Ancient Philosophy

Socrates, Plato, and Aristotle

Aristotle's Physics

In this chapter, I intend to examine two of Aristotle's works—one entitled "Physics" and the other "On the Heavens." These two texts are closely intertwined; the reasoning of the second begins precisely where that of the first concludes. Both have had an exceptional influence and dominated scientific thought until the time of Galileo. Terms such as "quintessence" and "sublunary" derive from theories articulated in these volumes. This is why the historian of philosophy must study them, despite the likelihood that not a single phrase from either text could be deemed correct in light of contemporary science.

To understand Aristotle's views, as well as those of most Greeks, in the realm of physics, it is essential to grasp their figurative underpinnings. Every philosopher, apart from the formal system he presents to the world, possesses another, far simpler one, of which he may be entirely unaware. If he does recognize it, he likely understands that it is not entirely suitable, and therefore conceals it, bringing to the fore something more sophisticated and devoid of naiveté, which he believes resembles his original, unrefined system. He proposes this system for acceptance, convinced that he has rendered it irrefutable. Such sophistication results from the refutation of refutations, yet that alone never yields a positive outcome. It merely suggests that a theory may be true, not that it must be so. A positive result, however dimly the philosopher may recognize this, owes its existence to his figurative, biased concepts, or what Santayana terms "animal faith."

In terms of physics, the figurative underpinnings of Aristotle's views differ significantly from those of the modern researcher. Today, a child begins studying physics with mechanics, a term that itself evokes notions of machinery. He is accustomed to automobiles and airplanes. Even in the most nebulous depths of his subconscious imagination, there lies no thought that inside a car resides something akin to a horse, or that an airplane flies because its wings are akin to those of a bird endowed with magical powers. Animals have lost their former significance in our imagined representations of the world, where humanity stands relatively alone as the master of a largely lifeless and fundamentally submissive material environment.

When the Greeks attempted to scientifically characterize motion, it is unlikely that a purely mechanical theory came to mind, except for a few individuals—such geniuses as Democritus and Archimedes. Two kinds of phenomena seemed particularly significant: the movements of animals and the movements of celestial bodies. For the modern scientist, an animal's body is an exceedingly complex machine with a sophisticated physical-chemical structure; each new discovery tends to minimize the apparent divide between animals and machines. For the Greeks, it seemed more natural to liken visible motions in inanimate nature to the movements of animals. Even a child still distinguishes animals from other entities by the fact that they can move independently; many Greeks, and especially Aristotle, saw this specificity as the foundation of a theory of physics.

But what of celestial bodies? They differ from animals in the regularity of their movements; yet perhaps this is solely due to their superior perfection. Every Greek philosopher, regardless of the views he would adopt later in life, was taught in childhood to consider the Sun and Moon as gods. Anaxagoras faced persecution for heresy because he believed that the Moon and Sun were not living beings. For a philosopher who could no longer regard celestial bodies as inherently divine, it was natural to think that they moved at the behest of a divine entity, characterized by the Greek love for order and geometric simplicity. Thus, the original source of motion becomes Will; on Earth, it is the capricious Will of human beings and animals, while in the heavens, it is the immutable Will of the Supreme Mechanic.

I do not mean to assert that this applies to every detail of Aristotle's conception, but I believe this figurative foundation underlies his thought—this is what he might have acknowledged as true at the outset of his investigations.

Having made these preliminary observations, let us consider what he actually stated. According to Aristotle, physics is the science of what the Greeks called "phusis" (or "physis")—a term that translates as "nature" but bears a slightly different meaning than we ascribe to it today. We still speak of "natural sciences," "natural history," yet "nature" itself—though a highly ambiguous term—rarely signifies precisely what "phusis" once denoted. "Phusis" related to growth; one could say that the "nature" of an acorn lies in its potential to grow into an oak, and in that sense, we would use the word as Aristotle did. The nature of a thing, Aristotle argues, is its purpose, that for which it exists. Thus, this term encompasses a teleological significance. Some things exist by nature, while others exist for other reasons. Animals, plants, and simple bodies (elements) exist by nature; they possess an internal principle of motion. (The word translated as "motion" had a broader significance than merely "movement"; it encompassed changes in quality or size as well.) Nature is the source of motion or rest. Things possess nature if they have such an internal principle. The phrase "according to nature" applies to these entities and their essential attributes. (It is precisely due to this understanding that "unnatural" has come to express something negative.) Nature manifests more in form than in matter; that which is potentially flesh or bone has yet to acquire its own nature, and a thing becomes more itself as it reaches full actualization. This viewpoint appears largely inspired by biology: the acorn is an oak "in potentiality."

Nature belongs to that class of causes which act for the sake of something. This leads to a consideration of the viewpoint that nature produces necessarily, without purpose, which prompts Aristotle to discuss the survival of the fittest in the manner taught by Empedocles. This cannot be correct, he asserts, because things occur in specific ways, and when a series is complete, it turns out that all preceding steps were made for that purpose. The things that are "natural" are those which, "moving continuously under the influence of some principle within themselves, reach a certain end." While this entire concept of "nature" may seem quite suitable for explaining the growth of animals and plants, it ultimately became a significant obstacle to the progress of science and a source of much that is detrimental in ethics. Its adverse influence persists even today.

Motion, we are told, is the realization of what exists in potentiality. This view, among other shortcomings, is incompatible with the relativity of movement. When A moves relative to B, then B moves relative to A, and it is nonsensical to assert that one of the two is in motion while the other is at rest. When a dog grabs a bone, it seems commonsensical to consider that the dog is in motion while the bone remains at rest (until it is seized), and that this movement has a purpose, namely to actualize, to realize the "nature" of the dog. Suddenly, it becomes evident that this perspective is inapplicable to inanimate matter, that for scientific physics, any notion of "purpose" is meaningless, and that, strictly scientifically, no movement can be considered otherwise than relative.

Aristotle denies the existence of a vacuum, a notion defended by Leucippus and Democritus. He then proceeds to a rather curious discussion on time. One might argue, he suggests, that time does not exist, as it is composed of the past and the future, one of which no longer exists and the other of which does not yet exist. However, he rejects this perspective. Time, he asserts, is motion that allows for the process of counting (why he considers this process essential remains unclear). It is legitimate to ask, he continues, whether time could exist without a soul, since nothing can be counted without someone to count, and time involves the process of counting. He seemingly envisions time as a certain quantity of hours, days, or years. Some things, he adds, are eternal in the sense that they lie beyond time; he is presumably referring to things such as numbers.

Movement has always existed and will continue to do so, for time cannot exist without movement; and all, save for Plato, concur that time is uncreated. In this regard, Christian followers of Aristotle felt compelled to distance themselves from him, as the Bible asserts that the universe had a beginning.

Aristotle’s work, Physics, concludes with an argument in favor of the immobility of the source of motion, which we have examined in conjunction with his other treatise, Metaphysics. There exists one unmoved mover, which directly initiates circular motion. Circular motion is primary; it alone can be continuous and infinite. The first mover is without parts or dimensions and resides at the circumference of the cosmos.

Having arrived at this conclusion, we turn our attention to the heavens.

The treatise On the Heavens presents a pleasing and straightforward theory. The things below the Moon undergo birth and decay; all that lies above the Moon is neither born nor destructible. The Earth, which is spherical, occupies the center of the universe. In the sublunary sphere, everything is composed of the four elements: earth, water, air, and fire; yet there exists a fifth element from which heavenly bodies are formed. The natural motion of earthly elements is linear, whereas the movement of the fifth element is circular. The heavens are entirely spherical, and their upper parts are more divine than their lower counterparts. The stars and planets are not composed of fire but of this fifth element; their motion is a result of the movement of the spheres to which they are affixed. (All of this is poetically expressed in Dante’s Paradise.)

The four earthly elements are not eternal; they generate one another. Fire is absolutely light in the sense that its natural motion is directed upward, while earth is absolutely heavy. Air is relatively light, and water is relatively heavy.

This theory gave rise to numerous difficulties in subsequent centuries. Comets, which had been deemed destructible, were to be classified within the sublunary sphere; however, in the seventeenth century, it was discovered that comets trace orbits around the Sun and are rarely at the same distance from the Earth as the Moon. Since the natural motion of terrestrial bodies is linear, it was asserted that a projectile launched horizontally would move horizontally for some time before suddenly beginning to fall vertically. The revelation made by Galileo, demonstrating that a projectile describes a parabola, shocked his colleagues—Aristotelian followers. Copernicus, Kepler, and Galileo had to contend with Aristotle as vigorously as they did with the Bible to affirm the perspective that the Earth is not the center of the universe but rotates on its axis within a day and revolves around the Sun in a year.

Yet let us turn to more general questions. Aristotle's physics is incompatible with Newton’s “first law of motion,” initially formulated by Galileo. This law asserts that every body, left to itself, will, if it is already in motion, continue to move in a straight line at constant speed. Thus, external causes are not required to explain motion, but rather to elucidate the change in motion—its speed or direction. Circular motion, which Aristotle deemed "natural" for celestial bodies, involved a constant change in the direction of motion and consequently necessitated a force directed toward the center of the circle, as articulated in Newton's law of gravitation.

Finally, it became necessary to abandon the view that celestial bodies are eternal and indestructible. The Sun and stars exist for a long time, but not eternally. They are born from nebulous clouds and ultimately either explode or perish as they cool. Nothing in the visible world is free from change and decay; Aristotle's belief to the contrary, though embraced by medieval Christians, is a product of pagan worship of the Sun, Moon, and planets.