Notes on Aristotelian Physics

Kevin T. Kelly

Department of Philosophy

Carnegie Mellon Univeristy

Historical Significance

Plato (427-347) was Socrates's student. Aristotle (384-322) was Plato's student. Alexander the Great was Aristotle's student. Plato set up a school called the Academy. Following this example Aristotle set up a school called the Lyceum. From what has survived, Plato seems more introspective and mathematical whereas Aristotle seems more like an empirical biologist. Of course, this is an oversimplification, but it is a useful one.

Much of the history of both Islamic and Christian civilization can be tracked as a swinging pendulum of popularity between the schools of Plato and Aristotle. This may sound extravagant, but here is how it happened. The early Christian Church had its doctrinal roots in Platonic mysticism, as some of the best educated and most influential advocates of Christianity (e.g., St. Augustine) hadPlatonic training.

The western Roman empire collapsed in the late 5th century, leading to a collapse of organized learning and a loss of nearly all original Greek texts. Christian documents embodying neo-Platonic ideas were maintaind by the Church. The eastern empire stood until the mid 15th c.

The eastern and western branches of the Church were drawn into early disagreement. The western branch maintained that Jesus was really a man and also really divine. Eastern theologians around Syria were more strongly influenced by neo-Platonism than their western colleagues, who preferred a more literal reading of the new testament. To a Platonic mind, the notion that Jesus' physical body would come back to life in the resurrection was an abomination. The earliest Platonic dialogues stressed the importance of separating one's soul from the body as soon as possible. That a virtuous, omnipotent God would recover his on purpose after being clear of it was positively incomprehensible. Islam eliminated the fussy dispute in favor of the Greek perspective by insisting on a clear, strict, monotheism. This helped to facilitate its rapid spread among disaffected Syrian Christians, who were persecuted by the Christian establishment in Constantinople for their beliefs.

The explosive expansion of Islam in the eastern Empire in the 7th century was accompanied by great respect for Greek mathematics, astronomy, and medicine. By the end of the 9th century, Aristotle and much of Plato had been assimilated, with extensive, learned commentaries. The most famous commentaries are by Avicenna and Averroes. The strictly monotheistic character of

After the fall of the western Roman empire in the 7th c. A.D., the original texts of both Aristotle and Plato were lost to western readers. When the eastern empire fell to the Muslims, they inherited the original texts. This resulted in a general project of translation into Arabic, along with interpretive commentaries. These translations led to a philosophical and scientific flowering in Islam, leading to new developments in astronomy and optics.

Some texts of Aristotle began to filter back into western Europe in in the 12th century. They caused a sensation among those capable of reading the new translations. It was as though a flying saucer were to come and leave a huge encyclopedia on our doorstep, leaving us to figure it out. There was already a tradition of cathedral and monastery schools to maintain sufficient literacy in Latin to allow for performance of the liturgy. These schools developed into Europe's first universities. The chief research activity of these new institutions was to assimilate the new arabic translations of Aristotle. Assimilation meant squaring the new Aristotelian science with the neo-Platonic tradition carried along in the works of the early Church. The result was an Aristotelian/Neo-Platonic/Christian synthesis, personified by St. Thomas Aquinas (1225-1274). As Aquinas put the matter, two truths cannot contradict one another, so the Aristotelian science and divine revelation as interpreted by Church authority must all fit together.

It is important to understand that in 13th century Europe, Aristotle represented the cutting edge of science. Indeed, in 1277 Bishop Tempier of Paris condemned the works of Aquinas for spreading pernicious Aristotelian views (e.g., that the universe never had a beginning). This condemnation provided incentive for overturning Aristotle's authority, leading to a new interest in skepticism, scientific methodology, experimentation, and physical theory. For example, medieval scholars in the Merton school arrived at a geometrical analysis of uniformly accelerated motion, presaging Galileo's analysis.

By the end of the 14th c., Italian scholars were learning Greek and translating new Greek texts arriving with scholars fleeing from Constantinople after its fall in 1453 (by Turkish artillery). By the mid 16th century, many texts of Plato and other ancient figures such as Archimedes had been translated. The rhetoric of the Renaissance is filled with sharp criticism of Aristotelian authority in the university establishment by scholars and engineers with external means of support. Archimedes' theory of buoyancy did not appear in Aristotle, revealing that Aristotle's word in natural science was not final. Archimedes' engineering perspective appealed to the new class of engineers employed by Italian princes to design buildings and fortifications. Especially important was a new interest in trajectories, as old-fashioned medieval fortifications fell before the advance of the cannon. Thus, there was a flurry of interest in applying archimedes theory of buoyancy to the problem of projectile motion. Galileo's first break with Aristotle was an attempt to construct such a theory. This is in sharp contrast with Galileo's undergraduate notebooks, filled with syllogisms proving Aristotelian propositions such as the immovability of the Earth and the immutability of the heavens.

Pythagoreanism and Platonic ideas also had renewed influence into the 17th century. Thus, Copernicus argues that the Sun has the right to be in the center of the universe, in view of its brightness and excellence. Kepler followed the Pythagorean numerological project explicitly in his attempt to explain the diameters of the planetary orbits using a construction based on the Platonic solids. Galileo and Descartes, who were inspired more by the mechanical example of Archimedes, ridiculted Kepler's and Newton's hypothesis of an "occult" force acting through empty space at a distance. More on this in later lectures.


Ancient Greece faced a formidable intellectual problem. The problem had its roots in religion. Greek mystery religions were replacing the Homeric pantheon. These cults emphasized meditative purification and ascentism as a means for purging the soul of the evil influences of the body. Followers of the quasi-mythical Pythagoras (who proved the famous theorem and who discovered irrational numbers) adovcated mathematical reasoning as a means to this purification. They also held the view that all reality is ultimately number, so that mathematics is indeed adequate for mystical encounter with Truth. This curious partnership between mysticism and mathematical science was characteristic of the Greek scientific tradition, and of the Renaissance of Greek thought two thousand years later.

Parmenides: Across a range of mystical religious cultures, meditative practice is held to give rise to a firm conviction that Being (everything that is) is a necessarily existing One and that all diversity and motion is an illusion. In the Greek spirit, Parmenides offered a logical proof of this curious thesis. The argument for necessary existence is not too clear, but it seems to run something like this:

Also, Being must be One, for there is nothing else separate from it. Being must be unchanging, for if it changed, it would not-be what it changed from. So all change and multiplicity are illusions. According to Plato's dialogues, the young Socrates held Parmenides' views in high esteem.

Zeno: Parmenides had an able ally in Zeno, who assisted his master by using logic to show that the assumption of motion leads immediately to a contradiction.

Heraclitus held the opposing view, that everything changes except for rules governing the change. His motto was that you can never step in the same stream twice. Early Greek philosophers, like American presidential candidates, seemed to score higher for boldness than for wishy-washy qualifications.

Plato, Socrates' student, attempted to synthesize the two views, with emphasis on the former. One of his motives was moral. His arch-opponents, the sophists, held that moral standards are socially relative and in constant change. Plato didn't like that kind of talk. But he didn't want to deny the rality of change altogether. According to Plato, genuine truth is unchanging and inaccessible by our senses. This genuine truth concerns the Forms, abstract, unchanging entities that serve as referents to predicates such as "is good" or "is even". Physical objects "resemble" the Forms without conveying genuine information about them, so experimentation and observation are sometimes suggestive but ultimately misleading. The body is is therefore the source of ignorance, distraction, and evil. The mind should distance itself from the body by contemplating the Forms. As practice, one should do geometry, for geometrical objects, like the forms, are unchanging and accessible by pure reasoning. The world of becoming emphasized by Heraclitus has its place, but the unchanging world of the Forms is the ultimate reality and philosophical goal. Philosophy leads to virtue, because to know the form of the Good is to desire it.

Aristotle's Science

Aristotle had a penchant for proving whatever was politically convenient. It is notorious that for him, philosophy is the culmination of human and social development. While residing on the island of Lesbos, he became interested in biology. Later, his school, the Lyceum would focus on biology and history. Since this concerned just the sort of digusting filth his teacher had gone to great lengths to deplore, he felt he needed some theoretical justification.

The most central idea of Aristotle's philosophy is to replace Plato's sharp separation between forms and the world of concrete physical things with the view that forms are in things and make them the kind of things that they are. This is called hylomorphism. According to hylomorphism, forms cannot exist without concrete individuals, reversing Plato's account of primacy. Nonetheless, forms remain important. They are the objects of natural science, to be investigated through observation rather than mathematics.

Aristotle also broke with Plato's sharp distinction between the good sould and the evil body. For Aristotle, souls are forms, and hence do not exist on their own any more than the shape of a cookie exists without the cookie.

Hylomorphism is the foundation of Aristotle's approach to metaphysics and science. Laws of motion were incidental and peripheral. I will develop his system in outline to illustrate this point.


Example: human generation

Sometimes it isn't easy to work out an example of change. Aristotle's favorite example was human generation.


Aristotle's laws of motion are half-hearted, sketchy, and hopeless. They are an empirical mish-mosh of sliding friction, buoyancy, turbulence, gravitational acceleration, projectile motion, and biological locomotion. But that is exactly what happens when one is too empirical. Nobody today would propose simple kinematical "laws of motion" that cover spacecraft, airplanes, wagons, sliding stones, fish, and bullets. These cases are separated by a sophisticated theoretical tradition. Without this tradition, the central distinctions between the cases are undetectable. The scientific revolution would turn be a revolution of abstraction, not a revolution of observation. By focusing on astronomy, simple rules could be found. We now know that there are no suc rles for friction and turbulence. The situation is essentially complicated and unpredictable.

Recommended Reading

G. E. R. Lloyd (1968) Aristotle: the Growth and Structure of his Thought. Chicago: University of Chicago Press.

T. S. Kuhn (1959) The Copernican Revolution. New York: Vintage Books.