The Unmoved Mover and Aristotle: Cosmology

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The Unmoved Mover and Aristotle: Cosmology


Dynamics. In his Physics (circa 335 b.c.e.), Aristotle was the first to formulate and then explore such common Western scientific concepts as force, movement, speed, place, weight, mass, distance, and resistance. He used them to develop a theory to account for what he called “forced” or “unnatural motion” here on earth. Natural motion occurs when an object obeys its immanent tendency to move, as when a rock falls down. Unnatural motion, on the other hand, is motion that is imparted to a body from the outside, as when a rock is thrown up in the air, against its nature. Aristotle’s Physics is the beginning of the science of dynamics.

Celestial Bodies. The laws that govern the motion of celestial bodies are the subject of three books also written around 335 b.c.e. In Meteorology, On the Heavens, and the Metaphysics, science and philosophy together reach their highest point of speculation. Aristotle’s observation of the movements of the planets and stars led him to postulate the existence of an entirely new, fifth element. His reasoning was influenced greatly by his discoveries in the study of dynamics. The four terrestrial elements, inherited from Empedocles, naturally move either up or down. Moreover, they tend to move along linear vectors—that is, in straight lines—and they always stop once they have reached their natural destinations. The stars and planets, however, behave quite differently. On the one hand, they all move in regular and circular patterns, as the mathematician and astronomer Eudoxus had so elegantly shown. On the other, and even more remarkable, the stars and planets never stop. Their movement is perpetual and ceaseless, unlike anything that can be observed here in the earthly world. Aristotle reasoned that they must therefore be made of something nonterrestrial, something much more refined and perhaps even divine. This “something” is aither—an eternal material in constant circular motion, unaffected by the impermanence that touches everything on earth.

The Universe. On the basis of these five elements, then, and in keeping with his new laws of dynamics, Aristotle proceeded to construct a comprehensive picture of the entire universe. (The theory would remain virtually the same for more than 1,500 years.) At the center of the cosmos is our dense planet. It is made of elemental earth (cold and dry) that has naturally collected here. The earth’s spherical shape is confirmed, among other things, by the shadow it casts on the moon during lunar eclipses. Ideally, if the four lower elements existed in an unmixed state, the earth would be surrounded by perfectly concentric rings of water, air, and fire. This configuration is how the elements naturally tend to collect, with the heaviest at the center and the lightest at the outermost ring. However, most things in the world are in reality mixtures of earth, water, air, fire, and water, and each element is rarely found in an unadulterated condition.

Rotational Tracks. In Aristotle’s cosmology, the orbit of the moon begins precisely where fire, the lightest terrestrial element, achieves its farthest reach. Everything above this point is made of aither, the fifth and purest element of all. Beginning with the moon, a system of aitherial bodies extends out to the largest sphere of the universe. Next after the moon comes the sun, then the five known planets, and finally the fixed stars. The stars are fixed because they were believed to be embedded in the inner wall of the outermost sphere and to turn along with its slow movement. Aristotle further refined the geometrical model that Eudoxus had designed, and his celestial bodies move along a total of fifty-six rotational tracks within tracks within tracks within tracks. In his system, moreover, each orbiting sphere touches both the sphere above and the sphere below it. In this way, celestial motion is communicated from heaven’s outermost circle down into the dense, sublunary region. The spheres turn in a complex pattern of rotations and counterrotations, producing what appears in the night sky to an observer on earth.

Mechanical Model. They were meant to do much more than just produce an illusion. Aristotle connected all fifty-six spheres together, made each touch the next sphere in line, and made motion pass all the way down the line of spheres from beginning to end. This changed the Eudoxan model of the universe in a radical way. Eudoxus, after all, had proposed a geometrical model. It was an abstract, theoretical answer to an intriguing puzzle that Plato had said was in need of some solution. His concern was principally with calculating the minimum number of circular tracks that were necessary to imitate the observed movement of the sun, planets, and stars. As a result, his answer was more like a design hypothesis than a blueprint for a model that could actually work. By physically linking each sphere to the next, Aristotle transformed this geometrical model into a dynamic and mechanical one. The fifty-six spheres in his system are like the cogs in a divine machine, and their turns and counterturns are really meant to show how the heavenly bodies actually behave as they move through celestial space. Rather than being merely hypothetical, the spheres in his model obey the laws of both physics and dynamics, at least as Aristotle conceived them. The result is a working model, not simply an abstract sketch. This set the precedent for over two millennia of astronomical models built to reflect the real shape of the universe.

Natural Movement. Aristotle’s theory of causes required that the eternal, perfectly circular movement of all the celestial bodies starting with the moon be natural, not forced. Forced motion, after all, is contrary to nature, and in the end it is always defeated by the tendency every object has to move in the way that is proper to it. That is the reason why rocks always fall to earth, despite how much effort we might put into throwing them up in the air. The moon, sun, planets, and

stars continue to move around and around forever because this is in keeping with their immanent teleology.

Displacement. However, where does this motion come from? What causes it? A rock, for instance, falls because it is made of the element earth, because earth is the heaviest element, and because the heaviest always moves toward the center. Once a rock reaches the center, however, it stops moving. If we ask why a certain specific rock actually moves, then, the only answer can be that someone or something at some time removed it from its natural place at the center. The rock falls because it has been displaced, and it is attempting to return to its proper “home.”

Infinite Regress. What about the celestial bodies, then, the ones that are all made of aither the pure fifth element? They eternally move in a circle because this is their natural way; it expresses their immanent teleology. Yet, where does their motion come from? What could have started it? Aristotle reasoned that the source of celestial motion must logically be something that does not itself move. Otherwise, any attempt to understand it gets stuck on an endless treadmill of what is called infinite regress, where each answer only raises the same question all over again. If the source of celestial movement is something that also moves, then it must get its own motion from some other source, and then that source must itself be either moving or unmoved. If it is moved, then the question is thrown back yet another step. To avoid spinning the logical wheels forever, then, reason demands that the ultimate source of motion is something that is unmoved.

Prime Mover. At the source of all movement, celestial and earthly, Aristotle theorized the existence of what he called a Prime or Unmoved Mover. He identified it with God, and this God is the most complete being imaginable. Unlike everything else in the universe, which naturally strives to fulfill its inner goal or immanent teleology, God is already fully actualized. In God there is nothing potential—no dunamis at all—but only energeia (pure reality). Since God is complete, God has no need of anything else; this means that God is perfectly autonomous and self-sufficient. God is entirely separate from all other things.

Function. What does God, the Unmoved Mover, do for all eternity? The only thing such a being could do, in keeping with its perfect reality, is to spend all of eternity contemplating itself. God cannot act, after all, since action would imply that God does not already have what it needs, or that God wants something else. It is impossible, however, for God to want anything because He is by definition perfect and complete. If God cannot act, then, God must think, and the universe offers no other object of thought worthy enough of God except for God himself.

Desire. How, then, does this most perfect being, entirely absorbed in thinking about itself, make the stars and planets move? It cannot do so in the way that an efficient or moving cause makes things happen. After all, it is not as if the Unmoved Mover turns some kind of heavenly crank, which turns the sphere of the stars, which turns the spheres of Saturn, which turn the spheres of Jupiter, and so on. Instead, God makes things move in the way that a final or telic cause works. God is a totally perfect being, fully actualized—in fact, the only such being in the entire universe. As a result, God embodies the desire that every natural thing has to realize its potential and thus achieve its own perfection. Since God represents what is ultimately real, God is the ultimate aim of immanent teleology. God is the goal that every existing thing—from rock to fire to seed to human soul to wandering planet—naturally seeks to become. The whole universe turns out of desire for God.

Connections. At this stage, Aristotle passes from science to theology and religion, and thus well beyond the limits of this chapter. For Aristotle, however, these boundaries are not as definite as they are for modern people, since his world was far more interconnected than ours. No clear line separates physics from theology. The fact is that theology is simply what every rational person should study after completing the study of physics. In turn, physics leads naturally to the study of God as the Unmoved Mover of the cosmos. Everything is bound together—biology, zoology, physics, astronomy, dynamics, theology—because the theory of the four causes offers a complete and comprehensive theory of every thing and action in the universe.

Influence. The range of his theories has much to do with Aristotle’s profound influence on subsequent generations of philosophers and scientists, especially throughout the ancient world and from the thirteenth century up through the Renaissance. First, his work addressed and offered answers to questions that had occupied Greek thought for upward of three centuries: What is reality made of? How does it change into the variety of things that we experience through our senses? What causes this change? How is the universe structured? How can phenomena be saved? How can reality be apprehended? How do we know that we know? How can we prove it? How can our knowledge be communicated? Here Aristotle did not just propose grand, unified solutions to specific problems. He also analyzed the language itself in which scientific claims should be cast in order to be genuinely objective.

Scientific Disciplines. Aristotle also contributed greatly to the content and in some cases even the definition of a variety of scientific disciplines. Many of the different branches of research that make up the various departments in a modern university were either created by Aristotle or else given their first, clear definition by him. The sheer range and scope of Aristotle’s scientific attention have seldom ever been matched.

Research. Moreover, in founding the Lyceum, Aristotle created the first genuinely scientific research community in the West. He also gave it a genuinely scientific research agenda—into physics, mechanics, dynamics, medicine, geometry, astronomy, zoology, biology, botany, chemistry, and mineralogy, to name the most important fields. That agenda has driven western scientific investigation ever since. Along with the agenda, finally, Aristotle bequeathed a set of methodological procedures—especially those of logical argument and detailed, empirical observation—by which all science still continues to be guided.


J. L. Ackrill, Aristotle the Philosopher (Oxford &, New York: Oxford University Press, 1981).

Jonathan Barnes, Aristotle (Oxford & New York: Oxford University Press, 1982).

D. R. Dicks, Early Greek Astronomy to Aristotle (Ithaca, N.Y.: Cornell University Press, 1970).

Pierre Pellegrin, Aristotle’s Classification of Animals, translated by Anthony Preus (Berkeley: University of California Press, 1986).

Samuel Sambursky, The Physical World of the Greeks (London: Routledge & Kegan Paul, 1956).

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The Unmoved Mover and Aristotle: Cosmology

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The Unmoved Mover and Aristotle: Cosmology