Natural Philosophy. The body of knowledge now called science was known to the Middle Ages as natural philosophy. It encompassed all the ways that one could reason about the world and come to a better understanding of it. From the twelfth century onward the most important medieval scientists tended to be associated with royal households or universities (which were initially offshoots from cathedrals in cities such as Paris, Oxford, and Padua). Before that time monks studied nature in their monasteries. In either case, these scientists divided their time between scientific studies and either teaching or church duties.
Ancient Roots. Roman science was never as developed as Roman technology, and the Romans generally depended on and preserved earlier Greek learning in astronomy, mathematics, medicine, and philosophy. When the administrative structures of Rome crumbled in the fourth and fifth centuries, Europe was left largely without centers of higher learning or repositories of knowledge. Monasteries held some ancient manuscripts, but—as far as historians have been able to tell—virtually no one looked at these sources for many centuries. In the twelfth century, however, European scholars traveled to southern centers of learning, notably Spain and Sicily, as well as to Greece and Constantinople, to translate Greek, Arabic, and Hebrew copies of ancient Greek manuscripts. Some manuscripts were sent to England, France, and Italy to be copied and translated as well. Two of the most important translators of Greek texts were Robert Grosseteste and William of Moerbeke, while Michael Scot, William of Luna, and Hermann the German were notable translators of Arabic manuscripts of Aristotle’s works. These Latin translations made nearly the entire body of Aristotle’s works accessible to Europeans, whereas only a limited number of works and some fragments had been available before. They also gave Europeans access to the writings of many other Greek thinkers, as well as Arabic and Hebrew “commentaries” on Aristotle. These commentaries were much more than explanations and interpretations of Aristotle’s thought, however. Especially in the areas of medicine and optics, Arabic commentaries provided Europeans with knowledge far beyond that available in the Greek sources alone. This so-called twelfth-century renaissance stands as a profound dividing line between the early and high Middle Ages, and the science of the early medieval period remains shrouded in mystery. By the thirteenth and fourteenth centuries, however, scholars at Paris and Oxford began re-examining
the world around them and recasting it in the framework that forms the basis of modern scientific method.
Plato. The works of Plato were not as well known in the Middle Ages as those of his student Aristotle, but as early as the eleventh century, and especially from the thirteenth century onward through the translations of scholars such as William of Moerbeke and Aristippus, Platonic ideas began to appear in medieval philosophers’ works as alternatives or complements to those of Aristotle. For scientists in the Middle Ages the most relevant of Plato’s works was the Timaeus, which includes his ideas about elements. It was known to medieval scholars mainly through the works of the third-century C.E. writers Plotinus and Porphyry, who are known as Neoplatonists. Indirect knowledge of Platonic ideas was also accessible to medieval intellectuals through classical Roman authors, mainly Cicero and Seneca, as well as St. Augustine, who might be regarded as the greatest Christian Platonist. The main seat of medieval Platonism was Chartres in northern France. By the end of the Middle Ages, scholars were associating Plato with the outlook that there were perfect forms in the world and that what people experience is an imperfect reflection of those perfect forms. Practically, this viewpoint meant that there were true, perfect expressions capable of describing nature and that mathematics was the language in which those expressions—or laws—could be described. In particular, medieval thinkers tended to consider geometrical ideas of motion and quantities to be Platonic or Neoplatonic, whether they knew the ideas of Plato directly, indirectly, or at all. This mathematical outlook on nature became predominant in the Scientific Revolution of the sixteenth and seventeenth centuries.
Aristotle. Throughout the Middle Ages, including the early medieval period, the main sources for natural philosophy were the Bible and Aristotle. In fact, medieval science was in many ways an attempt to reconcile Aristotle’s ideas and teachings with those of the Bible. Aristotle, the fourth-century B.C.E. teacher of Alexander the Great, left many works to posterity, including Physics and On the Heavens, as well as many on the subjects now considered biology and botany, including History of Animals, Parts of Animals, The Motion of Animals, On the Generation of Animals, and On Plants. Aristotle’s name was so revered that many other works, such as Mechanical Problems (on levers and pulleys) and On Stones, came to be associated with him, even though scholars are now quite sure he did not write them. In some areas, Aristotle’s ideas seem strange to the modern mind, but in others no one improved on his knowledge for many centuries. Several of his basic ideas are important for understanding medieval concepts of science.
Terrestrial and Celestial Regions. Aristotle taught that the universe is divided into the terrestrial region and the celestial region. The changeable terrestrial region includes the earth, its oceans, and its atmosphere up to and including the close side of the moon. The unchanging celestial region includes everything else: the stars, the heavens, and the dark side of the moon. This way of understanding the universe did not change until the fifteenth century. One of the features of the celestial and terrestrial regions that was much debated and interpreted by medieval Europeans was Aristotle’s notion of how motion occurs in different and complimentary ways in each region.
Terrestrial Motion. Aristotle divided motion into “natural” motion and “violent,” or unnatural, motion. In the terrestrial region the natural motion is in a straight line toward the center of the earth. For example, a rock falls in a straight line to reach its natural position, that is, at rest in the lowest position possible. Any motion in the terrestrial region that is not in a straight line, or in a straight line but not toward the center of the earth, is violent motion, and can occur only through the force of some outside agent. According to Aristotle, violent motion is eventually spent and natural motion takes over. For example, when a soldier shoots an arrow from his bow, it leaves the bow in a straight line, but horizontally or in an upward arc, not vertically. It therefore starts out with entirely violent motion, but as it flies, it begins to arc more and more downward until it falls more or less vertically, unless it hits a solid target first.
Celestial Motion. In the heavens, or the celestial region, the situation is different. There natural motion takes the form of circles centered on the middle of the earth. Any straight-line motion in the celestial region would be violent, but since humans could not reach the celestial region, there is nothing to cause violent motion there. Thus, in the Aristotelian worldview all motion in the heavens is circular. Anything in the sky that is moving in a straight line—such as a comet or meteor—must actually be below the moon and, therefore, in the terrestrial region. This theory is supported by the related Aristotelian idea that the natural state of motion in the terrestrial region is rest, while in the celestial region, the natural state of motion is perpetual, perfect circular motion.
Elements, Qualities, and Humors. The second important explanation of the natural world that Aristotle gave to the Middle Ages was the ideas of the four elements, the four qualities, and the four humors. Aristotle and many other ancient philosophers considered the physical world to be made of combinations of four elements: earth, water, air, and fire. There is also a fifth element, called the ether or the quintessence (which is the Latin word for the “fifth essence”), which exists only in the celestial region. The four qualities—hot, cold, moist, and dry—are independent entities that are directly related to the four elements. Each quality is opposed to one of the others—moist to dry and hot to cold—and a combination of two nonopposed qualities characterizes each element: fire is hot and dry; air is hot and moist; water is cold and moist; and earth is cold and dry. The four humors—blood, phlegm, black bile, and yellow bile—are the substances that make up the human body and are also related to the four elements and thus to pairs of
nonopposing qualities. Blood (or sanguin), like air, is hot and moist, and a person in whom blood is the dominant humor has a sanguine (happy and confident) disposition. Yellow bile (or choler), like fire, is hot and dry and creates a choleric (hot-tempered) disposition. Black bile (melancho-ler), like earth, is dry and cold and creates a melancholic (sad) disposition. Phlegm, like water, is wet and cold and creates a phlegmatic (stolid) disposition. Medieval physicians believed that the humors should be kept in balance, so, for example, if a patient was melancholy, they would deduce that he had too much black bile, making him too dry and cold, and prescribe a hot and wet medicament.
Pure versus Earthly Elements. The four pure elements are not necessarily the same as the earthly entities with the same names. Real earth, or dirt, obviously has a preponderance of the element earth in it, just as real water (H20) is mostly the element water. In all other things two, three, or all four of the pure elements combine to make terrestrial substances. Just as mud is a mixture of the elements earth and water, a granite rock is a mixture of the elements earth, fire (because granite is found in mountainous and volcanic regions), and perhaps air or water. Sandstone, because it is lighter and softer than granite, has a higher proportion of either water or air in it. Neither Aristotle nor his contemporaries were ever explicit about the proportions of earth, air, fire, and water in specific substances, but they believed that in theory those proportions could be determined.
The Motions of the Elements. Aristotle also passed on to the medieval world the notion that each of the elements in the progression earth, water, air, and fire was lighter than one before it and that the weight of an element accounted for its natural motion in the terrestrial region. It is the nature of fire, as the lightest element, to move upward, away from the center of the earth, as is obvious in the way that it carries the sparks and smoke upward from any burning object. Air, the next lightest element, desires to move upward but not as much as fire, which accounts for the swirling of the smoke as the element fire races past the air in its ascent toward the heavens. Water and earth are heavy and move naturally toward the center of the earth. Since stones sink in water, earth is obviously heavier than water, but there are some abnormalities: for example, wood floats. Since it is solid and heavy, wood must be made mostly of the element earth; yet, it must be on average lighter than water. The solution for the ancients and medievals was to consider wood a mixture of earth, air, and water (it contains sap, after all) in such unspecified proportions as to make its overall “heaviness” (gravitas in Latin) less than that of water. Also, of course, they reminded themselves that terrestrial water might be mainly the element water, but it also has some of the element earth in it too, which increases its heaviness. By such reasoning, medieval scientists used the Aristotelian system of elements, regions, and motions to explain the mysteries of the natural world.
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