Aristotle: Founder of the Genre

Natural philosophy has existed since the dawn of human history. It embraced all efforts to understand nature and the physical world from pre-literate societies to the sixteenth-century.

It was, however, Aristotle (384-322 BC), the great Greek philosopher and scientist, who gave natural philosophy its defining characteristics and provided its substantive content for the next 2000 years. He did this by writing what came to be known as the “natural books” (libri naturales). These included the following treatises:

  • Physics
  • On the Heavens (De caelo)
  • On the Soul (De anima)
  • On Generation and Corruption (De generatione et corruptione)
  • Meteorology
  • The Short Physical Treatises (Parva naturalia).

Although later thinkers rarely commented on them, Aristotle’s biological works were also a part of natural philosophy.

Aristotle’s conception of the cosmos lay at the core of his physics. He conceived of it as a huge uncreated, eternal spherical plenum that had no beginning or end and was divided into two basic parts, terrestrial and celestial.

plenum: a sphere with no empty space

The terrestrial region embraced everything below the concave surface of the moon to the geometric center of the earth lying in the exact center of the world. It was a region of continuous change in which the four elements (earth, water, air, and fire) continually came together and separated to form the vast array of beings that inhabit our sublunar region.

The celestial region was filled and wholly occupied by an incorruptible ether. All planets and stars, ranging from the moon to the outermost sphere of the fixed stars, were composed of this ether. The only change that occurred in the celestial region was change of place, observed in the motion of the stars and planets.

Aristotle distinguished three broad categories of knowledge that he regarded as scientific: the productive sciences, the practical sciences, and the theoretical sciences. The last was comprised of metaphysics, mathematics, and physics. Physics was concerned with animate and inanimate bodies and was applicable to the whole physical world.

It was Aristotle’s concept of physics that came to be called natural science (scientia naturalis), and by the thirteenth-century was known as natural philosophy (philosophia naturalis). Metaphysics played a role in natural philosophy, but mathematics did not. The exact mathematical sciences, such as optics, astronomy, harmonics, and mechanics, were assumed to lie between pure mathematics and natural philosophy, but belong to neither. Hence they were known as middle sciences (scientiae mediae).

Of great importance is the fact that Aristotle separated mathematics and natural philosophy and did not apply the former to the latter. Thus there were two aspects of science: natural philosophy without mathematics and the exact sciences based largely on mathematics. These two separate disciplines were gradually brought together in the late Middle Ages, but were not truly joined until the Scientific Revolution of the seventeenth-century.

Almost all the relevant translations of Aristotle’s works were done from the first half of the twelfth century to the second half of the thirteenth century. Prior to the translations, a number of Roman, late ancient, and early medieval figures wrote about the natural world as best they could, and as early as the ninth century dramatic changes occurred that would affect the reception of Aristotelian natural philosophy.

Introducing Aristotle to Medieval Western Europe

In the late eleventh and first half of the twelfth century, a number of authors began to argue that reason should be applied to all arguments, including those in theology. Among this group mention should be made of John Scotus Eriugena and Anselm of Canterbury in the eleventh-century, and, the most important of all, Peter Abelard in the twelfth-century. Between 1155 and 1158, Peter Lombard (ca. 1095-1160) wrote the Four Books of Sentences, or opinions, the primary theological textbook until the end of the seventeenth-century, which was almost as important in natural philosophy as it was in theology.

During the twelfth century European scholars, aware of the intellectual riches embedded in Arabic works, were attracted to cities in Spain that had been occupied for centuries by Arabic-speaking peoples of the Islamic faith. Most importantly, these treasures included virtually all the works attributed to Aristotle.

The most eminent translator of Aristotle’s works from Arabic to Latin, as well as numerous other works by Islamic scholars in the various sciences (mathematics, mechanics, optics, medicine, astronomy and alchemy) was Gerard of Cremona (ca. 1114-1187). He translated most of Aristotle’s libri naturales as well as Aristotle's philosophy of science, the Posterior Analytics. The De anima translation is more often attributed to Michael Scot.

Among the Greek to Latin translations, the most popular were prepared or revised by William of Moerbeke (ca. 1215-ca. 1286).[1] Not only did he translate most of Aristotle’s works, but he also translated numerous other Greek authors, including seven treatises by Archimedes.1

Natural Philosophy and the Universities

Many factors contributed to the rise of the universities. Among them was the reintroduction of Aristotle's thought to the West. With Aristotle’s books on natural philosophy widely available in the twelfth century, the urge to study those works, along with Aristotle’s treatises on logic, metaphysics, ethics, and politics, brought students together at the first major universities in the Western world: Paris, Oxford, and Bologna.

Aristotle’s natural philosophy was sufficiently rich and comprehensive to provide the basis for the curriculum studied by all students who matriculated in the faculties of arts. By 1500, more than 65 universities were scattered throughout Europe teaching Aristotle’s natural philosophy. Aristotle’s views about the world and its operations were destined to shape western thought until well into the seventeenth century.

The early medieval universities drew their students from all over Europe. A degree from the arts faculty, which was based on a thorough study of Aristotle’s natural philosophy, was a prerequisite for entering one of the other three higher faculties: theology, law, and medicine.

Because students and faculty of many medieval universities were foreigners in the cities in which their universities were located, they won the right to form themselves into corporations and to elect their own officers. The members of each university corporation formulated statutes and laws that were binding on their members. By governing themselves, the universities were able to become permanent institutions, many of which continue to exist today.

Various opinions and concepts in Aristotle’s natural philosophy were regarded as dangerous to the Christian faith and proved to be a major obstacle to the study of Aristotle’s treatises. Judging from its reception by the Church in the early thirteenth century, Aristotle’s entry into Western intellectual life was anything but smooth. Lectures on Aristotle’s natural philosophy were probably given at Oxford and Paris in the first decade of the thirteenth century. This seems evident from the fact that in 1210, the Parisian Synod decreed, under pain of excommunication, that no public or private lectures were permitted on Aristotle’s books on natural philosophy, or on commentaries on those books. Although the ban was reiterated in 1215, it appears to have become ineffective by 1238, when Richard Rufus of Cornwall delivered lectures at the University of Paris on Aristotle’s Physics and On Generation and Corruption. In the 1240s, Roger Bacon also lectured on Aristotle’s natural philosophy.

Most people now believe that the prohibitions effectively lapsed in 1238, though they were repeated pro forma in 1245 and 1263. See, for example, Mark Jordan's article "Aristotelianism, Medieval" in the Routledge Encyclopedia of Philosophy.

By the 1250s at the latest, when the University of Paris required its students to study them, the Church had realized it was futile to prevent scholars from studying Aristotle’s works. They were simply too popular and too important. Rather than ban Aristotle’s works, conservative churchmen chose to censor the dangerous ideas and concepts in Aristotle’s texts. In 1270, the bishop of Paris was persuaded to condemn thirteen propositions, which was followed in 1277 by a condemnation of 219 articles, many of them from the works of Aristotle and Averroes. Those found guilty of defending positions contrary to the intent of any of the condemned articles were subject to excommunication.

What was Natural Philosophy?

The Condemnation of 1277 had a significant impact on natural philosophy at the University of Paris. To appreciate that impact let us first briefly describe the study of natural philosophy in medieval universities.

For scholars and students in a university, natural philosophy was the subject of Aristotle’s libri naturales. Understanding the universe and its parts was the legitimate object of natural philosophy. In the most general sense, natural philosophy was concerned with mobile beings that could be either material or immaterial. These mobile beings were always in the process of changing, either by being in motion, or by undergoing bodily change. The changes involved all animate and inanimate objects:

  • animals
  • plants
  • oceans
  • lakes
  • rivers
  • mountains
  • weather phenomena

Indeed it covered all of medieval cosmology.

In dealing with the subjects of natural philosophy, medieval university scholars commented on the texts of Aristotle’s libri naturales. They would present a portion of Aristotle’s text and then comment on it. They would then present the next passage and comment upon it, continuing in this manner to the end of the treatise.

By far the most important kind of commentary treatise was one that was in the form of a sequence of questions, almost all of which began with an opening inquiry, the first word of which was usually “whether" (utrum). It was an author’s obligation to present the pros and cons of an argument and then to present his own opinion. The final step in responding to a question obligated an author to give arguments rejecting the positions of his opponents that he had presented at the beginning of the question.

In the long medieval tradition of posing questions on Aristotle’s five major libri naturales (Physics, On the Heavens, On the Soul, On Generation and Corruption, and Meteorology) hundreds of questions accumulated to confront scholars who wished to range over the whole of natural philosophy. A total of 310 questions appear in the libri naturales of four major fourteenth-century natural philosophers:

  • John Buridan: Questions on the Heavens
  • Albert of Saxony: Questions on Generation and Corruption and Questions on the Physics
  • Themon Judaeus: Questions on the Meteorology
  • Nicole Oresme: Questions on the Soul.

The best way to acquire a sense of the range of medieval natural philosophy is to present some reasonably representative questions based on Aristotle’s natural books (libri naturales). The questions below would be classified under a variety of modern sciences:

  • whether the whole earth is habitable
  • whether spots appearing in the moon arise from differences in parts of the moon or from something external
  • whether the earth is spherical
  • whether by their light the celestial bodies are generative of heat
  • whether a compound is possible
  • whether there are four elements, no more nor less
  • whether any element is pure
  • we inquire whether one element could be generated directly from another, so that water could be generated directly from air without something else being from it previously; and (the same question can be asked) of the other elements.
  • whether it is possible for an actual infinite magnitude to exist
  • whether the existence of a vacuum is possible
  • whether the mass of the whole earth — that is, its quantity or magnitude — is much less than certain stars
  • whether a comet is of a celestial nature or (whether it is) of an elementary nature, say, of a fiery exhalation
  • whether lightning is fire descending from a cloud
  • on the supposition that a rainbow can occur by reflection of rays, we inquire whether such reflection occurs in a cloud or whether it occurs in tiny dewdrops or raindrops.2

Medieval Natural Philosophers Challenge Aristotle

It is often thought that medieval scholastics followed Aristotle’s natural philosophy slavishly, as if he were the sole reliable source of information about the structure and operations of our physical world. Nothing could be further from the truth.

Medieval scholars rejected Aristotle’s interpretations on numerous issues, replacing them with their own views. Two major types of departures can be distinguished. The first involved the rejection of certain of Aristotle’s physical interpretations as flawed, and substituting for them a quite different account. A second kind of departure was manifested by the introduction of imaginary hypothetical ideas about the cosmos. Many of these imaginary departures were prompted by Aristotle’s insistence that certain concepts about the world were “naturally impossible.”

Among these ideas the most significant were Aristotle’s firm belief that nothing whatever could exist beyond our world — neither body, place, void, nor time — and his rejection of the possible existence of void space within the world. What made this second kind of departure significant for the history of science and natural philosophy is the fact that Aristotle’s insistence on “natural impossibilities” seemed to Christians to imply that not even God could produce them.

Such a limitation on God’s power was unacceptable to Christians, so they condemned 219 articles in 1277 loosely based on the teaching of Aristotle and his medieval interpreters. As a consequence, scholastic natural philosophers imagined the possible existence of some of Aristotle’s “natural impossibilities;” they sought to determine how such possibilities could be realized and what kind of world would result.

1. Projectile Motion

In Physics 8.10, Aristotle explains projectile motion — that is, the motion of a physical body, such as a stone, after it loses contact with its initial mover, such as a catapult. He assumed that when a projectile is launched into space, the air is activated and continues to move the projectile after its initial launch. Successive segments of the air gradually lose their force until the projectile reaches a segment that is no longer capable of moving the projectile. At this point the projectile ceases to move. Note that in Aristotle’s account, the air not only serves as the mover, but also resists the projectile’s motion, because without resistance a body’s motion would be instantaneous.

A few Greek and Islamic commentators on Aristotle’s Physics had already rejected Aristotle’s explanation of projectile motion. In the sixth century, John Philoponus , a Greek convert to Christianity, explained that if Aristotle’s explanation were true, one ought to be able to cause a stone to move simply by stirring the air behind it, which is absurd. Instead of air as a motive force, Philoponus believed that when a motive force moved into contact with the stone, the motive force impressed an incorporeal force into the stone that continued the motion of the stone.

Islamic commentators added important dimensions to Philoponus’s explanation. Avicenna (Ibn Sina) insisted that the impressed force was permanent and would endure forever if there were no resistance to the projectile’s motion. By contrast, Abu’l Barakat (d. ca. 1164) argued for a non-permanent, self-dissipating impressed force. Even if there were no resistance, a stone in motion would come to a halt when its impressed force ceased to exist.

With the translations of Greek and Arabic texts in natural philosophy, the impressed force theory came to be known in the West. In the thirteenth century, Richard Rufus, for example, posited impressions to explain projectile motion. Roger Bacon rejected this account, however, and it was not really accepted until the fourteenth century.

The most prominent expositor was John Buridan , one of the most important natural philosophers of the Middle Ages. Buridan may have been the first to characterize impressed force by the Latin term impetus, a term that came into general use and is routinely used by modern scholars when they discuss medieval concepts of impressed force. In Buridan’s exposition, as a stone or heavy body was propelled through the air by its impetus, the resistant air and the natural tendency of the body to fall toward the earth combined to gradually dissipate the projectile’s impetus.

Indeed, Buridan extended the use of impetus to explain the acceleration of freely falling bodies. In agreement with Aristotle, Buridan believed that a body’s heaviness caused it to fall naturally downward toward the earth. But departing from Aristotle, who believed that a body accelerated as it approached the earth, which is its natural place, Buridan argued that at each moment of its fall, the heaviness of the body produces an increment of impetus which causes the body to accelerate.

Note that Buridan seems to have inadvertently presented conflicting views of projectile motion and accelerated fall. As we saw, he said that a body’s heaviness contributes to the dissipation of impetus when the body is moved through space. Now he adds that that same heaviness causes successive increments of impetus as a body accelerates during its natural fall toward the earth.

Aristotle insisted that a stone, or heavy body, that was hurled upward and then fell to earth naturally did not move continuously. Instead, it had but actually moved with two distinct motions — up and down — separated by a “moment of rest” (quies media). Beginning in the fourteenth century, most scholastics rejected Aristotle’s “moment of rest,” largely because of a thought experiment that made it untenable. Assume that a millstone falling downward strikes a bean moving upward. At the point when the millstone strikes the bean, it begins its downward motion. No temporal interval could possibly intervene. Hence there could be no moment of rest.

2. The Immobile Earth

A fundamental tenet of Aristotle’s natural philosophy was an immobile earth that lay at the geometric center of our spherical cosmos. In the fourteenth century, scholars gave serious consideration to the possibility that the stellar sphere was at rest while the earth rotated daily on its axis to produce night and day. All eventually sided with Aristotle, but before doing so they resorted to arguments involving the relativity of motion to show that the same celestial effects we now perceive would occur whichever of the two hypotheses were adopted. Nicole Oresme went the furthest by offering more powerful and plausible arguments for a daily rotating earth. In the end, however, he too opted for Aristotle’s position. But where Aristotle would have regarded a rotating earth as absurd, or even impossible, a number of medieval scholastics regarded it as plausible and reasonable, though not in fact true.

At least one scholastic rejected the total immobility of the earth at the center of the world. John Buridan argued that the earth moved continuously with slight rectilinear movements. This occurred because the earth’s surface and interior constantly underwent changes. These changes caused the earth’s center of gravity to shift slightly with rectilinear motions in order to bring the earth’s constantly changing center of gravity into coincidence with the center of the world. This was a major departure from Aristotle.

3. Alternative Worlds

The most unusual aspect of medieval natural philosophy was the intense interest in counterfactual propositions. By the fourteenth century, scholastic natural philosophers sought to examine the way God might have done things, but probably did not. This was largely a byproduct of the Condemnations of 1277. It derives from Aristotle’s insistence that certain phenomena are naturally impossible, as, for example, that more than one world could exist or that a vacuum could exist.

Christian theologians interpreted Aristotle to mean that not even God could create more than one world, which they regarded as a restriction of God’s absolute power to do anything that did not involve logical contradiction. Article 34 of the 1277 Condemnations, which rejected the idea that God could not make a plurality of worlds, was condemned. Article 49 condemned the claim that God could not move the whole cosmos with a rectilinear motion, because a vacuum would be left where the world had been. Other condemned articles were similarly directed against Aristotle’s claims that this or that natural phenomenon was impossible.

Underlying such condemnations was the assumption that, considering only His absolute power, God could do anything that Aristotle deemed naturally impossible. Whether made explicit or only implied, God’s absolute power prompted various scholastic natural philosophers to imagine other worlds and situations in those worlds that were impossible in Aristotle’s world. Different kinds of pluralities of worlds were imagined:

  • God might create successive worlds, replacing one world after another.
  • Or He could create numerous identical worlds that existed simultaneously.

Nicole Oresme, perhaps the most imaginative of all medieval natural philosophers, conceived of a plurality of worlds that were arranged concentrically with respect to our own world: one or more worlds might be arranged concentrically within our world, or one or more worlds might completely surround our world to form a series of concentric worlds.

The simultaneous existence of a plurality of spherical worlds led Oresme and others to assume that between any two such worlds a void space had to exist, although Aristotle had regarded the existence of void spaces as naturally impossible. For Oresme, however, extracosmic void space was not hypothetical, but rather was supernaturally real. He was convinced that an infinite and indivisible void space existed beyond our world, which he equated with God’s infinite immensity. Thomas Bradwardine had arrived at the same conclusion some years before.

Oresme also responded to article 49 of the Condemnation of 1277. He was not disturbed by the fact that a vacuum would be left behind by rectilinear motion, since he assumed a void space existed beyond our world. Rather he argued that the movement of the cosmos through void space must be absolute motion, because there were no other bodies to which its motion could be related. By contrast, according to Aristotle, a body could only change places and be in motion, when its position could be related to the position of other bodies. Similar reflections on God moving the world with a rectilinear motion played a role in seventeenth-century discussions about space, void, and the infinite.

Rather he used the movement of the cosmos through void space to argue that it was an absolute motion, because there were no other bodies to which its motion could be related. For Aristotle a body could only be considered to change places, and therefore to be in motion, when it could be related to the position of other bodies. The idea of God moving the world with a rectilinear motion was destined to play a role in seventeenth-century discussions about space, void, and the infinite.

Medieval natural philosophers also imagined that God could make parts of our world vanish, thereby leaving void spaces behind. Could bodies move through such void spaces, and if so, how? Scholastics generated a lively body of literature as they inquired whether bodies in a vacuum would move instantaneously or successively. In the course of these discussions, scholastics introduced concepts that do not appear in Aristotle, but which made motion in a hypothetical void plausible. Walter Burley , Thomas Bradwardine , and Albert of Saxony concluded that homogeneous mixed bodies of unequal weight would fall with the same speed in a vacuum. In one of his early treatises (1590), Galileo also came to the same conclusion. But later in his Two New Sciences of 1638, Galileo abandoned this conclusion and rightly concluded that bodies of whatever weight and composition would fall with the same speed in a vacuum.

Towards a Merger of Natural Philosophy and the Exact Sciences

Although departures from Aristotle were common in medieval natural philosophy, those changes did not produce a transformation of Aristotle’s natural philosophy, which remained the basic interpretation of the physical world until the seventeenth-century. Nevertheless, medieval natural philosophy relied heavily on reason and imagination to explain and revise Aristotle. The “probing and poking around” that characterized their inquiry was nothing less than the scientific spirit.

The late Middle Ages not only produced a highly developed natural philosophy, but also witnessed the beginnings of the vital merger between natural philosophy and the exact sciences, a process that made early modern science and the Scientific Revolution possible.

1. Aristoteles Latinus editors point out that several of Moerbeke's `translations' were lightly revised versions of older versions by James of Venice, etc.

2. These questions appear in Edward Grant, “Scientific Imagination in the Middle Ages,” Perspectives on Science, Vol. 12, No. 4 (2004), 396-397. They are drawn from A Source Book in Medieval Science (Cambridge, MA: Harvard University Press, 1974), pp. 199-210.