In his use of critical reasoning, by his unwavering commitment to truth, and through the vivid example of his own life, fifth-century Athenian Socrates set the standard for all subsequent Western philosophy. Since he left no literary legacy of his own, we are dependent upon contemporary writers like Aristophanes and Xenophon for our information about his life and work. As a pupil of Archelaus during his youth, Socrates showed a great deal of interest in the scientific theories of Anaxagoras, but he later abandoned inquiries into the physical world for a dedicated investigation of the development of moral character. Having served with some distinction as a soldier at Delium and Amphipolis during the Peloponnesian War, Socrates dabbled in the political turmoil that consumed Athens after the War, then retired from active life to work as a stonemason and to raise his children with his wife, Xanthippe. After inheriting a modest fortune from his father, the sculptor Sophroniscus, Socrates used his marginal financial independence as an opportunity to give full-time attention to inventing the practice of philosophical dialogue.

For the rest of his life, Socrates devoted himself to free-wheeling discussion with the aristocratic young citizens of Athens, insistently questioning their unwarranted confidence in the truth of popular opinions, even though he often offered them no clear alternative teaching. Unlike the professional Sophists of the time, Socrates pointedly declined to accept payment for his work with students, but despite (or, perhaps, because) of this lofty disdain for material success, many of them were fanatically loyal to him. Their parents, however, were often displeased with his influence on their offspring, and his earlier association with opponents of the democratic regime had already made him a controversial political figure. Although the amnesty of 405 forestalled direct prosecution for his political activities, an Athenian jury found other charges—corrupting the youth and interfering with the religion of the city—upon which to convict Socrates, and they sentenced him to death in 399 B.C.E. Accepting this outcome with remarkable grace, Socrates drank hemlock and died in the company of his friends and disciples.

Our best sources of information about Socrates's philosophical views are the early dialogues of his student Plato, who attempted there to provide a faithful picture of the methods and teachings of the master. (Although Socrates also appears as a character in the later dialogues of Plato, these writings more often express philosophical positions Plato himself developed long after Socrates's death.) In the Socratic dialogues, his extended conversations with students, statesmen, and friends invariably aim at understanding and achieving virtue {Gk. αρετη [aretê]} through the careful application of a dialectical method that employs critical inquiry to undermine the plausibility of widely-held doctrines. Destroying the illusion that we already comprehend the world perfectly and honestly accepting the fact of our own ignorance, Socrates believed, are vital steps toward our acquisition of genuine knowledge, by discovering universal definitions of the key concepts governing human life.

The son of wealthy and influential Athenian parents, Plato began his philosophical career as a student of Socrates. When the master died, Plato travelled to Egypt and Italy, studied with students of Pythagoras, and spent several years advising the ruling family of Syracuse. Eventually, he returned to Athens and established his own school of philosophy at the Academy. For students enrolled there, Plato tried both to pass on the heritage of a Socratic The Academy style of thinking and to guide their progress through mathematical learning to the achievement of abstract philosophical truth. The written dialogues on which his enduring reputation rests also serve both of these aims.

In his earliest literary efforts, Plato tried to convey the spirit of Socrates's teaching by presenting accurate reports of the master's conversational interactions, for which these dialogues are our primary source of information. Early dialogues are typically devoted to investigation of a single issue, about which a conclusive result is rarely achieved. 

Although they continue to use the talkative Socrates as a fictional character, Platothe middle dialogues of Plato develop, express, and defend his own, more firmly established, conclusions about central philosophical issues. Beginning with the Μενων (Meno), for example, Plato not only reports the Socratic notion that no one knowingly does wrong, but also introduces the doctrine of recollection in an attempt to discover whether or not virtue can be taught. 

The masterpiece among the middle dialogues is Plato's Πολιτεια (Republic). It begins with a Socratic conversation about the nature of justice but proceeds directly to an extended discussion of the virtues (Gk. αρετη [aretê]) of justice (Gk. δικαιωσυνη [dikaiôsunê]), wisdom (Gk. σοφια [sophía]), courage (Gk. ανδρεια [andreia]), and moderation (Gk. σωφρσυνη [sophrosúnê]) as they appear both in individual human beings and in society as a whole. PlatoThis plan for the ideal society or person requires detailed accounts of human knowledge and of the kind of educational program by which it may be achieved by men and women alike, captured in a powerful image of the possibilities for human life in the allegory of the cave. 

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Born at Stagira in northern Greece, Aristotle was the most notable product of the educational program devised by Plato; he spent twenty years of his life studying at the Academy. When Plato died, Aristotle returned to his native Macedonia, where he is supposed to have participated in the education of Philip's son, Alexander (the Great). He came back to Athens with Alexander's approval in 335 and established his own school at the Lyceum, spending most of the rest of his life engaged there in research, teaching, and writing. His students acquired the name "peripatetics" from the master's habit of strolling about as he taught. Although the surviving works of Aristotle probably represent only a fragment of the whole, they include his investigations of an amazing range of subjects, from logic, philosophy, and ethics to physics, biology, psychology, politics, and rhetoric. Aristotle appears to have thought through his views as he wrote, returning to significant issues at different stages of his own development. The result is less a consistent system of thought than a complex record of Aristotle's thinking about many significant issues.

Although Aristotle's achievements in logic, metaphysics, and natural science are significant, he is perhaps best remembered for his several efforts to explain how moral conduct contributes to the good life for human agents, including the Εθικη Ευδαιμονης (Eudemian Ethics) and the Magna Moralia, but the most complete surviving statement of his views on morality occurs in the Εθικη Νικομαχοι (Nicomachean Ethics). There he considered the natural desire to achieve happiness, described the operation of human volition and moral deliberation, developed a theory of each virtue as the mean between vicious extremes, discussed the value of three kinds of friendship, and defended his conception of an ideal life of intellectual pursuit.

But on Aristotle's view, the lives of individual human beings are invariably linked together in a social context. In the Περι Πολις (Politics) he speculated about the origins of the state, described and assessed the relative merits of various types of government, and listed the obligations of the individual citizen. He may also have been the author of a model Πολιτειας Αθηναων (Constitution of Athens), in which the abstract notion of constitutional government is applied to the concrete life of a particular society.

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Born to an aristocratic family living in Roccasecca, Italy, Thomas Aquinas joined the Dominican order while studying philosophy and theology at Naples. Later he pursued additional studies in Paris and Köln, where he was exposed to Aristotelean thought by Albert the Great and William of Moerbeke. During the rest of his life, he taught at Paris and Rome, writing millions of words on philosophical and theological issues and earning his reputation among the scholastics as "the angelic doctor." 

Aquinas developed in massive detail a synthesis of Christianity and Aristotelian philosophy that became the official doctrine of Roman Catholic theology in 1879. His literary activity stopped abruptly as the result of a religious experience a few months before his death.Aquinas.

Aquinas's unfinished Summa Theologica (1265-1273) represents the most complete statement of his philosophical system. The sections of greatest interest for philosophy include his views on the nature of god, the five ways to prove god's existence, and his exposition of natural law. Although matters of such importance should be accepted on the basis of divine revelation alone, Aquinas held, it is at least possible (and perhaps even desirable) in some circumstances to achieve genuine knowledge of them by means of the rigorous application of human reason. 

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After receiving an education in mathematics, classics, and law at La Flèche and Poitiers, René Descartes embarked on a brief career in military service in Holland and Bavaria. Unsatisfied with scholastic philosophy and troubled by skepticism of the sort expounded by Montaigne, Descartes soon conceived a comprehensive plan for applying mathematical methods in order to achieve perfect certainty in human knowledge. During a twenty-year period of secluded life in Holland, he produced the body of work that secured his philosophical reputation. Descartes moved to Sweden in 1649, but did not survive his first winter there.

Descartes finally presented (in French) his rationalist vision of the progress of human knowledge in the Discours de la méthode pour bien conduire sa Raison et chercher la Vérité dans les Sciences (Discourse on Method) (1637). In this expository essay, Descartes assessed the deficient outcomes of a traditional education, proposed a set of rules with which to make a new start, and described the original experience upon which his hope for unifying human knowledge was based.

A few years later, Descartes offered (in Latin) a more formal exposition of his central tenets in Meditationes de Prima Philosophia (Meditations on First Philosophy) (1641). After an expanded statement of the method of doubt, he argued that even the most dire skepticism is overcome by the certainty of one's own existence as a thinking thing. From this beginning, he believed it possible to use our clear and distinct ideas to demonstrate the existence of god, to establish the reliability of our reason generally despite the possibility of error, to deduce the essence of body, and to prove that material things do exist.

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Soon after completing his studies at Edinburgh, Scottish philosopher David Hume began writing his comprehensive statement of the views he believed would contribute to philosophy no less than Newton's had to science. But the public reception for the three books of his magisterial Treatise of Human Nature (1739) was less than cordial, and Hume abandoned his hopes of a philosophical career in order to support his family as a librarian, historian, diplomat, and political essayist, a course of action he described in the autobiographical My Own Life (1776). Hume's Essays Moral and Political (1741-1742) found some success, and the multi-volume History of England (1754-1762) finally secured the modest livelihood for which he had hoped. Although he spent most of his life trying to produce more effective statements of his philosophical views, he did not live to see the firm establishment of his reputation by the criticisms of Kant and much later appreciation of the logical positivists.

The central themes of Book I of the Treatise receive a somewhat more accessible treatment in An Enquiry concerning Human Understanding (1748), a more popular summary of Hume's empiricism. HumeAccording to Hume, little human knowledge can be derived from the deductively certain relations of ideas. Since the causal interactions of physical objects are known to us only as inherently uncertain matters of fact, Hume argued, our belief that they exhibit any necessary connection (however explicable) can never be rationally justified, but must be acknowledged to rest only upon our acquired habits. In similar fashion, Hume argued that we cannot justify our natural beliefs in the reality of the self or the existence of an external world. Hume's graveFrom all of this, he concluded that a severe (if mitigated) skepticism is the only defensible view of the world.

Hume recast the moral philosophy of the Treatise's Book III in An Enquiry concerning the Principles of Morals (1751). In both texts Hume clearly maintained that human agency and moral obligation are best considered as functions of human passions rather than as the dictates of reason. In the posthumously published Dialogues concerning Natural Religion (1780), Hume discussed the possibility of arriving at certain knowledge of god through the application of reason and considered defense of a fideistic alternative.

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Immanuel Kant was born in the East Prussian city of Königsberg, studied at its university, and worked there as a tutor and professor for more than forty years, never travelling more than fifty miles from home. Although his outward life was one of legendary calm and regularity, Kant's intellectual work easily justified his own claim to have effected a Copernican revolution in philosophy. Beginning with his Inaugural Dissertation (1770) on the difference between right- and left-handed spatial orientations, Kant patiently worked out the most comprehensive and influential philosophical programme of the modern era. His central thesis—that the possibility of human knowledge presupposes the active participation of the human mind—is deceptively simple, but the details of its application are notoriously complex.

The monumental Kritik der reinen Vernunft (Critique of Pure Reason) (1781, 1787) Kant fully spells out the conditions for mathematical, scientific, and metaphysical knowledge in its "Transcendental Aesthetic," "Transcendental Analytic," and "Transcendental Dialectic," but Kant found it helpful to offer a less technical exposition of the same themes in the Prolegomena zu einer jeden künftigen Metaphysik die als Wissenschaft wird auftreten können (Prolegomena to any Future Metaphysic) (1783). Carefully distinguishing judgments as analytic or synthetic and as a priori or a posteriori, Kant held that the most interesting and useful varieties of human knowledge rely upon synthetic a priori judgments, which are, in turn, possible only when the mind determines the conditions of its own experience. 

Kant's moral philosophy is developed in the Grundlegung zur Metaphysik der Sitten (Grounding for the Metaphysics of Morals) (1785). From his analysis of the operation of the human will, Kant derived the necessity of a perfectly universalizable moral law, expressed in a categorical imperative that must be regarded as binding upon every agent. In the Third Section of the Grounding and in the Kritik der practischen Vernunft (Critique of Practical Reason) (1788), Kant grounded this conception of moral autonomy upon our postulation of god, freedom, and immortality.

In later life, Kant drew art and science together under the concept of purpose in the Kritik der Urteilskraft (Critique of Judgment) (1790), considered the consequences of transcendental criticism for theology in Die Religion innerhalb die Grenzen der blossen Vernunft (Religion within the Limits of Reason Alone) (1793), stated the fundamental principles for civil discourse in Beantwortung der Frage: Was ist Aufklärung? ("What is Enlightenment?" (1784), and made an eloquent plea for international cooperation in Zum ewigen Frieden (Perpetual Peace) (1795).

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The son of James Mill, a friend and follower of Jeremy Bentham, John Stuart Mill was subjected to a rigorous education at home: he mastered English and the classical languages as a child, studied logic and philosophy extensively, read the law with John Austin, and then embarked on a thirty-five career with the British East India Company at the age of seventeen. (He also suffered through a severe bout of depression before turning twenty-one.) Despite such a rich background, Mill credited the bulk of his intellectual and personal development to his long and intimate association with Harriet Hardy Taylor. They were devoted friends for two decades before the death of her husband made it possible for them to marry in 1852; she died in Avignon six years later. Mill continued to write and to participate in political affairs, serving one term in Parliament (1865-68). The best source of information about Mill's life is his own Autobiography (1873).Mill

Philosophically, Mill was a radical empiricist who held that all human knowledge, including even mathematics and logic, is derived by generalization from sensory experience. In A System of Logic, Ratiocinative and Inductive (1843) he explained in great detail the canons for reasoning inductively to conclusions about the causal connections exhibited in the natural world.

Mill's moral philosophy was a modified version of the utilitarian theory he had learned from his father and Bentham. In the polemical Utilitarianism (1861) Mill developed a systematic statement of utilitarian ethical theory. He modified and defended the general principle that right actions are those that tend to produce the greatest happiness of the greatest number of people, being careful to include a distinction in the quality of the pleasures that constitute happiness. There Mill also attempted a proof of the principle of utility, explained its enforcement, and discussed its relation to a principle of justice.Mill

Mill's greatest contribution to political theory occurs in On Liberty (1859), where he defended the broadest possible freedom of thought and expression and argued that the state can justify interference with the conduct of individual citizens only when it is clear that doing so will prevent a greater harm to others. Mill also addressed matters of social concern in Principles of Political Economy (1848) and Considerations on Representative Government (1861) and eloquently supported the cause of women's rights in The Subjection of Women (1869).

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Born the son of a Lutheran pastor in Röcken, Saxony, Friedrich Nietzsche was raised by female relatives after his father's death in 1849. He quickly abandoned his initial pursuit of theology in order to specialize in philology at Bonn and Leipzig, where he studied with Friedrich Ritschl. 

Nietzsche's mastery of classical literature led to an early academic appointment at Basel and the publication of Die Geburt der Tragödie aus dem Geiste der Musik (The Birth of Tragedy) (1872), with its distinction between Apollonian and Dionysian cultures. When ill health forced an early end to his teaching career, Nietzsche began to produce the less scholarly, quasi-philosophical, and anti-religious works for which he is now best known, including Human, All Too Human (1878), Thus Spoke Zarathustra) (1883), and The Gay Science. 

Nietzsche sharply criticized the Greek tradition's over-emphasis on reason in his Twilight of the Idols (1889). Reliance on abstract concepts in a quest for absolute truth, he supposed, is merely a symptom of the degenerate personalities of philosophers like Socrates. From this Nietzsche concluded that traditional philosophy and religion are both erroneous and harmful for human life; they enervate and degrade our native capacity for achievement.

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