This conflict was chiefly carried out in new European institutions of higher learning, the universities. One of the cathedral schools, at Paris, received a royal charter as a university in 1200. (There was a slightly older university at Bologna, but it specialized in law and medicine, and did not play an important role in medieval physical science.) Almost immediately, in 1210, scholars at the University of Paris were forbidden to teach the books of Aristotle on natural philosophy. Pope Gregory IX in 1231 called for Aristotle’s works to be expurgated, so that the useful parts could be safely taught.
The ban on Aristotle was not universal. His works were taught at the University of Toulouse from its founding in 1229. At Paris the total ban on Aristotle was lifted in 1234, and in subsequent decades the study of Aristotle became the center of education there. This was largely the work of two thirteenth-century clerics: Albertus Magnus and Thomas Aquinas. In the fashion of the times, they were given grand doctoral titles: Albertus was the “Universal Doctor,” and Thomas the “Angelic Doctor.”
Albertus Magnus studied in Padua and Cologne, became a Dominican friar, and in 1241 went to Paris, where from 1245 to 1248 he occupied a professorial chair for foreign savants. Later he moved to Cologne, where he founded its university. Albertus was a moderate Aristotelian who favored the Ptolemaic system over Aristotle’s homocentric spheres but was concerned about its conflict with Aristotle’s physics. He speculated that the Milky Way consists of many stars and (contrary to Aristotle) that the markings on the Moon are intrinsic imperfections. The example of Albertus was followed a little later by another German Dominican, Dietrich of Freiburg, who independently duplicated some of al-Farisi’s work on the rainbow. In 1941 the Vatican declared Albertus the patron saint of all scientists.
Thomas Aquinas was born a member of the minor nobility of southern Italy. After his education at the monastery of Monte Cassino and the University of Naples, he disappointed his family’s hopes that he would become the abbot of a rich monastery; instead, like Albertus Magnus, he became a Dominican friar. Thomas went to Paris and Cologne, where he studied under Albertus. He then returned to Paris, and served as professor at the university in 1256–1259 and 1269–1272.
The great work of Aquinas was the Summa Theologica, a comprehensive fusion of Aristotelian philosophy and Christian theology. In it, he took a middle ground between extreme Aristotelians, known as Averroists after Ibn Rushd; and the extreme anti-Aristotelians, such as members of the newly founded Augustinian order of friars. Aquinas strenuously opposed a doctrine that was widely (but probably unjustly) attributed to thirteenth-century Averroists like Siger of Brabant and Boethius of Dacia. According to this doctrine, it is possible to hold opinions true in philosophy, such as the eternity of matter or the impossibility of the resurrection of the dead, while acknowledging that they are false in religion. For Aquinas, there could be only one truth. In astronomy, Aquinas leaned toward Aristotle’s homocentric theory of the planets, arguing that this theory was founded on reason while Ptolemaic theory merely agreed with observation, and another hypothesis might also fit the data. On the other hand, Aquinas disagreed with Aristotle on the theory of motion; he argued that even in a vacuum any motion would take a finite time. It is thought that Aquinas encouraged the Latin translation of Aristotle, Archimedes, and others directly from Greek sources by his contemporary, the English Dominican William of Moerbeke. By 1255 students at Paris were being examined on their knowledge of the works of Aquinas.
But Aristotle’s troubles were not over. Starting in the 1250s, the opposition to Aristotle at Paris was forcefully led by the Franciscan Saint Bonaventure. Aristotle’s works were banned at Toulouse in 1245 by Pope Innocent IV. In 1270 the bishop of Paris, Étienne Tempier, banned the teaching of 13 Aristotelian propositions. Pope John XXI ordered Tempier to look further into the matter, and in 1277 Tempier condemned 219 doctrines of Aristotle or Aquinas.3 The condemnation was extended to England by Robert Kilwardy, the archbishop of Canterbury, and renewed in 1284 by his successor, John Pecham.
The propositions condemned in 1277 can be divided according to the reasons for their condemnation. Some presented a direct conflict with scripture—for instance, propositions that state the eternity of the world:
9. That there was no first man, nor will there be a last; on the contrary, there always was and always will be the generation of man from man.
87. That the world is eternal as to all the species contained in it; and that time is eternal, as are motion, matter, agent, and recipient.
Some of the condemned doctrines described methods of learning truth that challenged religious authority, for instance:
38. That nothing should be believed unless it is self-evident or could be asserted from things that are self-evident.
150. That on any question, a man ought not to be satisfied with certitude based upon authority.
153. That nothing is known better because of knowing theology.
Finally, some of the condemned propositions had raised the same issue that had concerned al-Ghazali, that philosophical and scientific reasoning seems to limit the freedom of God, for example:
34. That the first cause could not make several worlds.
49. That God could not move the heavens with rectilinear motion, and the reason is that a vacuum would remain.
141. That God cannot make an accident exist without a subject nor make more [than three] dimensions exist simultaneously.
The condemnation of propositions of Aristotle and Aquinas did not last. Under the authority of a new pope who had been educated by Dominicans, John XXII, Thomas Aquinas was canonized in 1323. In 1325 the condemnation was rescinded by the bishop of Paris, who decreed: “We wholly annul the aforementioned condemnation of articles and judgments of excommunication as they touch, or are said to touch, the teaching of blessed Thomas, mentioned above, and because of this we neither approve nor disapprove of these articles, but leave them for free scholastic discussion.”4 In 1341 masters of arts at the University of Paris were required to swear they would teach “the system of Aristotle and his commentator Averroes, and of the other ancient commentators and expositors of the said Aristotle, except in those cases that are contrary to the faith.”5
Historians disagree about the importance for the future of science of this episode of condemnation and rehabilitation. There are two questions here: What would have been the effect on science if the condemnation had not been rescinded? And what would have been the effect on science if there had never been any condemnation of the teachings of Aristotle and Aquinas?
It seems to me that the effect on science of the condemnation if not rescinded would have been disastrous. This is not because of the importance of Aristotle’s conclusions about nature. Most of them were wrong, anyway. Contrary to Aristotle, there was a time before there were any men; there certainly are many planetary systems, and there may be many big bangs; things in the heavens can and often do move in straight lines; there is nothing impossible about a vacuum; and in modern string theories there are more than three dimensions, with the extra dimensions unobserved because they are tightly curled up. The danger in the condemnation came from the reasons why propositions were condemned, not from the denial of the propositions themselves.
Even though Aristotle was wrong about the laws of nature, it was important to believe that there are laws of nature. If the condemnation of generalizations about nature like propositions 34, 49, and 141, on the ground that God can do anything, had been allowed to stand, then Christian Europe might have lapsed into the sort of occasionalism urged on Islam by al-Ghazali.
Also, the condemnation of articles that questioned religious authority (such as articles 38, 150, and 153 quoted above) was in part an episode in the conflict between the faculties of liberal arts and theology in medieval universities. Theology had a distinctly higher status; its study led to a degree of doctor of theology, while liberal arts faculties could confer no degree higher than master of arts. (Academic processions were headed by do
ctors of theology, law, and medicine in that order, followed by the masters of arts.) Lifting the condemnation did not give the liberal arts equal status with theology, but it helped to free the liberal arts faculties from intellectual control by their theological colleagues.
It is harder to judge what would have been the effect if the condemnations had never occurred. As we will see, the authority of Aristotle on matters of physics and astronomy was increasingly challenged at Paris and Oxford in the fourteenth century, though sometimes new ideas had to be camouflaged as being merely secundum imaginationem—that is, something imagined, rather than asserted. Would challenges to Aristotle have been possible if his authority had not been weakened by the condemnations of the thirteenth century? David Lindberg6 cites the example of Nicole Oresme (about whom more later), who in 1377 argued that it is permissible to imagine that the Earth moves in a straight line through infinite space, because “To say the contrary is to maintain an article condemned in Paris.”7 Perhaps the course of events in the thirteenth century can be summarized by saying that the condemnation saved science from dogmatic Aristotelianism, while the lifting of the condemnation saved science from dogmatic Christianity.
After the era of translation and the conflict over the reception of Aristotle, creative scientific work began at last in Europe in the fourteenth century. The leading figure was Jean Buridan, a Frenchman born in 1296 near Arras, who spent much of his life in Paris. Buridan was a cleric, but secular—that is, not a member of any religious order. In philosophy he was a nominalist, who believed in the reality of individual things, not of classes of things. Twice Buridan was honored by election as rector of the University of Paris, in 1328 and 1340.
Buridan was an empiricist, who rejected the logical necessity of scientific principles: “These principles are not immediately evident; indeed, we may be in doubt concerning them for a long time. But they are called principles because they are indemonstrable, and cannot be deduced from other premises nor be proved by any formal procedure, but they are accepted because they have been observed to be true in many instances and to be false in none.”8
Understanding this was essential for the future of science, and not so easy. The old impossible Platonic goal of a purely deductive natural science stood in the way of progress that could be based only on careful analysis of careful observation. Even today one sometimes encounters confusion about this. For instance, the psychologist Jean Piaget9 thought he had detected signs that children have an innate understanding of relativity, which they lose later in life, as if relativity were somehow logically or philosophically necessary, rather than a conclusion ultimately based on observations of things that travel at or near the speed of light.
Though an empiricist, Buridan was not an experimentalist. Like Aristotle’s, his reasoning was based on everyday observation, but he was more cautious than Aristotle in reaching broad conclusions. For instance, Buridan confronted an old problem of Aristotle: why a projectile thrown horizontally or upward does not immediately start what was supposed to be its natural motion, straight downward, when it leaves the hand. On several grounds, Buridan rejected Aristotle’s explanation that the projectile continues for a while to be carried by the air. First, the air must resist rather than assist motion, since it must be divided apart for a solid body to penetrate it. Further, why does the air move, when the hand that threw the projectile stops moving? Also, a lance that is pointed in back moves through the air as well as or better than one that has a broad rear on which the air can push.
Rather than supposing that air keeps projectiles moving, Buridan supposed that this is an effect of something called “impetus,” which the hand gives the projectile. As we have seen, a somewhat similar idea had been proposed by John of Philoponus, and Buridan’s impetus was in turn a foreshadowing of what Newton was to call “quantity of motion,” or in modern terms momentum, though it is not precisely the same. Buridan shared with Aristotle the assumption that something has to keep moving things in motion, and he conceived of impetus as playing this role, rather than as being only a property of motion, like momentum. He never identified the impetus carried by a body as its mass times its velocity, which is how momentum is defined in Newtonian physics. Nevertheless, he was onto something. The amount of force that is required to stop a moving body in a given time is proportional to its momentum, and in this sense momentum plays the same role as Buridan’s impetus.
Buridan extended the idea of impetus to circular motion, supposing that planets are kept moving by their impetus, an impetus given to them by God. In this way, Buridan was seeking a compromise between science and religion of a sort that became popular centuries later: God sets the machinery of the cosmos in motion, after which what happens is governed by the laws of nature. But although the conservation of momentum does keep the planets moving, by itself it could not keep them moving on curved orbits as Buridan thought was done by impetus; that requires an additional force, eventually recognized as the force of gravitation.
Buridan also toyed with an idea due originally to Heraclides, that the Earth rotates once a day from west to east. He recognized that this would give the same appearance as if the heavens rotated around a stationary Earth once a day from east to west. He also acknowledged that this is a more natural theory, since the Earth is so much smaller that the firmament of Sun, Moon, planets, and stars. But he rejected the rotation of the Earth, reasoning that if the Earth rotated, then an arrow shot straight upward would fall to the west of the archer, since the Earth would have moved under the arrow while it was in flight. It is ironic that Buridan might have been saved from this error if he had realized that the Earth’s rotation would give the arrow an impetus that would carry it to the east along with the rotating Earth. Instead, he was misled by the notion of impetus; he considered only the vertical impetus given to the arrow by the bow, not the horizontal impetus it takes from the rotation of the Earth.
Buridan’s notion of impetus remained influential for centuries. It was being taught at the University of Padua when Copernicus studied medicine there in the early 1500s. Later in that century Galileo learned about it as a student at the University of Pisa.
Buridan sided with Aristotle on another issue, the impossibility of a vacuum. But he characteristically based his conclusion on observations: when air is sucked out of a drinking straw, a vacuum is prevented by liquid being pulled up into the straw; and when the handles of a bellows are pulled apart, a vacuum is prevented by air rushing into the bellows. It was natural to conclude that nature abhors a vacuum. As we will see in Chapter 12, the correct explanation for these phenomena in terms of air pressure was not understood until the 1600s.
Buridan’s work was carried further by two of his students: Albert of Saxony and Nicole Oresme. Albert’s writings on philosophy became widely circulated, but it was Oresme who made the greater contribution to science.
Oresme was born in 1325 in Normandy, and came to Paris to study with Buridan in the 1340s. He was a vigorous opponent of looking into the future by means of “astrology, geomancy, necromancy, or any such arts, if they can be called arts.” In 1377 Oresme was appointed bishop of the city of Lisieux, in Normandy, where he died in 1382.
Oresme’s book On the Heavens and the Earth10 (written in the vernacular for the convenience of the king of France) has the form of an extended commentary on Aristotle, in which again and again he takes issue with The Philosopher. In this book Oresme reconsidered the idea that the heavens do not rotate about the Earth from east to west but, rather, the Earth rotates on its axis from west to east. Both Buridan and Oresme recognized that we observe only relative motion, so seeing the heavens move leaves open the possibility that it is instead the Earth that is moving. Oresme went through various objections to the idea, and picked them apart. Ptolemy in the Almagest had argued that if the Earth rotated, then clouds and thrown objects would be left behind; and as we have seen, Buridan had argued against the Earth’s rotation by reasoning that if the Earth rotated from west to east, t
hen an arrow shot straight upward would be left behind by the Earth’s rotation, contrary to the observation that the arrow seems to fall straight down to the same spot on the Earth’s surface from which it was shot vertically upward. Oresme replied that the Earth’s rotation carries the arrow with it, along with the archer and the air and everything else on the Earth’s surface, thus applying Buridan’s theory of impetus in a way that its author had not understood.
Oresme answered another objection to the rotation of the Earth—an objection of a very different sort, that there are passages in Holy Scripture (such as in the Book of Joshua) that refer to the Sun going daily around the Earth. Oresme replied that this was just a concession to the customs of popular speech, as where it is written that God became angry or repented—things that could not be taken literally. In this, Oresme was following the lead of Thomas Aquinas, who had wrestled with the passage in Genesis where God is supposed to proclaim, “Let there be a firmament above the waters, and let it divide the waters from the waters.” Aquinas had explained that Moses was adjusting his speech to the capacity of his audience, and should not be taken literally. Biblical literalism could have been a drag on the progress of science, if there had not been many inside the church like Aquinas and Oresme who took a more enlightened view.
Despite all his arguments, Oresme finally surrendered to the common idea of a stationary Earth, as follows:
Afterward, it was demonstrated how it cannot be proved conclusively by argument that the heavens move. . . . However, everyone maintains, and I think myself, that the heavens do move and not the Earth: For God has established the world which shall not be moved, in spite of contrary reasons because they are clearly not conclusive persuasions. However, after considering all that has been said, one could then believe that the Earth moves and not the heavens, for the opposite is not self-evident. However, at first sight, this seems as much against natural reason than all or many of the articles of our faith. What I have said by way of diversion or intellectual exercise can in this manner serve as a valuable means of refuting and checking those who would like to impugn our faith by argument.11
To Explain the World: The Discovery of Modern Science Page 14
