century philosophy its most publicly influential account of human freedom and responsibility, and The Second Sex became a founding text of second-wave feminist theory, arguing that categories long treated as natural, like womanhood, are in fact produced by history and society.

    How we know: Being and Nothingness and The Second Sex survive in their original 1943 and 1949 French editions and have been continuously translated and studied since publication; Sartre and de Beauvoir's decades-long intellectual partnership and mutual influence are documented in their extensive published correspondence and memoirs.

    Sartre's dates: 1905-1980 · de Beauvoir's dates: 1908-1986 · Being and Nothingness published: 1943 · The Second Sex published: 1949

  27. 1971
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    Best source: John Rawls
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    Rawls Reimagines Justice From Behind a Veil of Ignorance

    John Rawls, born in 1921, published A Theory of Justice in 1971, reviving the social contract tradition that had lain largely dormant in political philosophy since the 18th century. Rawls proposed a thought experiment, the original position, in which representatives of real citizens agree on principles of justice from behind a veil of ignorance that deprives them of knowledge of the race, class, gender, and social position of the people they represent, forcing them to choose principles that would be acceptable no matter which position in society they personally ended up occupying. From this position, Rawls argued, rational representatives would choose two principles: an equal claim to the fullest possible scheme of basic liberties for all, and a rule, the difference principle, permitting social and economic inequalities only when they work to the greatest benefit of society's least advantaged members.

    Why it matters: A Theory of Justice reestablished normative political philosophy as a central, rigorous field within academic philosophy after decades in which many analytic philosophers treated ethics and politics as outside philosophy's proper scope, and Rawls's two principles became the reference point that virtually all subsequent work in political philosophy, whether building on them or arguing against them, has had to address.

    How we know: A Theory of Justice survives in its original 1971 edition and Rawls's own later revised edition; his arguments and their reception are documented in the vast subsequent secondary literature responding directly to the book, beginning almost immediately after its publication.

    Rawls's dates: 1921-2002 · A Theory of Justice published: 1971 · Key device: The original position and veil of ignorance · Second principle: The difference principle

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