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(384 BC - 322 BC) In the early 1500s natural philosophy was still based primarily on the ideas of Aristotle, the great Greek philosopher of the fourth century B.C.E. Medieval theologians such as Thomas Aquinas brought Aristotelian philosophy into harmony with Christian doctrines. According to the revised Aristotelian view, a motionless earth was fixed at the center of the universe and was encompassed by ten separate concentric crystal spheres that revolved around it. In the first eight spheres were embedded, in turn, the moon, the sun, the five known planets, and the fixed stars. Then followed two spheres added during the Middle Ages to account for slight changes in the positions of the stars over the centuries. Beyond the tenth sphere was Heaven, with the throne of God and the souls of the saved. Angels kept the spheres moving in perfect circles. Even though Aristotle's cosmology made intellectual sense, it could not account for the observed motions of the stars and the planets and, in particular, provided no explanation for the apparent backward motion of the planets. However Ptolemy, a Hellenized Egyptian, offered a cunning solution to this dilemma. According to Ptolemy, the planets moved in small circles, called epicycles, each of which moved in turn along a larger circle, or deferent. Ptolemaic astronomy was less elegant than Aristotle's neat nested circles and required complex calculations, but it provided a surprisingly accurate model for predicting planetary motion. Aristotle's views, revised by medieval philosophers, also dominated thinking about physics and motion on earth. Aristotle had distinguished sharply between the world of the celestial spheres and that of the earth---the sublunar world. The spheres consisted of a perfect, incorruptible "quintessence," or fifth essence. The sublunar world, however, was made up of four imperfect, changeable elements. The "light" elements (air and fire) naturally moved upward, while the "heavy" elements (water and earth) naturally moved downward. These natural directions of motion did not always prevail, however, for elements were often mixed together and could not be affected by an outside force such as a human being. Aristotle and his followers also believed that a uniform force moved an object at a constant speed and that the object would stop as soon as that force was removed.
(1473-1543) The desire to explain and thereby glorify God's handiwork led to the first great departure from the medieval system. This was the work of the Polish cleric Nicolaus Copernicus. As a young man Copernicus was drawn to the vitality of the Italian Renaissance. After studies at the University of Kraków, he departed for Italy, where he studied astronomy, medicine, and church law at the famed universities of Bologna, Padua, and Ferrara. Copernicus noted that astronomers still depended on the work of Ptolemy for their most accurate calculations, but he felt that Ptolemy's cumbersome and occasionally inaccurate rules detracted from the majesty of a perfect creator. He preferred an alternative ancient Greek idea: that the sun, rather than the earth, was at the center of the universe. After finishing his university studies and returning to a position in church administration in East Prussia, Copernicus worked on his hypothesis from 1506 to 1530. The Copernican hypothesis had enormous scientific and religious implications, many of which the conservative Copernicus did not anticipate. First, it put the stars at rest, their apparent nightly movement simply a result of the earth's rotation. Thus it destroyed the main reason for believing in crystal spheres capable of moving the stars around the earth. Second, Copernicus' theory suggested a universe of staggering size. If in the course of a year the earth moved around the sun and yet the stars appeared to remain in the same place, then the universe was unthinkably large. Third, by using mathematics, instead of philosophy to justify his theories, he challenged the traditional hierarchy of the disciplines. Finally, by characterizing the earth as just another planet, Copernicus destroyed the basic idea of Aristotelian physics---that the earthly sphere was quite different from the heavenly one. Where then were Heaven and the throne of God? Religious leaders varied in response to Copernicus' theories. A few Protestant scholars became avid Copernicans, while others accepted some elements of his criticism of Ptolemy, but firmly rejected the notion that the earth moved, a doctrine that contradicted the literal reading of some passages of the Bible. Among Catholics, Copernicus' ideas drew little attention prior to 1600. Because the Catholic Church had never held to literal interpretations of the Bible, it did not officially declare the Copernican hypothesis false until 1616.
(1571-1630) The young assistant of Tycho Brahe who was left to rework Brahe's mountain of observations (that involved the stars and planets) after his sudden death in 1601. From a minor German noble family, Kepler suffered a bout of smallpox as a small child, leaving him with permanently damaged hands and eyesight. A brilliant mathematician, Kepler was inspired by his belief that the universe was built on mystical mathematical relationships and a musical harmony of the heavenly bodies. Kepler's examination of his predecessor's meticulously recorded findings convinced him that Ptolemy's astronomy could not explain them. Abandoning the notion of epicycles and deferents---which even Copernicus had retained in part---Kepler developed three new and revolutionary laws of planetary motion. First, largely through observations of the planet Mars, he demonstrated that the orbits of the planets around the sun are elliptical rather than circular. Second, he demonstrated that the planets do not move at a uniform speed in their orbits. When a planet is close to the sun it moves more rapidly, and it slows as it moves farther away from the sun. Kepler published the first two laws in his 1609 book, "The New Astronomy," which heralded the arrival of an entirely new theory of the cosmos. In 1619 Kepler put forth his third law: the time a planet takes to make its complete orbit is precisely related to its distance from the sun. Kepler's contribution was monumental. Whereas Copernicus had used mathematics to describe planetary movement, Kepler proved mathematically the precise relations of a sun-centered (solar) system. He thus united for the first time the theoretical cosmology of natural philosophy with mathematics. His work demolished the old system of Aristotle and Ptolemy, and with his third law he came close to formulating the idea of universal gravitation. In 1627 he also fulfilled Brahe's pledge by completing the "Ruldophine Tables" begun so many years earlier. Kepler was a genius with many talents. Beyond his great contribution to astronomy, he pioneered the field of optics. He was the first to explain the role of refraction within the eye in creating vision, and he invented an improved telescope. He was also a great mathematician whose work furnished the basis for integral calculus and advances in geometry. Kepler was not, however, the consummate modern scientist that these achievements suggest. His duties as court mathematician included casting horoscopes, and he based his own daily life on astrological principles. He also wrote at length on cosmic harmonies and explained, for example, elliptical motion through ideas about the beautiful music created by the combined motion of the planets. Kepler's fictional account of travel to the moon, written partly to illustrate the idea of a non-earth-centered universe, caused controversy and may have contributed to the arrest and trial of his mother as a witch in 1620. Kepler also suffered deeply as a result of his unorthodox brand of Lutheranism, which led to his rejection by both Lutherans and Catholics. His career exemplifies the complex interweaving of ideas and beliefs in the emerging science of his day.
(1564-1642) While Kepler was unraveling planetary motion, a young Florentine named Galileo Galilei was challenging all the old ideas about motion. Like Kepler and so many early scientists, Galileo was a poor nobleman first marked for a religious career. Instead, his fascination with mathematics led to a professorship in which he examined motion and mechanics in a new way. His great achievement was the elaboration and consolidation of the experimental method. That is, rather than speculate about what might or should happen, Galileo conducted controlled experiments to find out what actually did happen. In his early experiments, Galileo focused on deficiencies in Aristotle's theories of motion. He measured the movement of a rolling ball across a surface, repeating the action again and again to verify his results. In his famous acceleration experiment, he showed that a uniform force---in the case, gravity---produced a uniform acceleration. Through another experiment, he formulated the law of inertia. He found that rest was not the natural state of objects. Rather, an object continues in motion forever unless stopped by some external force. His discoveries proved Aristotelian physics wrong. Galileo then applied the experimental method to astronomy. On hearing details about the invention of the telescope in Holland, Galileo made on for himself and trained it on the heavens. He quickly discovered the first four moons of Jupiter, which clearly suggested that Jupiter could not possibly be embedded in any impenetrable crystal sphere as Aristotle and Ptolemy maintained. This discovery provided new evidence for the Copernican theory, in which Galileo already believed. Galileo then pointed his telescope at the moon. He wrote in 1610 in "The Sidereal Messenger", "By the aid of a telescope anyone may behold [the Milky Way] in a manner which so distinctly appeals to the senses that all the disputes which have tormented philosophers through so many ages are exploded by the irrefutable evidence of our eyes, and we are freed from wordy disputes upon the subject." Reading these famous lines, one feels a crucial corner in Western civilization being turned. No longer should one rely on established authority. A new method of learning and investigating was being developed, one that proved useful in any field of inquiry. A historian investigating documents of the past, for example, is not so different from a Galileo studying stars and rolling balls. In 1597, when Kepler sent Galileo an early publication defending Copernicus, Galileo replied that it was too dangerous to express his support his heliocentrism publicly. The rising fervor of the Catholic Reformation increased the church's hostility to such radical ideas, and in 1616 the Holy Office placed the works of Copernicus and his supporters, including Kepler, on a list of books Catholics were forbidden to read. The accompanying decree declared that belief in a heliocentric world was "foolish and absurd, philosophically false and formally heretical." Galileo was a devout Catholic who sincerely believed that his theories did not contract from the perfection of God. Out of caution he silenced his beliefs for several years, until in 1623 he saw new hope with the ascension of Pope Urban VIII, a man sympathetic to developments in the new science. However, Galileo's 1632 "Dialogue on the Two Chief Systems of the World" went too far. Published in Italian and widely read, this work openly lampooned the traditional views of Aristotle and Ptolemy and defended those of Copernicus. The papal Inquisition placed Galileo on trial for heresy. Imprisoned and threatened with torture, the aging Galileo recanting. "renouncing and cursing" his Copernican errors.
(1642-1727) Despite the efforts of the church, by about 1640 the work of Brahe, Kepler, and Galileo had been largely accepted by the scientific community. The old Aristotelian astronomy and physics were in ruins, and several fundamental breakthroughs had been made. But the new findings failed to explain what forces controlled the movement of the planets and objects on earth. That challenge was taken up by English scientist Isaac Newton. Newton was born into the lower English gentry in 1642, and he enrolled at Cambridge University in 1661. A genius who spectacularly untied the experimental and theoretical-mathematical sides of modern science, Newton was an intensely devout, albeit non-orthodox Christian, who privately rejected the doctrine of the Trinity. Newton was also fascinated by alchemy. He left behind thirty years' worth of encoded journals recording experiments to discover the elixir of life and a way to change base metals into god and silver. He viewed alchemy as one path, alongside mathematics and astronomy, to the truth of God's creation. Like Kepler and other practitioners of the Scientific Revolution, he studied the natural world not for its own sake, but to understand the divine plan. Newton arrived at some of his most basic ideas about physics between 1664 and 1666, during a break from studies at Cambridge caused by an outbreak of plague. As he later claimed, during this period he discovered his law of universal gravitation as well as the concepts of centripetal force and acceleration. Not realizing the significance of his findings, the young Newton did not publish them, and upon his return to Cambridge he took up the study of optics. It was in reference to his experiments in optics that Newton outlined his method of scientific inquiry most clearly, explaining the need for scientists "first to enquire diligently into the properties of things, and to establish these properties by experiment, and then to proceed more slowly to hypotheses for the explanation of them." In 1684 Newton returned to physics and the preparation of his ideas for publication. The result appeared three years later in "Philosophicae Naturalis Principia Mathematica". The key feature of the Newtonian synthesis was the law of universal gravitation. According to this law, every body in the universe attracts every other body in the universe in a precise mathematical relationship, whereby the force of attraction is proportional to the quantity of matter of the objects and inversely proportional to the square of the distance between them. The whole universe---from Kepler's elliptical orbits to Galileo's rolling balls---was unified in one coherent system. The German mathematician and philosopher Gottfried von Leibniz, with whom Newton contested the invention of calculus, was outraged by Newton's claim that the "occult" force of gravity could allow bodies to affect one another at great distances. Newton's religious faith, as well as his alchemical belief in the innate powers of certain objects, allowed him to dismiss such criticism.
(1596-1650) The French philosopher Rene Descartes was a multitalented genius who made his first great discovery in mathematics. As a twenty-three-year-old soldier serving in the Thirty Years' War, he experienced a life-changing intellectual vision one night in 1619. Descartes saw that there was a perfect correspondence between geometry and algebra and that geometrical spatial figures could be expressed as algebraic equations and vice versa. A major step forward in the history of mathematics, Descartes' discovery of analytic geometry provided scientists with an important new tool. Descartes used mathematics to elaborate a highly influential vision of the workings of the cosmos. Accepting Galileo's claim that all elements of the universe are composed of the same matter, Descartes began to investigate the basic nature of matter. Drawing on ancient Greek atomist philosophies, Descartes developed the idea that matter was made up of identical "corpuscules" that collided together in an endless series of motions. All occurrences in nature could be analyzed as matter in motion and, according to Descartes, the total "quantity of motion" in the universe was constant. Descartes' mechanistic view of the universe depended on the idea that a vacuum was impossible, which meant that every action had an equal reaction, continuing in an eternal chain reaction. Although Descartes' hypothesis about the vacuum was proved wrong, his notion of a mechanistic universe intelligible through the physics of motion proved inspirational. Decades later, Newton rejected Descartes' idea of a full universe and several of his other ideas, but retained the notions of a mechanistic universe as a key element of his own system. Descartes' greatest achievement was to develop his initial vision into a whole philosophy of knowledge and science. The Aristotelian cosmos was appealing in part because it corresponded with the evidence of the human senses. When the senses were proven to be wrong, Descartes decided it was necessary to doubt them and everything that could reasonably be doubted, and then, as in geometry, to use deductive reasoning from self-evident truths, which he called "first principles," to ascertain scientific laws. Descartes' reasoning ultimately reduced all substances to "matter" and "mind"---that is, to the physical and the spiritual. The devout Descartes believed that God had endowed man with reason for a purpose and that rational speculation could provide a path to the truths of creation. His view of the world as consisting of two fundamental entities is known as Cartesian dualism. Descartes' thought was highly influential in France and the Netherlands, but less so in England, where experimental philosophy won the day.
(1712-1778) In the early 1740s Jean-Jacques Rousseau, the son of a poor Swiss watchmaker, made his way into the Parisian Enlightenment through his brilliant intellect. He contributed articles on music to the "Encyclopedia" and became friends with its editors. Appealing but neurotic, Rousseau came to believe that the philosophes were plotting against him. In the mid-1750s he broke with them, living thereafter as a lonely outsider with his uneducated common-law wife and going in his own highly original direction. Like other Enlightenment thinkers, Rousseau was passionately committed to individual freedom. Unlike them, however, he attacked rationalism and civilization as destroying, rather than liberating, the individual. Warm, spontaneous feeling had to complement and correct cold intellect. Moreover, the basic goodness of the individual and the unspoiled child had to be protected from the cruel refinements of civilization. Rousseau's ideals greatly influenced the early romantic movement, which rebelled against the culture of the Enlightenment in the late eighteenth century. Rousseau also called for a rigid division of gender roles. According to Rousseau, women and men were radically different beings. Destined by nature to assume a passive role in sexual relations, women should also be subordinate in social life. Women's love for displaying themselves in public, attending social gatherings, and pulling the strings of power was unnatural and had a corrupting effect on both politics and society. Rousseau thus rejected the sophisticated way of life of Parisian elite women. His criticism led to calls for privileged women to renounce their frivolous ways and stay at home to care for their children.
(ca. 1690-1789) The Scientific Revolution was a crucial factor in the creation of the new worldview of the eighteenth-century Enlightenment. This worldview, which has played a large role in shaping the modern mind, grew out of a rich mix of diverse and often conflicting ideas that were debated in international networks. Despite the diversity, three central concepts stand at the core of Enlightenment thinking. The first and foremost idea was that the methods of natural science could and should be used to examine and understand all aspects of life. This was what intellectuals meant by "reason," a favorite word of Enlightenment thinkers. Nothing was to be accepted on faith; everything was to be submitted to rationalism, a secular, critical way of thinking. A second important Enlightenment concept was that the scientific method was capable of discovering the laws of human society as well as those of nature. These tenets led to the third key idea, that of progress. Armed with the proper method of discovering the laws of human existence, Enlightenment thinkers believed, it was at least possible for human beings to create better societies and better people. Loosely united by certain key ideas, the European Enlightenment was a broad intellectual and cultural movement that gained strength gradually and did not reach its maturity until about 1750. Yet it was the generation that came of age between the publication of Newton's "Principia" in 1687 and the death of Louis XIV in 1715 that tied the crucial knot between the Scientific Revolution and a new outlook on life. Whereas medieval and Reformation thinkers had been concerned primarily with abstract concepts of sin and salvation, and Renaissance humanists had drawn their inspiration from the classical past, Enlightenment thinkers believed that their era had gone far beyond antiquity and that intellectual progress was very possible. Talented writers of that generation popularized hard-to-understand scientific achievements and set an agenda of human problems to be addressed through the methods of science. Like the Scientific Revolution, the Enlightenment was also fueled by Europe's increased contacts with the wider world. In the wake of the great discoveries of the fifteenth and sixteenth centuries, the rapidly growing travel literature taught Europeans that the peoples of China, India, Africa, and the Americas all had their own very different beliefs and customs. Europeans shaved their faces and let their hair grow. In Europe a man bowed before a women to show respect. In Siam a man turned his back on a women when he met her because it was disrespectful to look disrespectful to look directly at her. Countless similar examples discussed in travel accounts helped change the perspective of educated Europeans. They began to look at truth and morality in relative, rather than absolute, terms. If anything was possible, who could say what was right or wrong? The excitement of the Scientific Revolution also generated doubt and uncertainty, contributing to a widespread crisis in late-seventeenth-century European thought. In the wake of the devastation wrought by the Thirty Years' War, some people asked whether ideological conformity in religious matters was really necessary. Others skeptically asked if religious truth could ever be known with absolute certainty and concluded that it could not. The atmosphere of doubt spread from religious to political issues. This was a natural extension, since many rulers viewed religious dissent as a form of political opposition and took harsh measures to stifle unorthodox forms of worship. Thus, questioning religion inevitably led to confrontations with the state.
(1689-1755) One of the greatest philosophes, the baron de Montesquieu, brilliantly pioneered the approach of the philosophes in "The Persian Letters," an extremely influential social satire published in 1721 and considered the first major work of the French Enlightenment. It consisted of amusing letters supposedly written by two Persian travelers who as outsiders saw European customs in unique ways, thereby allowing Montesquieu a vantage point for criticizing existing practices and beliefs. Having gained fame by using wit as a weapon against cruelty and superstition, Montesquieu turned to the study of history and politics. His interest was partly personal, for, like many members of the French robe nobility, he was disturbed by the growth in absolutism under Louis XIV. But Montesquieu was also inspired by the example of the physical sciences, and he set out to apply the critical method to the problem of government in "The Spirit of Laws" (1748). The result was a complex, comparative study of republics, monarchies, and despotisms. Showing that forms of government were shaped by history and geography, Montesquieu focused on the conditions that would promote liberty and prevent tyranny. He argued for a separation of powers, with political power divided and shared by a variety of classes and legal estates. Admiring greatly the English balance of power, Montesquieu believed that in France the thirteen high courts---the "parlements"---were frontline defenders of liberty against royal despotism. Apprehensive about the uneducated poor, Montesquieu was clearly no democrat, but his theory of separation of powers had a great impact on the constitutions of the young United States in 1789 and of France in 1791.
(1694-1778) The most famous and perhaps most representative philosophe was Francois Marie Arouet, who was known by the pen name Voltaire. In his long career, this son of a comfortable middle-class family wrote more than seventy witty volumes, hobnobbed with royalty, and died a millionaire through shrewd speculations. His early career, however, was turbulent, and he was arrested on two occasions for insulting noblemen. Voltaire moved to England for three years in order to avoid a longer prison term in France, and there he came to share Montesquieu's enthusiasm for English liberties and institutions. While living with Madame du Chatelet in Cirey, Voltaire wrote works praising England and popularizing English science. He had witnessed Newton's burial at Westminster Abbey in 1727, and he lauded Newton as history's greatest man, for he had used his genius for the benefit of humanity. In the true style of the Enlightenment, Voltaire mixed the glorification of science and reason with an appeal for better individuals and institutions. Yet, like almost all philosophes, Voltaire was a reformer, not a revolutionary, in politics. He pessimistically concluded that the best one could hope for in the way of government was a good monarch, since human beings "are very rarely worthy to govern themselves." He lavishly praised Louis XIV and conducted an enthusiastic correspondence with King Frederick the Great of Prussia, whom he admired as an enlightened monarch. Nor did Voltaire believe in social and economic equality, insisting that the idea of making servants equal to their masters was "absurd and impossible." The only realizable equality, Voltaire thought, was that "by which the citizen only depends on the laws which protect the freedom of the feeble against the ambition of the strong." Voltaire's philosophical and religious positions were much more radical than his social and political beliefs. In the tradition of Bayle, his writings challenged the Catholic Church and Christian theology at almost every point. Voltaire clearly believed in God, but, like many eighteenth-century Enlightenment thinkers, he was a deist, envisioning God as akin to a clockmaker who set the universe in motion and then ceased to intervene in human affairs. Above all, Voltaire and most of the philosophes hated all forms of religious intolerance, which they believed led to fanaticism. Simple piety and human kindness---as embodied in Christ's commandments to "love God and your neighbor as yourself"---were religion enough.
(1713-1784) The ultimate strength of the philosophes lay in their dedication and organization. The philosophes felt keenly that they were engaged in a common undertaking that transcended individuals. Their greatest and most representative intellectual achievement was, quite fittingly, a group effort---the seventeen-volume "Encyclopedia: The Rational Dictionary of the Sciences, the Arts, and the Crafts," edited by Denis Diderot and Jean le Rond d' Alembert. From different circles and with different interests, the two men set out to find coauthors who would examine the rapidly expanding whole of human knowledge. Even more fundamentally, they set out to teach people how to think critically and objectively about all matters. As Diderot said, he wanted the "Encyclopedia" to "change the general way of thinking." The "Encyclopedia" survived initial resistance from the French government and the Catholic Church. Published between 1751 and 1772, it contained seventy-two thousand articles by leading scientists, writes, skilled workers, and progressive priests, and it treated every aspect of life and knowledge. Not every article was daring or original, but the overall effect was little short of revolutionary. Science and the industrial arts were exalted, religion and immortality questioned. Intolerance, legal injustice, and out-of-date social institutions were openly criticized. The encyclopedists were convinced that greater knowledge would result in greater human happiness, for knowledge was useful and made possible economic, social, and political progress. Summing up the new worldview of the Enlightenment, the "Encyclopedia" was widely read, especially in less-expensive reprint editions, and it was extremely influential.