In the later Middle Ages, learned study of the natural world, usually termed “natural philosophy,” had gone on primarily in Europe’s universities, where it was seen as an appropriate part of understanding the glory of God. Such study revolved around ancient Greek ideas and texts, particularly those of Aristotle and Ptolemy. Aristotle (384–322 bce ) viewed the cosmos as centered on a motionless earth, with the planets (including the moon and sun) revolving around it in fi xed spheres made up of a crystalline substance, and the fi xed stars at its outer perimeter. The planets moved, he thought, in exactly circular orbits at a uniform speed, and were perfectly round bodies made of ether, a substance completely different from the four terrestrial elements – earth, air, fi re, and water. Above the moon – the heavenly body closest to earth – the cosmos was changeless, so that objects that did change in the skies, such as comets and meteors, must be closer than the moon. Things on earth did change, and each element had a tendency to move in a specifi c direction; things made primarily of the element earth tended to move toward the center of the earth, while water fl owed sideways around the earth and air went upward. The earth was round, the perfectly spherical center of a perfectly spherical cosmos. There were problems with Aristotle’s view. For one, it did not fi t with the motions of the planets observable from earth – the planets often appear to move backwards or reverse direction – but this was solved in the second century by Ptolemy ( c . 100– c . 165 ce ), a Greek astronomer working at Alexandria. Ptolemy held that the moon, sun, planets, and stars move around the motionless earth at various rates of speed in spiral-like paths he called epicycles. Based on observation, he calculated the epicycles of the major heavenly bodies, and the Ptolemaic system gained wide and long-lasting acceptance. The rediscovery of Greek writings other than those of Aristotle and Ptolemy led scholars in a different direction. In the fi fteenth and sixteenth centuries, the works of Pythagoras ( c . 582–c . 496 bce ), Plato ( c . 428–c . 348 bce ), and Archimedes ( c . 287– c . 212 bce ), were copied, translated, and ultimately printed. All of these ancient writers emphasized the importance of mathematics as the underlying structure of the universe, an idea that was echoed by their later admirers. Johannes Kepler (1571–1630), a German astronomer who calculated the laws of planetary motion, wrote: “Geometry, which before the origin of things was coeternal with the divine mind and is God himself … supplied God with patterns for the creation of the world.” 2 Kepler and other scholars saw the mathematical patterns of the universe as a mystical harmony, created by God and ultimately understandable to humans. Among the ancient texts rediscovered in the fi fteenth century was a body of writings attributed to Hermes Trismegistus, a god-like Egyptian sage thought to have lived at the time of Moses. These Hermetic writings – now known to have been written in the second and third centuries ce – were revered as ancient wisdom, and offered suggestions on how to exploit the hidden divine powers of minerals, plants, the planets, and other natural objects. Through processes of distillation, heating, and sublimation (cooking something to a gaseous state and then resolidifying it), these hidden powers could be tapped to transform lead into gold or cure disease and prolong life, practices usually termed alchemy. The Swiss physician Theophrastus Bombastus von Hohenheim, who called himself Paracelsus (1493?–1541), fully embraced the Hermetic tradition, as did many other scientists, who sometimes linked Hermeticism with Christian ideas about the power of angels. Paracelsus rejected the Aristotelian elements and the Galenic notion that disease is caused by an imbalance of bodily humors, and introduced the use of drugs made from small doses of purifi ed minerals, especially sulfur, antinomy, and mercury. Hoping to fi nd one powerful agent – often called the “philosopher’s stone,” or the “elixir of life” – that was capable of healing all illnesses and transforming all less perfect substances into more perfect ones, Paracelsus and other alchemists experimented with ways to extract pure elements (termed magisteria ) and divine essences (termed arcana ). With its peculiar properties as a metal that is liquid at normal room temperature, mercury was often part of alchemical theories, as was gold distilled in various liquids so that it was drinkable ( aurum potabile ). Alchemists such as Paracelsus were rooted in ancient texts, but they were innovative in their methods, advocating experimentation as the best way to discover the hidden properties of various substances. They were often the earliest to make extensive use of what was later called the “scientifi c method,” in which a hypothesis to explain a phenomenon is developed, tested, the results recorded and measured, and the hypothesis confi rmed, rejected, or modifi ed. They invented equipment still used in laboratories today, such as beakers and balance scales, and discovered new ways of producing chemical changes, such as the application of acids and alcohols. The development of the scientifi c method is often associated with the English philosopher and statesman Francis Bacon (1561–1626), who took his inspiration and procedures straight from alchemy. In The Advancement of Learning (1605) and Novum organum (New Instrument, 1620), Bacon rejected earlier claims of knowledge as based on faulty reasoning, and called for natural philosophy that began with the empirical observation of many similar phenomena. Those studying the phenomena would then use their powers of reason to propose a generalized explanation or hypothesis for the phenomena, a process called induction. This generalization would then be tested with further empirical and inductive inquiry. Like any good alchemist, Bacon was a fi rm believer in the practical value of science in promoting human progress and greater control of nature. He called for national support for scientifi c investigations, which led the founders of the English Royal Society in 1660 to see him as an inspiration. The nineteenth-century historians of science who developed the idea of a “Scientifi c Revolution” often tried to ignore the alchemical and magical interests of the thinkers they championed, but most major fi gures in the Scientifi c Revolution believed fi rmly in alchemy, astrology, and other examples of what are now often judged to be fringe occult beliefs. In his work on the motion of the planets, Kepler hoped to discover the mystical proportions underlying the universe and an explanation for how the heavenly bodies infl uenced human life. The Danish astronomer Tycho Brahe (1546–1601) constructed an advanced observatory and kept careful records of the skies, and also had an alchemical laboratory with multiple ovens for cooking and distilling plants and minerals to gain their spiritual essence. The Irish chemist Robert Boyle (1627–91) developed laws about the behavior of gases and disproved the theory that air, earth, fi re, and water were the basic elements, asserting instead that everything was made of very small particles in motion, which he called “corpuscles.” In long laboratory reports written in code, Boyle also reported to have witnessed elixirs that turned lead into gold and gold into lead. By the eighteenth century, those who experimented on the natural world tended to defi ne forces or substances they could not see in material rather than spiritual or mystical terms. George Ernst Stahl (1660–1734), a German chemist and physician, proposed that combustion and other processes resulted in the release and absorption of a substance he called phlogiston. The phlogiston theory led other chemists to study gases – what they called “airs” – and in the middle of the eighteenth century carbon dioxide and hydrogen were both identifi ed as substances different than the air that surrounds us. In the early 1770s, the Swedish apothecary Carl Wilhelm Scheele (1742–86) and the English cleric and theologian Joseph Priestley (1733–1804) both discovered an air in which substances burned more easily. Viewing his discovery within the context of the phlogiston theory, Priestley called it “dephlogisticated air.” The French chemist Antoine Lavoisier (1743–94) performed similar experiments, but interpreted the results differently. He recognized that the same substance allows for combustion, the action of acids, and respiration in living things, and called this substance “oxygen.” Lavoisier’s oxygen theory came to replace the phlogiston theory, particularly as Lavoisier made it part of a radically new way of discussing chemical compounds and processes in his Elementary Treatise on Chemistry (1789), the first modern textbook on chemistry. Like Bacon, Lavoisier regarded science as a way of providing solutions to real-world problems, and he experimented on crop rotation, the quality of drinking water, the military and scientific use of balloons, and the production of gunpowder. He also proposed reforms for the French economy and prison system, but his involvement in tax collection ultimately outweighed his contributions and he was sent to the guillotine during the French Revolution.