On the Heavens

Overview

Four books, roughly a hundred and ten pages, composed by Aristotle around 350 BCE as a direct sequel to the Physics, whose general theory of motion, place and the four terrestrial elements it now extends outward, from the earth beneath our feet to the outermost boundary of the cosmos. Together with the Physics and the Meteorology, it forms one continuous project of natural philosophy, moving systematically from the most general principles of change down through the structure of the heavens and finally to the disorderly region of weather and earthquakes just above the earth’s surface.

Its historical importance is difficult to overstate. The cosmological picture worked out here, a finite, spherical universe with a motionless spherical earth fixed at its exact center, surrounded by concentric heavenly spheres made of a fifth, incorruptible element, became, after being given precise mathematical form by Ptolemy several centuries later, the standard astronomical and physical framework of the ancient and medieval world, absorbed by Islamic philosophy, by Christian scholasticism, and by university curricula across Europe, and not seriously displaced until Copernicus proposed a heliocentric alternative in the sixteenth century and Galileo’s telescopic observations and new physics helped bring the whole Aristotelian edifice down in the early seventeenth.

The argument

Book I: the fifth element and the eternity of the heavens. Aristotle opens with his most consequential move: arguing that the visible heavens cannot be made of any of the four terrestrial elements, earth, water, air and fire, each of which has a natural rectilinear motion, either straight down toward the center of the universe, as earth and water do, or straight up away from it, as air and fire do.

The heavenly bodies, by contrast, are observed to move in circles, and circular motion, Aristotle argues, is a third kind of natural motion distinct from either up or down, requiring a fifth, distinct element to be its natural bearer. This element, later called aether, is by its nature different from the terrestrial four in a crucial respect: because circular motion has no contrary, no opposite motion to reverse or undo it, whatever moves naturally in a circle can undergo no substantial generation or destruction. The heavens, made of aether, are therefore ungenerated and indestructible, existing eternally, in sharp contrast to the terrestrial world beneath the moon, where the four elements are constantly generated from and destroyed into one another.

Aristotle also argues here that the universe is finite in size, spherical in overall shape, and unique, there being, on his account, no possibility of other worlds existing beyond or alongside our own, since any additional matter would necessarily be drawn toward the single natural center that already defines this cosmos.

Book I continued: no void, no place, no time beyond the heavens. A further, philosophically ambitious argument closes out the first book: Aristotle contends that a void, empty space containing nothing, is strictly impossible, and extends this into a striking claim about what lies, if anything, beyond the outermost heavenly sphere. His answer is that nothing lies beyond it, not even empty space, because place itself, on his technical definition, is always the inner boundary of a containing body, and there is no body outside the outermost sphere to serve as such a container. It follows, he argues, that there is neither place nor void nor even time outside the heavens, since time, for Aristotle, is bound up with motion and change, neither of which can occur in a region that does not exist at all.

Book II: the shape and position of the earth, and the motion of the spheres. Turning to the earth’s own place within this finite cosmos, Aristotle argues, from several independent lines of evidence, that the earth is spherical rather than flat, citing the curved shadow the earth casts on the moon during a lunar eclipse and the fact that travelers moving north or south see different stars appear and disappear along the horizon, both correctly interpreted as consequences of a curved surface.

He further argues that the earth is stationary at the exact center of the universe rather than in motion, appealing to the observed behavior of falling bodies, which move straight toward what appears to be a single fixed center, and to what he takes as the evident symmetry and order of the heavens circling around that fixed point. This book also addresses the number and arrangement of the heavenly spheres carrying the sun, moon, planets and fixed stars, engaging with and modifying the geometric models of concentric spheres that earlier astronomers, particularly Eudoxus and Callippus, had developed to account for the observed motions of the planets.

Books III and IV: the sublunary elements, weight and lightness. The treatise’s second half shifts attention downward, from the incorruptible heavens to the changeable region of the four terrestrial elements, working out in more detail than the Physics itself the theory of natural place that governs their behavior.

Aristotle argues that heavy and light are not, as a simpler view might suppose, intrinsic properties measuring how much matter a body contains, but relative tendencies each element has toward or away from the single natural center of the universe. Earth, the heaviest element, has its natural place at the very center and so moves straight downward when displaced from it; fire, the lightest, has its natural place at the outer boundary of the sublunary region, just beneath the orbit of the moon, and so moves straight upward when displaced; water and air occupy intermediate natural places and exhibit correspondingly intermediate tendencies. Motion, on this account, is fundamentally a matter of each element seeking to return to or remain in its own proper place within a hierarchically ordered cosmos, earth at the center, then water, then air, then fire, then the incorruptible celestial spheres beyond.

How to read it

Read Book I first and read it slowly. The argument for a fifth element distinct from the terrestrial four, and the consequent eternity of the heavens, is the treatise’s foundational and most historically consequential claim, and everything that follows depends on accepting or at least understanding this starting point.

Notice how much of the reasoning proceeds from what strikes Aristotle as observationally evident, that circular motion has no contrary, that falling bodies move toward a single center, rather than from anything resembling a controlled experiment. This is characteristic of ancient natural philosophy generally, and seeing exactly how Aristotle moves from ordinary observation to sweeping cosmological conclusion is more instructive than simply noting that the conclusions turned out to be wrong.

Pay attention to the earth’s sphericity argument in Book II specifically. Unlike the treatise’s cosmology as a whole, this particular argument, from the shape of the earth’s shadow during a lunar eclipse and from the changing visibility of stars at different latitudes, was correct and remained part of the standard case for a spherical earth for the rest of antiquity and the medieval period, despite a persistent and largely mistaken modern belief that educated people before Columbus thought the earth was flat.

Read this treatise alongside the Physics, which it presupposes and extends, and, if you want to see the framework given precise mathematical and predictive form, alongside Ptolemy’s Almagest, which built a working, centuries-durable astronomical model on top of the basic Aristotelian cosmological picture established here.

Read Galileo’s Two New Sciences afterward for the other end of the story: the direct, sustained challenge, grounded in new observation and a fundamentally different physics of motion, that finally brought this entire cosmological edifice down after roughly nineteen centuries of authority.

Questions it raises

• What made the argument for a fifth celestial element, distinct in kind from the four terrestrial ones, seem so compelling for so long, and what kind of evidence would it have taken, within Aristotle’s own framework, to overturn it?
• Aristotle’s argument that nothing, not even empty space, exists beyond the outermost heavenly sphere follows from his particular definition of place. What does this suggest about how much a cosmological conclusion can depend on a prior, seemingly technical definition?
• The earth’s sphericity argument was correct while the geocentric argument was not, though both proceed from similarly careful observation. What distinguishes the two cases?
• How much of the treatise’s staying power for nearly two thousand years owed to genuine argumentative strength, and how much to its convenient fit with later theological pictures of a hierarchically ordered creation?
• Is there a meaningful difference between Aristotle’s method here, reasoning from broad observed regularities to sweeping metaphysical conclusions, and what would later be recognized as the experimental method?
• What does the eventual overthrow of this cosmology suggest about how confidently we should hold our own best current physical theories of the universe’s overall structure?