Why did the ancients believe celestial matter is of a totally different type than terrestrial matter?

From a footnote in Christopher A. Decaen's The Thomist 68 (2004): 375-429 article "Aristotle's Aether and Contemporary Science:"

  1. Emphasizing the dialectical character of these arguments for aether, St. Thomas notes that although all the evidence suggests that the heavenly bodies and their motions are incorruptible, it remains a possibility that we have not observed them long enough (I De Caelo, lect. 7, nn. 5-6). Aristotle himself does not try to show that the Earth is immobile at the center of the universe until De Caelo, 2.13-14, and St. Thomas points out that until this issue is settled it is possible that the Earth, not the heavens, is moving (II De Caelo, lect. 11, n. 2). While Aristotle could not be accused of circular reasoning here, since the later arguments against the Earth's mobility do not appear to rest on the assumption that the heaven is a different kind of matter, nevertheless these arguments are not demonstrative.

Thus, one argument "that the heavenly bodies and their motions are incorruptible" is the uniformity and constancy of their apparent motions over long periods of time.

Did ancient astronomers advance any other arguments "that the heavenly bodies and their motions are incorruptible"?

A plausible argument, which I don't know if any ancient astronomers have made, is that astronomy's objects of study are so far removed from the senses; thus, why would they be akin to touchable matter?

  • $\begingroup$ Are you referring only to the ancient Greeks? $\endgroup$
    – HDE 226868
    Commented Jul 19, 2015 at 13:41
  • $\begingroup$ @HDE226868 No, I'm including those all the way up to the High Middle Ages or even later. $\endgroup$
    – Geremia
    Commented Jul 20, 2015 at 8:01

5 Answers 5


One argument is that terrestrial objects are made up of the four elements earth, water, air, fire, and out of these the first two are "heavy" elements whose natural motion is rectilinear motion toward the center of the universe, while the latter two are "light" elements whose natural motion is rectilinear motion upwards. The natural motion of the heavenly bodies, on the other hand, is evidently circular. Therefore they are unlike any terrestrial matter. If they were made of stone they would fall down, and if they were made of fire they would fly away. There is a discussion more or less to this effect in Aristotle, De Caelo I.2.


It's difficult for us who have learned that the laws of physics are constant for all time and space to put ourselves in the minds of "the ancients". But all the reading I've done in this area, of the classical Greeks and the early, middle and late Middle Ages, indicates that they had absolutely no reason to believe that the sky had the same laws as the Earth. The idea of unified physics was unknown. Even two hundred years ago, anyone who claimed that electricity, magnetism and light were the "same stuff" would not have been believed.

More importantly, the ancient Greeks strictly split physics and astronomy into mutually exclusive subjects. The Heath book (in Dover Publications) about "Greek Astronomy" contains passages which scold astronomers for going outside their job of making numerical calculations to speculate on the actual nature of what they were describing numerically. So all speculations about the nature of things were non-numerical. So they never got to the point that Newton did, where he showed that the numbers for describing terrestrial and extra-terrestrial physics were a near-perfect match. It was this numerical match in the gravity equations which finally unified sky physics with Earth physics. And even then, most people found this difficult to accept. It is only because we are taught the modern viewpoint in school that we find it difficult to see how odd it used to be to think of earth and sky as a combined system obeying the same laws.

So the answer to your question is that it is the wrong question. The "correct" question is when and how did people get the idea that a single set of physical laws governs both Earth and sky. And the answer to that is that the calculations of astronomers finally led to Newton demonstrating that apples and moons obey the same laws. That clinched it!


From my studies on the Western, Chinese and Meso-American cultures(still working on ancient India) from the classical eras up to early Renaissance, the ancient deduced that heavenly bodies were made of different materials than that of the earth because the observers qualities and behaviors of heavenly objects differed radically from the observed qualities and behavior of mundane objects and materials.

Prior to the invention of modern materials like stainless steel and plastics, all known materials save gold began to degraded constantly. All metals, even bronze oxidized, woods dried, split or decayed under microbial attack, stones cracked and eroded. Producing pure substances was difficult (save for gold and sulfur) and the vast majority of pure substance either degraded or became contaminated. Pure white substances such a fresh snow, ice or purified salt, seemed to have special properties. Salt was considered a magical and material purification agent because of its pure white color and its know ability to prevent decay.

By contrast, all observable celestial objects except the moon, appeared as eternally unblemished and never decaying objects. For many traditions and schools, the observable features on the face of moon indicated it belonged to the mundane realm. Medieval western (Judaeo-christian-Islamic) astronomy divided the heavens into the sub-lunar, the corruptible earth, atmosphere and moon and the trans-lunden, the incorruptible regions beyond. The sun appeared to be made of molten cold and the fixed and wandering stars (planets) appeared a dots of pure usually white light.

Based on the available observed data, the only logical deduction was that that the materials of the heavens differed from the materials of the mundane.

Likewise, the behavior of the mundane world seemed utterly chaotic and unpredictable while the behavior of the heavens appeared a paragon of order and predictability. Cartography as we know it today did not exist so all ancient cultures had better maps of the entire heavens than they did of the earth in just a couple of hundred miles away.

Astronomy became the first science precisely because the heavens could be universally and repeatable observed. Different cultures with no communication developed the same predictive models for the motions of the sun, moon, planets and stars.

Again, based on the available observed data, the only logical deduction was that the material objects of the heavens followed different rules than material objects in the mundane realm. (It helped that measurements in astronomy are made in angles, which can be easily and repeatedly measures with tools made of many different materials but which still produce the same measure angles. Linear measurement tools are far more susceptible to the error from the composition of the tool and its deformation from changing environmental conditions.)

Observations about the different quality of materials of the heavens and their different behaviors preceded the philosophical and theological explanations for those differences.

There is a famous anecdote, usually attributed to the great British scientist Lord Kelvin. After giving a speech on ptolemaic geocentrism, a member of audience came up after and said, "It sure was silly of the Ptolemy to say that the sun went around the earth. Why to do you think he believed something so silly."

Supposedly, Lord Kelvin blinked and replied, "Because good Sir/Madam that's what it looked like." He meant that prior to the invention of the telescope, no scientific evidence existed that the sun didn't orbit the earth. All heliocentric arguments up to that point e.g. Bruno, were based on philosophy or theology, not observation and science.

Because Ptolemy got caught up in the scientific wars of the Reformation, he has gone down in history a foolish foil to Copernicus and Kepler but in reality he was one of humanities greatest scientist. His complex model of the heavens remained the most predictive and useful model of the heavens up for centuries up to Kepler. His model even made better predications than Copernicus. That stands as humanities longest standing scientific model.

It's important to remember that science isn't about producing absolute truth but rather producing the most predictive model possible. All scientific knowledge is only true or useful within a strict range of precision. Newtonian physics is only valid to a certain precision of IIRC, 6 decimal points in the motions of of all the planets save mercury and fails utterly to predict the orbit of Mercury because the Newton did not know to account for gravities distortion of space itself.

So, the usually answer answer as to why the ancient held some particular model of natural phenomena usually boils down to Lord Kelvin's answer, "because that's what it looked like to them." That divergent cultures often arrived at very similar models supports this assertions.

There ancient were hampered not by superstition, a lack of imagination of intellectual courage but by a lack of sufficiently precise and accurate scientific instruments with which to extend human senses and make repeatable measurements. They usually made the correct logical deductions and inferences based on what they could actually observe with the tools they had. The same way science works today.

  • $\begingroup$ Hi, TechZen, good to see you on HSM. We require our answers to have sources whenever possible, especially when quotes are used. Can you add in citations? Thanks. $\endgroup$
    – HDE 226868
    Commented Jul 28, 2015 at 17:30
  • $\begingroup$ This post makes a number of bold, unsubstantiated claims. Until they are backed up with trustworthy sources, I think it deserves a downvote. -1 $\endgroup$
    – Danu
    Commented Jul 29, 2015 at 22:19

The question is rather misleading. The different clestial matter was a successful theory devised by Aristotle. The sources being what they are, it is rather speculative to tell what the ancients believed. Obviously many presocratics, atomists, earlier and later, did not believe such a thing. Anaxagoras is a famous exemple. But he was tried for impiety and Aristotle apparently approved that.

Aristotle presented his view as taditional piety but it allowed him to reject all variants of atomism and to explain why mathematics is unapplicable in the sublunar work. Seen thus De Coelo is his alternative to Plato's Timaeus, something scholars have noted.

Refs: the Timaeus connection is a leading theme in Bowen, A. and Wildberg, C. (ed.). 2009. New Perspectives on Aristotle’s De Caelo. Leiden/Boston: Brill; also see Solmsen, F. 1960. Aristotle's System of the Physical World. A Comparison with his Predecessors, Cornell Studies in Classical Philology 33


The underlying issue, in today's language, was that he needed two different models to explain observations on Earth and in the sky.

For instance in his model about earthly matter he wrote that a constant force is needed in order to have a constant motion, a thing that matches many observations where attrition is manifest. Celestial bodies did not seem to work that way, so they needed to be excluded from the scope of application of that model.

Why he needed "celestial matter" to do that is a different philosophical question. Currently modern physics has disagreement between general relativity and quantum mechanics; we realize both are different approximations of what really happens and just ascribe the inconstintency to stuff we don't understand yet. Apparently Aristotle really wanted his laws to be intrinsecally true and valid everywhere.

If laws are universal and absolute, a way to reconcile the models is to differentiate among the objects themselves and saying that each obeys its own rules.


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