On the Heavens

Written in the fourth century BCE by the Greek philosopher Aristotle, On the Heavens is a treatise on cosmology and natural philosophy that investigates the structure, composition, and behavior of the physical universe. Divided into four books, the work moves from the nature of the celestial realm to the properties of earthly matter, constructing an interconnected account of why the cosmos has the form it does.

Aristotle opens Book 1 by defining the scope of natural science: It concerns bodies, magnitudes, their properties, their movements, and the principles underlying them. He establishes that a body is that which is divisible in all three dimensions and, drawing on the Pythagorean principle that three constitutes completeness, concludes that body alone among magnitudes is complete. The Pythagoreans were an ancient Greek philosophical school that emphasized mathematical relationships in nature.

From this foundation, Aristotle turns to locomotion. All natural movement, he argues, is either straight, circular, or a combination of the two. Upward motion moves away from the center of the cosmos, downward motion moves toward it, and circular motion revolves about it. Simple bodies such as fire and earth possess inherent principles of movement: Fire moves upward, earth downward. Because circular motion is simple yet cannot belong to any of the four known elements, Aristotle reasons that a fifth element must exist whose natural motion is circular. He argues that circular motion is prior to straight-line motion because the circle is complete and self-bounded, whereas any straight line is either infinite or extendable. This fifth element, which the ancients called aither, is the substance of the celestial region, more divine and prior to the four terrestrial elements.

Aristotle then attributes extraordinary properties to this celestial substance. Defining heavy as what moves naturally toward the center and light as what moves naturally away from it, he argues that the circularly moving body possesses neither heaviness nor lightness. Because generation and destruction require contraries, and circular motion has no contrary, the fifth element must be ungenerated, indestructible, and exempt from increase, diminution, and alteration. He supports this by noting that no change has ever been observed in the outermost heaven throughout recorded history.

A sustained sequence of arguments establishes that no infinite body can exist. Aristotle presents seven proofs that the circularly revolving body must be finite, centering on the impossibility of traversing infinite distances in finite time. He extends the argument to rectilinear bodies, demonstrating that infinite weight is impossible and that proportional relationships cannot hold between infinite and finite quantities.

Aristotle next argues that only one heaven, or universe, can exist. If multiple universes shared the same elements with the same natural motions, elements in one world would move toward the center of another, creating contradictions. He contends that this universe contains the entirety of matter, so no material remains from which another could be constituted. Beyond the outermost circumference, there is no place, void, or time, only an eternal, unchangeable, divine existence.

The remainder of Book 1 examines whether the heaven is generated or eternal. Aristotle refutes the view that the world was generated yet is eternal, arguing that observation consistently shows generated things are destroyed. He rejects theories of alternating cosmic combination and dissolution as describing changes in disposition rather than genuine creation and destruction. Through detailed logical analysis, he demonstrates that whatever always exists is imperishable and ungenerated, and that the terms generated and destructible are coincident, as are ungenerated and indestructible.

Book 2 reaffirms the heaven's eternity and explores its characteristics. Aristotle rejects mythological explanations such as the tale of Atlas supporting the sky and criticizes the pre-Socratic philosopher Empedocles's claim that the world is preserved by the speed of its whirling. He argues that the heaven, being animate, exhibits spatial distinctions analogous to those in animals: above and below, right and left, front and back.

The necessity of the four terrestrial elements follows from the heaven's nature: Since something must rest at the center of the revolving body, earth must exist; fire must exist as earth's contrary; and the intermediates, air and water, must exist as well. Their mutual contrariety entails generation and destruction, which requires multiple celestial motions beyond the outermost revolution to account for cyclical changes in the sublunary world, or the region below the moon's orbit. Aristotle demonstrates that the heaven must be spherical, since the sphere is the primary solid shape and any non-spherical revolving body would create void. Its motion must also be regular and uniform, because circular motion has no intrinsic beginning or end at which acceleration or deceleration could occur.

Aristotle argues that the stars are composed of the fifth element rather than fire, and that their observed warmth results from friction as the celestial sphere's motion heats the air beneath it. The stars do not move independently but are fixed to their revolving spheres. He refutes the Pythagorean theory of celestial harmony, reasoning that sounds from such enormous moving bodies would shatter physical objects, and that bodies fixed to spheres produce no independent friction.

Aristotle then addresses the earth. He catalogues rival views: The Pythagoreans placed fire at the center with the earth orbiting it; the early Greek thinker Thales held the earth floats on water; Anaximander proposed the earth remains at the center through equal relation to every extreme point. Aristotle rejects each view and argues that the earth rests at the center because its natural motion is toward the center and constrained circular motion cannot be eternal. He demonstrates the earth's spherical shape through the curved outline visible during lunar eclipses, changes in visible stars with shifts in latitude, and the convergence of all heavy matter toward a common center. He reports that mathematicians calculated the earth's circumference at 400,000 stades, an ancient Greek unit of length roughly equivalent to 185 meters.

Book 3 transitions to the sublunary world. Aristotle critiques theories that deny generation entirely, such as those of the pre-Socratic philosophers Parmenides and Melissus, and those that compose bodies from geometric planes or numbers. He establishes that every simple body must have a natural movement, that generation cannot occur from nothing, and that elements must be generated from one another through genuine transformation. He argues the elements are finite in number, criticizing Anaxagoras's theory that bodies with uniform parts throughout are fundamental and the atomist Democritus's theory of infinite atomic shapes, and refutes monistic theories that derive everything from a single element.

Book 4 presents Aristotle's account of heaviness and lightness. He defines earth as absolutely heavy, always moving toward the center, and fire as absolutely light, always moving toward the extremity, with air and water as intermediates possessing both weight and lightness. Each body's movement to its natural place is the actualization of its form, analogous to other kinds of change. Four distinct kinds of matter must exist to explain the four elements' behaviors. The treatise concludes by explaining how shape affects the speed but not the direction of motion: Broad surfaces resist the disruption of the continuous medium beneath them, allowing flat heavy objects to float, while narrow bodies part the medium easily and sink.