Aristotle’s De caelo was his major contribution to cosmology, embracing the celestial and terrestrial regions into which he divided the cosmos. By translations, it passed from Greek to Arabic, and was finally translated from Arabic, and then from Greek into Latin in the twelfth century. Numerous commentaries and treatises known as Questions were written on the De caelo from the thirteenth to the seventeenth century. One of the most problematic questions deriving from De caelo was whether the world is eternal, as Aristotle argued, or whether it was created as Christians insisted. This difficult question, and others, produced a theological reaction in the form of a Church condemnation of 219 articles in 1277. The central issue that emerged was about God’s absolute power. Scholastic natural philosophers assumed that God could do anything short of a logical contradiction. By His supernatural powers, it was therefore assumed that God could do anything that Aristotle had regarded as naturally impossible. This gave rise to numerous hypothetical discussions about what things would be like if God created other worlds and what form that might take, or if He moved our world with a rectilinear motion, or if He made matter disappear and created a vacuum, and so on. In commentaries and Questions on De caelo in the sixteenth and seventeenth centuries - following the momentous contributions by Copernicus, Tycho Brahe, and Galileo that had completely subverted Aristotle’s physical cosmos - scholastics sought, as best they could, to adjust to the new cosmology.
In his basic works on natural philosophy, Aristotle devoted De caelo (On the Heavens) to cosmology. Of the four books comprising the treatise, Aristotle devotes the first two to the celestial region, which includes all the stars and planets. Because celestial bodies are incapable of selfmotion, they are carried around the heavens by spheres that are concentric with the earth’s center. The celestial region extends downward from the outermost sphere of the celestial region - the sphere of the fixed stars - to the innermost sphere of the moon. Aristotle regarded the celestial region as eternal and therefore assumed it to be composed of an ether that was eternal and incorruptible. The third and fourth books treat of the terrestrial region that ranges from just below the moon to the geometric center of the earth. Here we find four terrestrial elements in descending concentric circles of fire, air, water, and earth, the last mentioned constituting a sphere that lies at the center of our spherical universe.
The first significant commentaries on De caelo were written in Byzantium. In the fourth century CE, Themistius (fl. late 340s-c. 385), a Peripatetic philosopher, wrote a commentary on De caelo, as did Simplicius (fl. first half of sixth century CE), a Neoplatonic philosopher. Aristotle’s natural philosophical works were virtually unknown in the Latin west until the twelfth century. But they were known east of Byzantium, in Syria and Persia, brought there by Nestorian and Monophysite Christians who had been driven from the Byzantine Empire as heretics. Nestorian Christians translated Aristotle’s treatises on natural philosophy into Syriac and from the latter they were often translated into Arabic.
Aristotle’s works played a significant role in Islamic science and natural philosophy. By far the most important Islamic natural philosopher to comment on Aristotle’s works was Averroes (1126-1198), in the twelfth century, who usually made three different types of commentaries on most ofAristotle’s natural philosophical works, including De caelo. There was the short commentary, which was really a paraphrase or summary, of Aristotle’s text. The second, or middle commentary, was also not a direct translation, but gave the meaning of a brief segment of Aristotle’s text intermingled with comments by Averroes. The third and last type of commentary was the Long Commentary, which included all of Aristotle’s text presented sequentially section by section, with each section followed by Averroes’ commentary on that section. In the short and middle commentaries, it was often difficult to distinguish Aristotle’s text from Averroes’ comments.
During the twelfth and thirteenth centuries, Aristotle’s De caelo finally reached the West in Latin translation. Gerard of Cremona (c. 1114-1187), the greatest medieval translator of Aristotelian texts from Arabic to Latin, translated De caelo, while in the thirteenth century William of Moerbeke (c. 1215-c. 1286) translated some 49 works from Greek into Latin, including De caelo and virtually all works attributed to Aristotle (Grant 1974). Two important commentaries on Aristotle’s De caelo were also available to medieval authors, namely Michael Scot’s (d. c. 1235) translation from Arabic to Latin of Averroes’ (Ibn Rushd) Long Commentary on De caelo, which was popular between the thirteenth and sixteenth centuries (as evidenced by 30-40 extant manuscripts), and William of Moerbeke’s translation from Greek to Latin of Simplicius’ Commentary on De caelo.
In the thirteenth century, Albert the Great (c. 12001280) and Thomas Aquinas (c. 1224/1225-1274) followed Averroes in their commentaries on De caelo. Albert wrote a paraphrase and Thomas an Exposition, although instead of including the whole relevant text, he substituted a few words - known as cue-words - from the beginning of the passage that would presumably enable the reader to identify the proper text in Aristotle’s De caelo.
In the fourteenth and fifteenth centuries, the most popular form of commentary on De caelo, and other of Aristotle’s texts, was the Questiones. The questions format was developed at the medieval universities from a type of classroom disputation known as the ‘‘ordinary disputation.’’ In such a disputation, the teaching master proposed a question and then selected one student to defend the affirmative side and another to defend the negative side. After both sides had presented their arguments, the master proposed a solution, thus resolving or ‘‘determining’’ the question. This formed the basis for the written questions on Aristotle’s books on natural philosophy. They all bore the title Questions on [title of Aristotle’s treatise].
In the fourteenth century, Nicholas Oresme (c. 13201382) performed an extraordinary feat that was never duplicated. He produced two strikingly different interpretations of De caelo. In his earlier career, as a master of arts at the University of Paris, he wrote a questions treatise on De caelo between 1346 and 1356. Around 1370, King Charles V of France requested Oresme to translate four of Aristotle’s treatises from Latin into French. The fourth treatise was De caelo, which Oresme translated around 1376 with the title Le Livre du ciel et du monde. Oresme not only translated Aristotle’s text into French, though usually by paraphrasing the text, but added a section-bysection French commentary. Each commentary was preceded by the entire relevant Aristotelian passage. It was
Oresme’s final treatise and was undoubtedly the most brilliant Commentary on De caelo in the whole of the Middle Ages. As recognition for his four French translations, King Charles made Oresme the Bishop of Lisieux in 1377.
To arrive at some idea of the number of extant commentaries on De caelo between the years 1200 and 1650, one should consult the indispensable catalogs compiled by Charles Lohr (Lohr 1967-1974,1988). A count reveals 278 authors who commented on De caelo treatises. Of these, 40 are known only from manuscript catalogs or were mentioned by other authors, which leaves 238 authors represented by a manuscript or a printed edition, or both; 139 are only in manuscript form; 76 are only in printed editions; 21 exist in both manuscript form and printed editions. If we add 76 and 21, we obtain 97 printed editions of De caelo (Grant 1994:29-30). Of the 278 authors who commented on De caelo, a total of 71 wrote between the thirteenth and fifteenth centuries (18 in the thirteenth, 21 in the fourteenth, and 32 in the fifteenth centuries), while a total of 207 composed their commentaries during the sixteenth and seventeenth centuries (108 in the sixteenth and 99 in the seventeenth century). It is obvious that De caelo was still frequently commented upon well into the seventeenth century. Its popularity may have derived from the impact of Nicholas Copernicus’ (1473-1543) heliocentric theory published in 1543 (On the Revolutions of the Heavenly Bodies).
Commentaries and Questions on De caelo ranged over numerous cosmological issues. Of great significance was Aristotle’s argument that the world is eternal, having neither beginning nor end. Here Aristotle was in direct conflict with the Christian faith, which assumed that God had created the world from nothing and would eventually destroy it. In the Condemnation of 1277, of the 219 articles condemned, some 27 denounced the eternity of the world in many contexts. In their Questions on De caelo, natural philosophers such as John of Jandun (1285/1289-1328), John Buridan (d. 1361), and Nicholas Oresme argued that the world could not have been brought into being by natural means, because no natural means could be identified. Hence, they argued that in terms of natural causation, the world should indeed be eternal, as Aristotle had argued. Nevertheless, almost all commentators on De caelo believed on faith that the world is not eternal, but was created by God (Grant 1994:76).
A number of comments on De caelo were influenced by the Condemnation of 1277. In his De caelo, and in many of his treatises, Aristotle had argued that certain phenomena were naturally impossible, as, for example: an infinite world, the existence of other worlds, the existence of anything beyond our world, and so on. Christian theologians viewed Aristotle’s claims as effectively denying God the power to perform any of these acts, even if He chose to do so. Thus, they viewed Aristotle’s arguments as a denial of God’s absolute power to do anything short of a logical contradiction. Thereafter, scholastics allowed that God could do all the things Aristotle had said were impossible. They showed this by hypothetically assuming that these naturally impossible conditions were real and arguing accordingly. Many De caelo commentaries included a number of hypothetical questions and many arguments were thereafter formulated ‘‘according to the imagination” (secundum imaginationem). Although God could do naturally impossible things, none believed that He had actually done any of them, or would do them. But since they were possible by God’s absolute power, scholastics envisioned worlds that were radically different from Aristotle’s, even though they agreed with him that these hypothetical phenomena were indeed naturally impossible. The alternative was to abandon Aristotle’s cosmos, a possibility that was never seriously entertained during the Middle Ages.
De caelo commentators often asked whether a body moved circularly could be infinite, and similarly for a body moved rectilinearly. If an infinite body moved circularly and completed one circulation in a finite time, it would follow that an infinite body traversed an infinite space in a finite time, which is absurd (Grant 1994:109). It was also usual to inquire whether an infinite body could move rectilinearly. Buridan denied that it could, because many infinite places would have to exist as the body moved into and out of them successively. Moreover, all bodies moving rectilinearly must be in a place. But an infinite body cannot be in a place, because no body could contain it (Grant 1994:109-110). These were Aristotle’s arguments. But Buridan concedes that, according to article 49 of those condemned in 1277, it is an error to say that God could not move the whole world with a rectilinear motion. Thus, God could move the entire world rectilinearly even though the world is not in a place, because nothing lies outside of it to serve as a place.
In a question asking whether an infinite body is possible, Buridan denies that it is naturally possible, and also denies that God could create an infinite body, because then He could not create a body greater than that infinite body, since there is nothing greater than an infinite body. Thus would God’s absolute power be limited (Grant 1994:111) A number of scholastic theologians - for example, William of Ockham (c. 1285-1347), Gregory of Rimini (c. 1300-1358), Robert Holkot (c. 1290-1349),
Henry of Harclay (...), and others - argued the opposite: God could indeed create an actual infinite, although none believed he had done so. Those who adopted this interpretation were called ‘‘Infinitists.’’
Article 34 of the 219 articles condemned in 1277 declared, ‘‘That the first cause could not make several worlds’’ (Grant 1974:48). This was Aristotle’s opinion and was condemned because it denied to God the absolute power to create as many worlds as He wished. In the fourteenth century, Questions on De caelo always included a question as to whether God could create other worlds. Although medieval scholastics did not really believe that God had created other worlds, or would ever do so, they routinely argued that God could indeed create other worlds. Many assumed hypothetically that God did indeed create other worlds and then raised questions about the characteristics of those worlds. In refuting the existence of other worlds, Aristotle had assumed they were all alike and were somehow distant from one another. But medieval natural philosophers posed questions that probably never occurred to Aristotle. If other worlds existed, did they exist simultaneously? Or did they exist successively, one world at a time? If simultaneously, were they all distinct and independent of one another? Or were they concentric, with one spherical world inside another?
Aristotle had claimed that if other worlds existed simultaneously, the earth of one world would move toward the earth of another world. This implied that an earth would have to rise up against its natural inclinations and somehow get through space and enter another world where it would seek to join the earth at the center of that world. To avoid this absurd consequence, Buridan and Albert of Saxony (c. 1316-1390) insisted, as did others, that each earth would remain at the center of its own world and have no inclination to join the earth of another world. In their view, each world is independent of every other world.
In his French commentary on De caelo, Nicholas Oresme used a plurality of worlds to introduce the subject of void space lying beyond our world. On the assumption of a plurality of spherical worlds, Oresme insisted that a void space would exist between any two of them. Thus did Oresme reject Aristotle’s arguments that it was impossible that anything - place, plenum, void, or time - exist beyond our world. It was supernaturally possible. Indeed, Oresme argued that the void space beyond our finite world is infinite and is God’s infinite immensity, although God is not an extended being (Oresme 1968:177). Oresme’s identification of God with infinite void space would be repeated in the seventeenth century by such figures as Nicolas
Malebranche (1638-1715), John Locke (1632-1704) (Grant 1981:239), Henry More (1614-1687) - although More viewed God as extended in His infinite space - and Samuel Clarke (1675-1729) (Grant 1981:248).
Article 49 of the Condemnation of 1277 denied that God could move the entire spherical world with a rectilinear motion because a vacuum would be left in the place vacated by the world. This was a common question in De caelo commentaries. Buridan and Oresme insisted that by his supernatural power, God could indeed move the world with a rectilinear motion even though the world is not in a place and would be moved through a vacuum, leaving a vacuum behind in its former location. Because there is nothing outside of our world, Oresme regarded the rectilinear motion of the world as an absolute motion, a concept that made no sense in Aristotle’s world. Indeed such a motion would deny Aristotle’s concept of place and motion.
Long before Copernicus, medieval natural philosophers were discussing whether or not the earth lies immobile in the geometric center of the world, as Aristotle argued. Indeed, it was one of the most popular questions in medieval cosmology. One form of the question inquired whether the earth rotates daily on its axis. Although numerous scholastic commentators posed this question, none accepted the earth’s daily axial rotation as physically true. In his French commentary on De caelo, after extensive arguments in favor of the earth’s rotation, Oresme rejected it, insisting, however, that it was nonetheless a viable hypothesis, because neither reason nor experience could resolve the issue.
Another aspect concerning the earth’s motion was whether it might move slightly in the position it occupied at the center of the world. On this issue, John Buridan departed radically from Aristotle. Buridan agreed that if the earth were homogeneous and spherical, its center of magnitude and its center of gravity would coincide with the geometric center of the world. But the earth is obviously not homogeneous and therefore its center of magnitude does not coincide with the center of the world. However, the earth’s center of gravity coincides with the true center of the world. But it does so in a very strange manner. Continual geological processes throughout the earth cause its center of gravity to change incessantly as it seeks to coincide with the geometric center of the world. Buridan concluded that the earth continually moves with small rectilinear motions that never cease because the earth incessantly undergoes geologic changes. Albert of Saxony and John Major (1467/1468-1550) adopted this position in the fourteenth and sixteenth centuries respectively. After the Copernican theory was condemned in 1616, very few commentators on De caelo adopted Buridan’s idea of a slightly moving earth.
When commenting on De caelo medieval scholars overwhelmingly emphasized the first two cosmological books, paying much less attention to the third and fourth books that were devoted to the heaviness and lightness of bodies and their motions. Some, like John Buridan, formulated a question in the third book that was concerned with whether the motive force that projects a stone, or continues the motion of an arrow, are moved by an external force or by an internal force that was usually called impetus.
Commentaries on De caelo continued to the end of the seventeenth century. The Catholic Church’s condemnation of the Copernican theory in 1616, and of Galileo in 1633, made the heliocentric system unacceptable to Catholic commentators. But the geoheliocentric theory proposed by Tycho Brahe (1546-1601) in the sixteenth century was adopted by some scholastics. Brahe denied the daily rotation of the earth and retained its position in the center of the world. However, he assumed that all the planets - except the earth - moved with circular motions around the sun as their geometric center. The Jesuit natural philosopher, Melchior Cornaeus (1598-1665), adopted Brahe’s geoheliocenric system in his De caelo commentary and thus made a dramatic departure from the traditional Aristotelian medieval system.
Tycho Brahe caused another major change in De caelo commentaries. By his observation of the comet of 1577, Brahe concluded that the comet moved in the celestial region and that the heavenly region was composed of a fluid substance, not hard celestial orbs, as had been generally assumed since the Middle Ages. If the orbs were hard, the comet would have smashed through them in ways that would have been detectable, which, of course, did not happen. As a consequence of Brahe’s rejection of hard orbs, at least ten scholastic authors in the seventeenth century added a question to their De caelo commentaries that had never been discussed in the Middle Ages: ‘‘Whether the heavens are fluid or solid’’ (Grant 1994:705). Those who opted for a fluid heaven also abandoned Aristotle’s fundamental concept that the celestial ether was incorruptible and unchangeable.
The new questions that were incorporated into early modern scholastic Questions on De caelo as a result of the cosmological changes proposed by Copernicus and Brahe were quite dramatic. Many scholastics, especially among the Jesuits, adopted Tycho’s system in which the earth remained stationary in the center of the world, while the other planets moved around the Sun as their center of motion. Thus, Questions on De caelo underwent
R
Significant changes. But this should not obscure the fact that the great majority of questions first presented during the late Middle Ages, continued to be discussed to the end of the seventeenth century. We may conclude, therefore, that although traditional questions continued to dominate among De caelo authors, there was a genuine attempt to take cognizance of the revolutionary cosmological changes of the early modern era.
See also: > Albert of Saxony > Albert the Great > Gerard of Cremona > Ibn Rushd (Averroes), Latin Translations of > Impetus > John Buridan > John of Jandun > Nicholas Oresme > Parisian Condemnation of 1277 > Thomas Aquinas
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