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Aristotelian physics

Aristotelian physics is a form of natural science described in the works of the Greek philosopher Aristotle (384–). In his work Physics, Aristotle intended to establish general principles of change that govern all natural bodies, both living and inanimate, celestial and terrestrialincluding all motion, change with respect to place, change with respect to size or number, qualitative change of any kind; and "coming to be" (coming into existence, "generation") and "passing away" (no longer existing, "corruption"). To Aristotle, "physics" was a broad field that included subjects such as the philosophy of mind, sensory experience, memory, anatomy and biology. It constitutes the foundation of the thought underlying many of his works.


Aristotle's Physica, a book addressing a variety of subjects including the philosophy of nature and topics now part of its modern-day namesake: physics.]] |Aristotle|Physics VIII.1}} While consistent with common human experience, Aristotle's principles were not based on controlled, quantitative experiments, so, while they account for many broad features of nature, they do not describe our universe in the precise, quantitative way now expected of science. Contemporaries of Aristotle like Aristarchus rejected these principles in favor of heliocentrism, but their ideas were not widely accepted. Aristotle's principles were difficult to disprove merely through casual everyday observation, but later development of the scientific method challenged his views with experiments and careful measurement, using increasingly advanced technology such as the telescope and vacuum pump. |Michael J. White|"Aristotle on the Infinite, Space, and Time" in Blackwell Companion to Aristotle}}


s and comets were believed to originate. The surrounding celestial spheres from inner to outer are those of the Moon, Mercury, Venus, Sun, Mars, Jupiter, and Saturn, each indicated by a planet symbol. The eighth sphere is the firmament of fixed stars, which include the visible constellations. The precession of the equinoxes caused a gap between the visible and notional divisions of the zodiac, so medieval Christian astronomers created a ninth sphere, the Crystallinum which holds an unchanging version of the zodiac. linkhttp://www.medievalastrologyguide.com/medieval-universe.html History of Science The tenth sphere is that of the divine prime mover proposed by Aristotle (though each sphere would have an unmoved mover). Above that, Christian theology placed the "Empire of God". What this diagram does not show is how Aristotle explained the complicated curves that the planets make in the sky. To preserve the principle of perfect circular motion, he proposed that each planet was moved by several nested spheres, with the poles of each connected to the next outermost, but with axes of rotation offset from each other. Though Aristotle left the number of spheres open to empirical determination, he proposed adding to the many-sphere models of previous astronomers, resulting in a total of 44 or 55 celestial spheres.]]

Elements and spheres

Aristotle divided his universe into "terrestrial spheres" which were "corruptible" and where humans lived, and moving but otherwise unchanging celestial spheres. Aristotle believed that four classical elements make up everything in the terrestrial spheres: earth, air, fire and water. He also held that the heavens are made of a special weightless and incorruptible (i.e. unchangeable) fifth element called " aether". Aether also has the name "quintessence", meaning, literally, "fifth being". Aristotle considered heavy substances such as iron and other metals to consist primarily of the element earth, with a smaller amount of the other three terrestrial elements. Other, lighter objects, he believed, have less earth, relative to the other three elements in their composition. The four classical elements were not invented by Aristotle; they were originated by Empedocles. During the Scientific Revolution, the ancient theory of classical elements was found to be incorrect, and was replaced by the empirically tested concept of chemical elements.

Celestial spheres

According to Aristotle, the Sun, Moon, planets and starsare embedded in perfectly concentric " crystal spheres" that rotate eternally at fixed rates. Because the celestial spheres are incapable of any change except rotation, the terrestrial sphere of fire must account for the heat, starlight and occasional meteorites.Aristotle, meteorology. The lowest, lunar sphere is the only celestial sphere that actually comes in contact with the sublunary orb's changeable, terrestrial matter, dragging the rarefied fire and air along underneath as it rotates. Like Homer's æthere (αἰθήρ)the "pure air" of Mount Olympuswas the divine counterpart of the air breathed by mortal beings (άήρ, aer). The celestial spheres are composed of the special element aether, eternal and unchanging, the sole capability of which is a uniform circular motion at a given rate (relative to the diurnal motion of the outermost sphere of fixed stars). The concentric, aetherial, cheek-by-jowl " crystal spheres" that carry the Sun, Moon and stars move eternally with unchanging circular motion. Spheres are embedded within spheres to account for the "wandering stars" (i.e. the planets, which, in comparison with the Sun, Moon and stars, appear to move erratically). Mercury, Venus, Mars, Jupiter, and Saturn are the only planets (including minor planets) which were visible before the invention of the telescope, which is why Neptune and Uranus are not included, nor are any asteroids. Later, the belief that all spheres are concentric was forsaken in favor of Ptolemy's deferent and epicycle model. Aristotle submits to the calculations of astronomers regarding the total number of spheres and various accounts give a number in the neighborhood of fifty spheres. An unmoved mover is assumed for each sphere, including a "prime mover" for the sphere of fixed stars. The unmoved movers do not push the spheres (nor could they, being immaterial and dimensionless) but are the final cause of the spheres' motion, i.e. they explain it in a way that's similar to the explanation "the soul is moved by beauty".

Terrestrial change

Unlike the eternal and unchanging celestial aether, each of the four terrestrial elements are capable of changing into either of the two elements they share a property with: e.g. the cold and wet ( water) can transform into the hot and wet ( air) or the cold and dry ( earth) and any apparent change into the hot and dry ( fire) is actually a two-step process. These properties are predicated of an actual substance relative to the work it is able to do; that of heating or chilling and of desiccating or moistening. The four elements exist only with regard to this capacity and relative to some potential work. The celestial element is eternal and unchanging, so only the four terrestrial elements account for "coming to be" and "passing away"or, in the terms of Aristotle's De Generatione et Corruptione (Περὶ γενέσεως καὶ φθορᾶς), "generation" and "corruption".

Natural place

The Aristotelian explanation of gravity is that all bodies move toward their natural place. For the elements earth and water, that place is the center of the ( geocentric) universe; De Caelo II. 13-14. the natural place of water is a concentric shell around the earth because earth is heavier; it sinks in water. The natural place of air is likewise a concentric shell surrounding that of water; bubbles rise in water. Finally, the natural place of fire is higher than that of air but below the innermost celestial sphere (carrying the Moon). In Book Delta of his Physics (IV.5), Aristotle defines topos (place) in terms of two bodies, one of which contains the other: a "place" is where the inner surface of the former (the containing body) touches the outer surface of the other (the contained body). This definition remained dominant until the beginning of the 17th century, even though it had been questioned and debated by philosophers since antiquity.For instance, by Simplicius in his Corollaries on Place. The most significant early critique was made in terms of geometry by the 11th-century Arab polymath al-Hasan Ibn al-Haytham ( Alhazen) in his Discourse on Place.

Natural motion

Terrestrial objects rise or fall, to a greater or lesser extent, according to the ratio of the four elements of which they are composed. For example, earth, the heaviest element, and water, fall toward the center of the cosmos; hence the Earth and for the most part its oceans, will have already come to rest there. At the opposite extreme, the lightest elements, air and especially fire, rise up and away from the center. Tim Maudlin (2012-07-22). Philosophy of Physics: Space and Time: Space and Time (Princeton Foundations of Contemporary Philosophy) (p. 2). Princeton University Press. Kindle Edition. "The element earth's natural motion is to fall— that is, to move downward. Water also strives to move downward but with less initiative than earth: a stone will sink though water, demonstrating its overpowering natural tendency to descend. Fire naturally rises, as anyone who has watched a bonfire can attest, as does air, but with less vigor." The elements are not proper substances in Aristotelian theory (or the modern sense of the word). Instead, they are abstractions used to explain the varying natures and behaviors of actual materials in terms of ratios between them. Motion and change are closely related in Aristotelian physics. Motion, according to Aristotle, involved a change from potentiality to actuality.Bodnar, Istvan, "Aristotle's Natural Philosophy" in The Stanford Encyclopedia of Philosophy (Spring 2012 Edition, ed. Edward N. Zalta). He gave example of four types of change. Aristotle proposed that the speed at which two identically shaped objects sink or fall is directly proportional to their weights and inversely proportional to the density of the medium through which they move. While describing their terminal velocity, Aristotle must stipulate that there would be no limit at which to compare the speed of atoms falling through a vacuum, (they could move indefinitely fast because there would be no particular place for them to come to rest in the void). Now however it is understood that at any time prior to achieving terminal velocity in a relatively resistance-free medium like air, two such objects are expected to have nearly identical speeds because both are experiencing a force of gravity proportional to their masses and have thus been accelerating at nearly the same rate. This became especially apparent from the eighteenth century when partial vacuum experiments began to be made, but some two hundred years earlier Galileo had already demonstrated that objects of different weights reach the ground in similar times.Lindberg, D. (2008), The beginnings of western science: The European scientific tradition in philosophical, religious, and institutional context, prehistory to AD 1450 (2nd ed.), University of Chicago Press.

Unnatural motion

Apart from the natural tendency of terrestrial exhalations to rise and objects to fall, unnatural or forced motion from side to side results from the turbulent collision and sliding of the objects as well as transmutation between the elements ( On Generation and Corruption).


In his Physics Aristotle examines accidents (συμβεβηκός, symbebekòs) that have no cause but chance. "Nor is there any definite cause for an accident, but only chance (τύχη, týche), namely an indefinite (ἀόριστον, "aóriston") cause" ( Metaphysics V, 1025a25). It is obvious that there are principles and causes which are generable and destructible apart from the actual processes of generation and destruction; for if this is not true, everything will be of necessity: that is, if there must necessarily be some cause, other than accidental, of that which is generated and destroyed. Will this be, or not? Yes, if this happens; otherwise not ( Metaphysics VI, 1027a29).

Continuum and vacuum

Aristotle argues against the indivisibles of Democritus (which differ considerably from the historical and the modern use of the term " atom"). As a place without anything existing at or within it, Aristotle argued against the possibility of a vacuum or void. Because he believed that the speed of an object's motion is proportional to the force being applied (or, in the case of natural motion, the object's weight) and inversely proportional to the viscosity of the medium, he reasoned that objects moving in a void would move indefinitely fastand thus any and all objects surrounding the void would immediately fill it. The void, therefore, could never form.Land, Helen, The Order of Nature in Aristotle's Physics: Place and the Elements (1998). The " voids" of modern-day astronomy (such as the Local Void adjacent to our own galaxy) have the opposite effect: ultimately, bodies off-center are ejected from the void due to the gravity of the material outside.

Speed, weight and resistance

The ideal speed of a terrestrial object is directly proportional to its weight. In nature however, vacuum does not occur, the matter obstructing an object's path is a limiting factor that is inversely proportional to the viscosity of the medium.

Four causes

According to Aristotle, there are four ways to explain the aitia or causes of change. He writes that "we do not have knowledge of a thing until we have grasped its why, that is to say, its cause."Aristotle, Physics 194 b17–20; see also: "Four Causes". Falcon, Andrea. Aristotle on Causality. Stanford Encyclopedia of Philosophy 2008. Aristotle held that there were four kinds of causes. Aristotle in 23 Volumes, Vols. 17, 18, Cambridge, MA, Harvard University Press; London, William Heinemann Ltd. 1933, 1989; (hosted at perseus.tufts.edu.) Aristotle also discusses the four causes in his Physics, Book B, chapter 3.


The material cause of a thing is that of which it is made. For a table, that might be wood; for a statue, that might be bronze or marble.


The formal cause of a thing is the essential property that makes it the kind of thing it is. In Metaphysics Book Α Aristotle emphasizes that form is closely related to essence and definition. He says for example that the ratio 2:1, and number in general, is the cause of the octave. |R.J. Hankinson|"Cause" in Blackwell Companion to Aristotle}}


The efficient cause of a thing is the primary agency by which its matter took its form. For example, the efficient cause of a baby is a parent of the same species and that of a table is a carpenter, who knows the form of the table. In his Physics II, 194b29—32, Aristotle writes: "there is that which is the primary originator of the change and of its cessation, such as the deliberator who is responsible for the action and the father of the child, and in general the producer of the thing produced and the changer of the thing changed".

Life and death of Aristotelian physics

.]] The reign of Aristotelian physics, the earliest known speculative theory of physics, lasted almost two millennia. After the work of many pioneers such as Copernicus, Tycho Brahe, Galileo, Descartes and Newton, it became generally accepted that Aristotelian physics was neither correct nor viable. Despite this, it survived as a scholastic pursuit well into the seventeenth century, until universities amended their curricula. In Europe, Aristotle's theory was first convincingly discredited by Galileo's studies. Using a telescope, Galileo observed that the Moon was not entirely smooth, but had craters and mountains, contradicting the Aristotelian idea of the incorruptibly perfect smooth Moon. Galileo also criticized this notion theoretically; a perfectly smooth Moon would reflect light unevenly like a shiny billiard ball, so that the edges of the moon's disk would have a different brightness than the point where a tangent plane reflects sunlight directly to the eye. A rough moon reflects in all directions equally, leading to a disk of approximately equal brightness which is what is observed.Galileo Galilei, Dialogue Concerning the Two Chief World Systems. Galileo also observed that Jupiter has moonsi.e. objects revolving around a body other than the Earthand noted the phases of Venus, which demonstrated that Venus (and, by implication, Mercury) traveled around the Sun, not the Earth. According to legend, Galileo dropped balls of various densities from the Tower of Pisa and found that lighter and heavier ones fell at almost the same speed. His experiments actually took place using balls rolling down inclined planes, a form of falling sufficiently slow to be measured without advanced instruments. In a relatively dense medium such as water, a heavier body falls faster than a lighter one. This led Aristotle to speculate that the rate of falling is proportional to the weight and inversely proportional to the density of the medium. From his experience with objects falling in water, he concluded that water is approximately ten times denser than air. By weighing a volume of compressed air, Galileo showed that this overestimates the density of air by a factor of forty.Galileo Galilei, Two New Sciences. From his experiments with inclined planes, he concluded that if friction is neglected, all bodies fall at the same rate (which is also not true, since not only friction but also density of the medium relative to density of the bodies has to be negligible. Aristotle correctly noticed that medium density is a factor but focused on body weight instead of density. Galileo neglected medium density which led him to correct conclusion for vacuum). Galileo also advanced a theoretical argument to support his conclusion. He asked if two bodies of different weights and different rates of fall are tied by a string, does the combined system fall faster because it is now more massive, or does the lighter body in its slower fall hold back the heavier body? The only convincing answer is neither: all the systems fall at the same rate. Followers of Aristotle were aware that the motion of falling bodies was not uniform, but picked up speed with time. Since time is an abstract quantity, the peripatetics postulated that the speed was proportional to the distance. Galileo established experimentally that the speed is proportional to the time, but he also gave a theoretical argument that the speed could not possibly be proportional to the distance. In modern terms, if the rate of fall is proportional to the distance, the differential expression for the distance y travelled after time t is: {dy\over dt} \propto y with the condition that y(0)=0. Galileo demonstrated that this system would stay at y=0 for all time. If a perturbation set the system into motion somehow, the object would pick up speed exponentially in time, not quadratically. Standing on the surface of the Moon in 1971, David Scott famously repeated Galileo's experiment by dropping a feather and a hammer from each hand at the same time. In the absence of a substantial atmosphere, the two objects fell and hit the Moon's surface at the same time. The first convincing mathematical theory of gravityin which two masses are attracted toward each other by a force whose effect decreases according to the inverse square of the distance between themwas Newton's law of universal gravitation. This, in turn, was replaced by the General theory of relativity due to Albert Einstein.

As listed in the Corpus Aristotelicum

See also


a Here, the term "Earth" does not refer to planet Earth, known by modern science to be composed of a large number of chemical elements. Modern chemical elements are not conceptually similar to Aristotle's elements; the term "air", for instance, does not refer to breathable air.



  • H. Carteron (1965) "Does Aristotle Have a Mechanics?" in Articles on Aristotle 1. Science eds. Jonathan Barnes, Malcolm Schofield, Richard Sorabji (London: General Duckworth and Company Limited), 161-174.

Further reading

  • Katalin Martinás, “Aristotelian Thermodynamics” in Thermodynamics: history and philosophy: facts, trends, debates (Veszprém, Hungary 23–28 July 1990), .
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