Physics: Aristotelian Physics

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Physics: Aristotelian Physics


No other philosopher had such a deep and long-standing impact on Western science as the Greek philosopher Aristotle (384–322 BC). In the fourth century BC he developed a fully comprehensive worldview that would, with only a few modifications, stand for about 2,000 years. Rather than merely collect isolated facts, he posed fundamental questions about nature and the methods needed to study it. Physics in the Aristotelian sense was a fundamental understanding of matter, change, causality, time, and space, all of which had to be consistent with logic and experience. From this he derived a cosmology that allowed him to explain all phenomena from everyday life to astronomy, including both natural phenomena and technology.

Historical Background and Scientific Foundations

Aristotle was born in Stagira, on the Chalcidic peninsula of Macedonia; his father, Nichomachus, was physician to King Amyntas III of Macedonia. Aristotle lived in a time of extreme political turbulence that deeply influenced his life. When the 17-year old Macedonian moved to Athens to enroll at the famous Academy of Plato (c.428–c.348 BC), the city-state had lost its former political hegemony, but still had an international reputation in education. When Amyntas's son, King Philip II, began to conquer the Greek states in 359 BC, it spawned a wave of anti-Macedonian sentiment that almost certainly made Aristotle's life difficult. When his patron Plato died in 347 BC and Athens declared war against Macedonia, Aristotle left for the city of Assos in Asia Minor (modern-day Turkey), where he led a group of philosophers.

Around 343 BC, Philip brought Aristotle to his court as a tutor for his son Alexander (356–323 BC), a brilliant young man who would go on to conquer the world's largest empire to date, ranging from Greece eastwards to India and southwards to Egypt. Under Alexander the Great's rule, Aristotle returned to Athens at the age of 49 to found a new school called the Lyceum. When Alexander died only 13 years later and his huge empire fell apart, Aristotle left for Chalcis, where he died shortly afterward.

Aristotle's intellectual work was truly encyclopedic. His writings cover fields as diverse as logic, epistemology, metaphysics, rhetoric, physics, chemistry, biology, psychology, political studies, ethics, and literature studies; many of these disciplines, most notably logic and biology, claim Aristotle as their founding figure. Even in mathematics, which Aristotle conspicuously neglected (although it was then a major topic at Plato's Academy), he influenced Euclid's (c.325–c.265 BC) geometry through his axiomatic approach to logic. Moreover, Aristotle's general approach became the standard scientific method for about 2,000 years.

Unlike other philosophers, who presented their views in aphorisms or narratives, Aristotle developed a systematic approach. For each issue, he first collected all the views and arguments by his predecessors, which makes his work a rich source for historical studies. Then he clarified the meaning of all pertinent concepts and analyzed the various views and their conflicts. To resolve a fundamental issue, Aristotle drew on different sources. Were the views in accordance with available empirical data? Were the arguments sound? Did the views appeal to common sense? Finally, did they fit with knowledge previously established by the same method? Working incrementally through the entire realm of knowledge with this method, Aristotle built a stable philosophical system that covered almost every discipline and stood for about two millennia with only slight modifications.

The Causality of Nature

The English term “physics” is derived from the Greek physike episteme, the knowledge and study of nature, or physis. Even in the early nineteenth century, physics was a blanket term for natural philosophy, which covered all scientific disciplines. In antiquity, however, the fields of modern physics (e.g., electricity, magnetism, and thermodynamics) were either undeveloped or misunderstood. For instance, mechanics was considered a craft like carpentry, and optics was either a theory about visual sensation, or a branch of mathematical geometry. For Aristotle and his followers, mathematics was clearly distinct from physics, because it described nature in purely numerical terms. The task of physics was to explain nature.

Aristotle's approach is still appealing today because of his straightforward reasoning. For him, explaining nature meant answering “why” questions, insisting that scientists have fulfilled their duty only if all questions have been answered satisfactorily. He observed that people ask four different types of why questions, each of which requires an answer that reflects a distinct cause. Consider an example that covers the four different causes: “Why does a knife cut meat?” If you respond that the knife is made of iron, which is harder than meat, you are referring to the material cause. Arguing that the knife has a sharp blade provides the form cause. If you explain the mechanism by which the knife takes the meat apart, you give the efficient cause. And if you say that the knife can cut meat because that is the purpose for which it has been made, you provide the final cause. For a satisfying answer, you must refer to all four causes, although their relative importance may differ from case to case.

Of course, a meat-cutting knife is not an example of physics (the study of nature) in the ancient sense, because knives are artifacts and not natural things. Aristotle, however, was convinced that we ask the same four kinds of why questions for both. In particular, unlike modern physicists, he thought that scientists must not forget the final cause to provide satisfying answers. For instance, a blooming flower could not sufficiently be explained simply by explaining the mechanism that makes the flower bloom. A satisfying answer, according to Aristotle, must refer to the bloom's purpose—enabling the flower to reproduce—which he thought was embedded in the flower like an unfolding program. Moreover, the flower's proper form develops only in the state of blooming, and this is not only part of our concept of flowers, it is also part of the flower itself throughout its development.

Aristotle defined natural things as those that develop and are what they are only by virtue of causes internal to them. Artifacts, in contrast, are made by humans according to human goals, which are external to the objects. Examples of natural things are stars, animals, plants, stones, clouds, and basic materials; examples of artifacts are houses, furniture, cloth, and tools. However, the distinction is not a simple one. For instance, when a rotting chair looses its original form, it is still an artifact insofar as it is a piece of furniture, but it becomes a natural thing, a piece of matter, insofar as rotting is a natural process determined by its basic material properties. A hedge is natural insofar as it is a plant that grows according to its own principles, but artificial insofar as humans have shaped it into a certain form for human ends. Hence, the world cannot simply be divided into natural and artificial things—the division depends on how we perceive them.

The Dynamics of Nature

Aristotle was convinced that nature is essentially dynamic, and that natural things are under continuous development. Thus, understanding a natural thing requires two perspectives: we need to know 1) what the thing is composed of, and 2) how and why the thing alters. In response to the first question, Aristotle developed a metaphysical scheme that shaped his entire philosophy: Every real thing, whether natural or artificial, is composed of matter and form. For instance, a brick consists of clay shaped like a rectangle. Rectangular forms that are not materialized, as in geometry, are not real things but simply mathematical ideas. On the other hand, real things can be the material of which other real things consist if they are arranged in a certain form. For instance, bricks are the material for building houses and houses are the material of cities. Aristotle used this scheme to build up the entire cosmos.

To understand the dynamics of natural things, Aristotle distinguished between four kinds of processes. First, a thing can simply move in space without being changed. Second, it can grow or shrink, i.e. increase or decrease in size, without changing its characteristics. Third, it can undergo qualitative changes—such as when a tadpole is transformed into a frog—without losing its identity. Finally, it can undergo substantial change when it emerges out of or turns into something entirely different; this occurs, for instance, when an animal dies and decomposes into basic materials or when basic materials undergo chemical transformation. Once we have identified the kind of change—spatial, quantitative, qualitative, or substantial—we can investigate its cause, which for Aristotle are both the efficient and final causes.

In every change, Aristotle believed, something must persist throughout the process. While this is obvious with spatial and quantitative changes, it is more difficult to identify in qualitative and, particularly, substantial changes. According to Aristotle, the matter of each real thing persists even though its form changes. For instance, when we form a mug from a lump of clay, the clay persists and gradually changes its form, going from a lump to a mug. Since some forms can't be made from clay (spider webs, for instance), matter and form are related. Thus, clay has the potential to assume the form of a mug, but not that of a spider web. This is more important for natural processes, where the causes of change are internal. For instance, a tadpole has the hidden potential to assume the form of a frog instead of a bird or something else. Therefore, Aristotle also described any process as a change from potentiality (a potential frog) to reality (a real frog).

Furthermore, Aristotle thought that change always requires some interaction between the changing thing and its cause, and that the change ends when the interaction stops, an idea that was revised in early modern mechanics. For instance, if we heat water with fire, fire acts on water because water is susceptible to the action of fire; as soon as we stop heating it, the water cools down. Similarly, if a change is driven by a final cause, the object of change needs to be susceptible to this final cause and stop changing as soon as the final cause is removed.

The Elements of Nature before Aristotle

One of Aristotle's most persistent contributions to science, and indeed the core of his physics, was his theory of the elements, which endured until the end of the eighteenth century and the dawn of the chemical revolution. Apart from astronomy, the theory of the elements was the core of ancient natural philosophy. It explained the plurality and change of all matter, disciplines now called chemistry and particle physics. Unlike today's scientists, however, ancient philosophers rarely conducted experiments, but searched instead for rational systems that were in accordance with all available and observable data. Before we deal with Aristotle's solution, we will briefly look at those of his predecessors.

Little is known of the pre-Socratic philosophers; only indirect reports and a few extant fragmentsremain, and those are difficult to understand. By the seventh century BC, Greek philosophers had broken with their religious traditions, rejecting the idea that natural phenomena were caused by supernatural forces. Instead they characterized the ultimate principles of nature by their material properties. Many preSocratics were monists, who argued that a single material principle underlay the plurality and change of all matter. For Thales (c.624–c.546 BC) this principle was water, for Anaximenes (c.585–c.528 BC) it was air, and for Her-aklitus (c.540–c.480 BC), fire.

Pluralists, like Anaxagoras (c.500–c.428 BC), assumed that the infinite plurality of things required infinite principles, and that any change is caused by the mixing and separating of the elements. Working from Pythagoras' (c.582–c.500 BC) idea that everything is founded in the dualism of opposing principles, Empedocles(c.495–c.435 BC) developed the first ancient synopsis on which Aristotle would later draw. He combined the earlier suggestions of water, air, and fire with earth into a system of four elements that interacted with each other by the opposing principles of attraction and repulsion to form the plurality of all things.

The most interesting account may be that of atomism, expressed by Democritus (c.460–c.370 BC) and based on the earlier ideas of Leukippus (fl. 5th c. BC). On one hand, atomism resembled Anaxagoras' pluralism, because Democritus claimed that there were an endless number of atoms that form the variety of things, and that all change is caused by their separation and mixing. On the other, ancient atomism was a dualistic doctrine, because its proper principles were matter and void. Thus, atoms (from Greek atomos, indivisible, uncut) were thought to be a certain distribution of matter and void, such that matter forms invisibly small regions of irregular shapes that persist through all changes in time.

Atomism remained a prominent but much-contested doctrine throughout its history. Its critics, first among them Aristotle, had many objections. Since matter, according to Democritus and unlike all the other philosophies of nature, had no material properties, it was unclear how it differed from void. When Democritus argued that matter was full whereas void was empty, critics objected that the empty void was not a principle of nature but merely nothing, and that claiming the existence of nothing was a contradiction. The debate continued up to early modern times as the question of whether or not vacuums could exist.

Others argued that there was no empirical evidence for the existence of atoms. Further, since matter had no material properties, every explanation of material properties based on supposed atomic shape was highly speculative. Indeed, Democritus and his followers arbitrarily claimed various shapes to explain differences in color, taste, or any other empirical properties. Finally, atomism was a difficult concept for many people to grasp.

The idea that matter could be indivisible and that it had no intrinsic properties were counterintuitive, because evidence suggested just the opposite.

Plato had developed his own version of atomism that drew on earlier Pythagorean ideas, some sophisticated mathematics, and the doctrine of Empedocles. Although it was esoteric even for contemporaries, it became influential because Plato set his theory in the form of a creation myth. In it, the divine creator builds the world according to geometrical ideas by shaping not matter but space. Empedocles' elements of fire, air, water, and earth consisted of four invisibly small regular polyhedra. These were not atoms but consisted of indivisible triangles of two different types. Plato selected their mathematical construction in such a way that several material changes (e.g., fire boils water to become air-like steam) could be explained by a quasi-geometrical mechanism. For instance, the sharp-edged tedrahedra of fire could split the blunt-edged dodecahedra of water into their composing triangles, which could then reassemble to form the octahedra of air.

Aristotle's Elements of Nature

Aristotle rejected both kinds of atomism, and argued that Plato's system confused mathematical ideas with real things. Instead, he added a new foundation to Em-pedocles' four elements. In Aristotle's view, the elements of nature must represent the fundamental characteristics of nature, i.e., they must bear the basic properties of matter that drive the dynamics of nature. Matter's basic characteristic is its tangibility, which for Aristotle included two tactile properties: matter is more or less dry (hard) or wet (soft) and more or less cold and hot.

To cover the whole realm of these two property dimensions, each element had one extreme property from each dimension, producing four pairs of properties to which Aristotle related Empedocles' four elements: dry and cold were the characteristics of earth, wet and cold those of water, wet and hot those of air, and dry and hot those of fire. Moreover, for Aristotle, hard and soft were passive properties, because they determined the malleability of materials, whereas hot and cold were active properties because they could act on other materials.

For instance, water expands if it is heated by fire and shrinks if it is cooled. The two pairs of properties thus represented both the empirical characteristics of matter and the basic interactions between materials.

Aristotle used his theory of the elements to explain a wealth of natural phenomena ranging from chemistry, physics, and meteorology to biology and medicine. Moreover, his theory allowed him to write the first treatise on what we would call the chemical processing of materials, including metallurgy and cooking. He made no fundamental distinction between natural and technological phenomena, because the materials and their interactions were essentially the same in natural and artificial processes. Furthermore, the elements structured the entire world in two different approaches.

The Hierarchical Structure of the World

As in Plato's theory, Aristotle's elements could interact with and transform each other. When an excess of fire (hot and dry) acted on water (cold and wet) to neutralize the elemental property cold, water turned into a kind of air (hot and wet). For Aristotle, the elements were real things, although they occurred only in impure forms or mixtures. According to his metaphysical doctrine, both elements and real things must be composed of form and matter, and their elemental properties were their specific form.

In elemental transformation, his paradigm for substantial change, the elemental form was replaced. His theory of substantial change required that a primary matter, devoid of any qualities, persisted through the change. Since primary matter had no qualities and no form, it was not a real thing but only the bearer of elemental properties and the substratum of substantial change that united the physical world. Nonetheless Aristotle's primary matter would later inspire numerous misunderstandings, particularly among alchemists in their experimental search for the basic principle of matter.

Starting with the elements composed of primary matter and their specific form of elemental properties, Aristotle developed a hierarchy of the physical world in which each step provided the matter for the next. For basic compounds, the elements served as matter and their composition as specific form. In the next step, heterogeneous compounds such as wood could be combined and structured to form parts of living beings, like a human arm or the trunk of a tree. If combined and organized according to certain forms and ends, they would form a living being. For Aristotle this required at least a “vegetative soul” to serve as the organizing principle and to control the metabolism. Animals differed from plants by an additional higher-order soul that allowed living beings to move and feel. Humans were endowed with an additional “intellectual soul” that enabled them to organize their life according to ideas and goals.

The inorganic world, including air, water, and earth, was spatially and chronologically structured to form regular and periodical phenomena like the weather and the seasons, for which Aristotle identified the sun, the moon, and the stars as their structuring and moving principle. Finally, since for Aristotle every movement must have a cause, he postulated gods as the ultimate cause of the regular motion of the stars. Like primary matter, these gods were not real things composed of matter and form. Rather, like the human intellect, which can organize real events through its nonmaterial existence and activity, the gods were pure form and so-called “unmoved movers.” Entities of complete independence and modesty, they also served as models for human beings.


For the foundation of his work, Aristotle turned to the ideas of his predecessor Empedocles (c.495–c.435 BC). Empedocles thought that change was the result of the interaction of four elements: fire, water, earth, and air. The forces of love and hate acted upon these elements, and their interaction first led to minerals, then plants, then animals from a long series of trial-and-error interactions.

Aristotle also thought that all matter in the terrestrial part of the universe, comprised of the area below the sphere of the moon to Earth (sublunar region), was made by interactions between the four elements. Earth, which was made of the heaviest earthly element, rested immovably in its natural place, at the center of the sublunar region. The watery sphere surrounded Earth, but the boundaries between the earth and water were irregular, because the higher parts of the land projected above the oceans that surround our globe. The sphere of the air was next. Above it, but below the moon, was the sphere of fire, which was the lightest of the elements and the transition to the eternal realms of the planets. This sublunar realm was a realm of change and corruption.

In the area of the universe beyond the moon, everything was made of a fifth element he called the ether. Planetary orbits were solid crystalline spheres made of the ether, to which the perfectly polished planetary bodies, also made of the ether, were firmly attached. The heavens were immutable.

The forces behind the movement of the elements were ultimately due to an eternal being called the Prime Mover. Taking a cue from Empedocles, Aristotle's Prime Mover functioned as an object of love and desire for the soul that animated the body of the outermost sphere of fixed stars, the primum mobile. The primum mobile rotated at an enormous speed every twenty-four hours and communicated this motion to the planetary orbital spheres. Later Christian commentators such as St. Thomas Aquinas (c.1225–1274) adapted Aristotle's idea of the Prime Mover to their conception of God, another reason for the durability of Aristotelian philosophy.

Anna Marie Eleanor Roos

The Cosmological Structure of the World

Aristotle viewed the cosmos as a series of spherical shells, each related to one element, around Earth. Ancient Greeks knew that the planet was a sphere, and even measured it with some precision. It consisted mainly of the element earth, with a surface largely covered by water, and its atmosphere was dominated by air. The lower atmosphere was filled with moisture—clouds and rain—owing to turbulence at the interface between the water and air shells that determined the weather. Above the atmosphere, the next sphere, reaching up to the height of the moon, was filled mainly with the element fire.

Aristotle saw ample empirical evidence of this shell model. In particular, earth was heavier than water, which in turn was much heavier than air; and flames obviously rose up into the air. In water, a stone sank down whereas a bubble of air rose up. Based on such empirical regularities he drew the general conclusion that each element tended to move to its specific shell, which he called its proper place. This theory could also explain any ordinary phenomenon on Earth that we now explain by the force of gravity.

Above the moon, things were obviously different, since the sun, stars, and planets appeared to move in semiregular circles around Earth, a motion that was, without the help of additional forces, impossible on Earth. Because of this, Aristotle postulated that the stars and their surroundings were composed of an entirely different matter, unknown to humans, which he called ether and which enabled circular rather than straight-line motion.

Aristotle's model was based on the theories the astronomer Eudoxos (c.395–342 BC), who developed a complex geometric model that explained the stars' irregular motions by the superposition of many regular circles. This geocentric cosmological model, with Earth at its center and all the celestial bodies moved around it in circular orbits, was later developed in greater detail by the Greek mathematician and astronomer Claudius Ptolemy (c.AD 90–c.168). As early as the third century BC, however, an astronomer from Aristotle's own school, Aristarchus of Samos (c.310–c.230 BC), suggested that the sun was at the center of the cosmos, with Earth moving around it. This heliocentric model, although known to many succeeding astronomers, did not gain acceptance until Polish astronomer Nicolaus Copernicus (1473–1543) developed it with a mathematical rigor that could explain the irregular orbits with greater precision.

Aristotle's cosmology would be incomplete without his views on time and space. If you ask, “What is in space beyond the sphere of the stars?” Aristotle would have responded that this question has no meaning because there is no space beyond the sphere of the stars. For him the entire cosmos was a huge but finite sphere composed of matter, with each element, including the ether, in its specific place. For Aristotle, space without matter did not exist, in either cosmology or in atomism.

Unlike space, however, he saw time as infinite, without beginning or end. The cosmos was eternal, without beginning or end, because both its emergence out of nothing and its vanishing into nothing violated the basic principles of his metaphysics of change. Moreover, owing to the regular movement of the stars, and ultimately to the eternal nature of the gods, neither radical nor evolutionary changes were possible. Indeed, Aristotle believed that biological species did not evolve but were stable, in the same way that minerals were. Even if, by some natural disaster, some species disappeared, the long-term balanced conditions on Earth would enable its reemergence.

Aristotle's Natural Philosophy in the Middle Ages

After the fall of the Roman empire in the fifth century AD, most of Aristotle's philosophy was lost to the West for centuries. His works were translated into Arabic in the Muslim world, where they helped form the basis of much Islamic science from the eighth century onward. As Europe made contact with the eastern world in the late Middle Ages, the writings of classical philosophers were rediscovered.

The earliest translations of Aristotle's scientific works from Arabic to Latin in the twelfth century shocked many medieval Christians. Until then only fragments of his logic had been known, but even that was enough to make him the unquestioned authority in all logical and philosophical matters. Now they learned that the revered philosopher had taught that the world was not created by God, as the Bible said, but eternal, without beginning and end. Moreover, Aristotle had defined gods as “unmoved movers” who guaranteed the eternal movements of the stars, but did not intervene in worldly events, effectively dismissing the idea of miracles or the role of angels.

The writings of the greatest Arabic Aristotlean scholars included those of Averroes (1126–1198), whose numerous commentaries on Aristotle's teachings were particularly influential. He posited that the human soul could not, according to Aristotle, survive physical death, which contradicted the Christian doctrine of the soul's immortality. Thus, Christian authorities' first reaction was to ban the teaching of Aristotle's science altogether on pain of death. However, the German philosopher Albertus Magnus (c.1200–1280), and particularly his pupil Thomas Aquinas (c.1225–1274), undertook enormous efforts to reconcile Aristotle's natural philosophy with Christian doctrine by writing voluminous commentaries that explained in great detail how Christians should interpret Aristotle's texts.

Thanks to these commentaries, Aristotle's revised natural philosophy moved into the core curricula of the newly established European universities, where it remained for at least four centuries. Furthermore, Aquinas's blend of Aristotelian and Christian views, which came to be known as Thomism, was made the official doctrine of natural philosophy and metaphysics by the Roman Catholic Church and has remained so up to today.

This theological assimilation gave Aristotle's natural philosophy an extraordinary status. On the one hand, any criticism or differing views were threatened by official sanctions, ranging from a ban on teaching

and publishing to excommunication and even death. The Italian philosopher Giordano Bruno (1548–1600), for example, was burned at the stake for his rejection of Aristotelean cosmology and his proposition of a heliocentric solar system and infinite universe. On the other, it closely related natural philosophy to theology, infusing all debates on natural philosophy, including attempts to overcome the Aristotelian system and to establish what we call modern science, with religion. Since Aristotelian natural philosophy was administered by the church, however, criticism grew with the Protestant Reformation.

Early Attempts to Overthrow the Aristotelian System

Apart from its Christian assimilation, Aristotle's natural philosophy was a strong system based on intertwined metaphysical principles that could not easily be altered. Radical changes were required to build a new system, but any such change was threatened with persecution. The French philosopher and mathematician René Descartes (1596–1650) solved this paradox by building a new system based on selected and remodeled Aristotelian principles. Where Aristotle had claimed four different causes in nature (formal, material, efficient, and final) with which scientists must explain natural phenomena, Descartes selected only the efficient cause.

The scientist's task, according to Descartes, was to explain all natural phenomena solely by its causal mechanism. Similarly, of Aristotle's four kinds of change (spatial, quantitative, qualitative, and substantial), Descartes choose only spatial motion, declaring that any qualitative or substantial change could ultimately be reduced to the motion and collision of particles in space. He remodeled Aristotle's principles of form and matter to become geometrical form and spatial extension, and characterized the elements by the geometrical form and size of their particles rather than the elemental qualities of hot, cold, wet, and dry. In the end, Descartes's universe strongly resembled ancient atomism, with invisible particles swirling around, but he rejected both the ideas of an empty space or vacuum and of indivisible particles.

Descartes's new emphasis, however, was the idea that the mechanism of any particle motion (and thus any natural phenomena) could be expressed mathematically. He developed a set of mathematical theories that would strongly influence English physicist and mathematician Isaac Newton's (1642–1727) later principles of mechanics. Indeed, Descartes, along with Italian mathematician and astronomer Galileo Galilei (1564–1642), formulated what we now call the principle of inertia, according to which a body set in motion tends to continue its motion in a straight line as long as no other external cause interferes.

This was an important departure from Aristotelian physics in two regards. First, Aristotle taught that motion or change continues only as long as the moving cause is effective; inertia required only a moving cause at the beginning of the motion. With respect to the entire universe, an initial impetus would suffice to cause all the succeeding dynamics of the universe. That idea was theologically appealing to Descartes and his followers of mechanical philosophy because it could convert Aristotle's “unmoved mover” into God, who started the dynamics of the universe at creation.

Second, since Descartes (unlike Galileo) claimed his principles were valid for all motions, he rejected the Aristotelian distinction between earthly and celestial physics. In particular, he dismissed the prominent idea that the natural motion of the stars was circular rather than straight and instead tried to explain the quasicircular movement of celestial bodies by gigantic vortices of celestial particles. This approach, despite its weaknesses, would later inspire Newton to unite earthly and celestial mechanics with the common force of gravitation.

The science of ballistics began to develop in the sixteenth century, following the model of the ancient mathematician and engineer Archimedes (c.287–212 BC), who used empirical measurements to solve engineering problems. In an effort to maximize the range of

projectiles, the Italian military engineer Niccolò Tarta-glia (1499–1557) studied their paths. He was the first to analyze the curved trajectory as being simultaneously caused by the (artificial) impetus in the direction of the shot and the (natural) gravity down to the earth.

Once separated analytically, the two components of motion yielded to further empirical studies. The Dutch engineer Simon Stevin (1548–1620) dropped two lead projectiles of different size from the same height; he concluded that their velocity was the same regardless of their weight. This contradicted the physics of Aristotle, who had reached the opposite conclusion from the different velocities of, say, a piece of metal and a feather. Galileo, to whom Stevin's experiment has wrongly been attributed, further studied the motion of falling bodies by combining metaphysically inspired mathematical hypothesis with measurements.

Galileo reasoned that all natural motions must be mathematically simple. This meant that the simplest motion, with constant velocity or distance proportional to time, was reserved for celestial bodies. Freely falling bodies on Earth moved according to the second-simplest motion: constant acceleration or velocity proportional to time. Because sixteenth-century clocks were too inaccurate to prove his hypothesis, he modified the experiment to measure the time that a ball needed to roll down an inclined plain. The measurements confirmed his mathematical hypothesis, which came to be known as the law of free fall. It allowed Galileo to describe Tartaglia's trajectories as parabolic curves and to prove mathematically what Tartaglia had shown only by empirical tests: The maximum range of projectiles was achieved when the shot was made at a 45° angle. Newton integrated this into his laws of general mechanics, which combined celestial and ballistic motion in a uniform mathematical theory centered on the force of gravitation.

Mechanics was only a marginal part of Aristotle's comprehensive natural philosophy, because outside of astronomy it did not apply to natural phenomena. Although the rise of mathematically based mechanics by Descartes, Galileo, English natural philosopher Robert Boyle (1627–1691), Newton, and others is now called the scientific revolution, it did not touch on most topics covered by Aristotle. Indeed these subjects, the bulk of modern scientific disciplines, remained deeply influ-enced by Aristotle's philosophy for centuries. His biology in particular stood almost unmodified well into the nineteenth century, when it was finally eclipsed by Darwin's theories of evolution and natural selection.

Aristotle's theory of elements and compounds was the foundation of eighteenth-century chemistry, mineralogy, meteorology, geology, and medicine, even though in retrospect it more accurately covered what we today call thermodynamic phenomena, e.g. the boiling or freezing of water, rather than truly chemical transformations. Since it also claimed that one element could be transformed into another, it became the theoretical basis of alchemy, which proposed the very non-Aristotelian idea of studying nature by trying to transform it. Eventually, however, that became the approach of modern experimental laboratory science.

Despite their inability to transform base materials into gold, alchemists or “chymists,” in sixteenth-century parlance, did create a plethora of new materials and chemical phenomena in their laboratories that defied Aristotelian explanation. For centuries Aristotelian elements had been supplemented only by additional “chymical principles” to account for such phenomena as burning, calcination, or acid-forming. It was not until the late eighteenth century that the theory of matter was put on a new, experimental basis. Instead of placing the elements in a metaphysical system, as Aristotle had done, French chemist Antoine Lavoisier (1743–1794) defined elements as any material of matter that resisted experimental efforts to take it apart.

Modern Cultural Connections

Even though most of Aristotle's scientific answers are now outdated, his texts provide compelling reading. He poses “common sense” questions that provide a benchmark


On the surface, Aristotle's explanations agree with most basic sensory observations of motion, but there were some troubling exceptions that had important implications for the development of modern physics. If a rock is hurled from a catapult, it continues to travel even after it has left the arm of the machine and does not drop to the ground immediately, as Aristotelian physics would predict. Later commentators claimed that the air in front of the rock was disturbed by the motion of the rock and swirled behind the rock and pushed it along. Other physicists, such as John Philoponus (AD 490–570) modified Aristotle's theory with the concept of impetus. Philoponus claimed that a projectile moves on account of a force or impetus the mover gives it, which exhausts itself in the course of the movement. Impetus would keep a projectile moving after it left the catapult. Although erroneous, Philoponus' idea did demonstrate some of the inconsistencies of Aristotelian physics. Galileo later solved this problem by demonstrating that projectile or parabolic motion was the result of inertial and gravitational forces.

Aristotle also attempted to explain the speed of objects in free fall, claiming that the weight of the object divided by the resistance of the medium in which it traveled would result in its speed. This was why, he reasoned, a rock seemed to fall faster in the air than a feather, a claim that would not be disproved until the work of Galileo in the seventeenth century. Aristotle's concept of the speed of motion also did not explain free-fall acceleration, as the speed of objects in his scheme should be constant. Later commentators attempted to explain acceleration in Aristotelian terms as due to the increasing desire of the object to reach its natural place, a concept also shown to be false by Galileo.

According to his equation for speed, Aristotle also realized that if an object moved in a medium without resistance (no friction), it would go infinitely fast and move forever, a concept he rejected as absurd. The concept he rejected was the principle of inertia, later discovered by French philosopher and mathematician René Descartes (1596–1650) and comprising Newton's First Law—the property of an object to remain at constant velocity unless acted upon by outside forces. In a frictionless environment, a pushed object will move forever at a constant velocity.

Aristotle's concept of the natural circular motion of the planets also did not agree with empirical observation, as planets move irregularly in the sky in retrograde or looping fashion along the band of the zodiac. So influential however was Aristotle's cosmological system though, that the purpose of astronomy until the European Renaissance in the sixteenth century was to provide a mathematical theory that preserved the circle as a means of calculating planetary positions, yet explained observed deviations from those circular orbits. By using the devices of the epicycle and the equant point, Roman astronomer Ptolemy (c.AD 90–168) created a set of compounded circles to account for the irregularities in the apparent motions of all of the planets. Ptolemy's book The Almagest was the first mathematical treatise that systematically gave a complete and quantitative account of all the celestial motions, and it was based on Aristotle's concept of the heavens. The Almagest was not replaced until the work of Polish astronomer Nicolas Copernicus (1473–1543) which demonstrated the solar system was sun-centered, not Earth-centered. German astronomer Johannes Kepler's (1571–1630) first planetary law in the early seventeenth century also demonstrated that planets move in elliptical—not strictly circular—orbits.

Though sometimes internally inconsistent or incompatible with sense observation, Aristotle's physics remained the dominant paradigm in scientific thought for two thousand years.

AnnaMarie Eleanor Roos

of early human understanding of the Cosmos that are increasingly challenged by modern science (which is increasingly counterintuitive, especially in areas of quantum physics).

See Also Astronomy and Cosmology: A Mechanistic Universe; Astronomy and Cosmology: Big Bang Theory and Modern Cosmology; Astronomy and Cosmology: Cosmology; Astronomy and Cosmology: Western and Non-Western Cultural Practices in Ancient Astronomy:; Astronomy and Space Science: Astronomy Emerges from Astrology; Physics: Articulation of Classical Physical Law; Physics: Newtonian Physics.



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Joachim Schummer

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Physics: Aristotelian Physics

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